EXECUTIVE SUMMARY

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The purpose of this Stormwater Planning chapter is to provide a summary of fundamental principles and guidelines that should be considered when planning an urban stormwater drainage system.

Benefits of Stormwater Planning – If drainage planning is incorporated into the initial stages of an urban design, the benefits that result from a well-planned storm drainage system are numerous and include improved functionality of the drainage system, reduced development costs, and improved building sites for residential and commercial development with increased opportunities to make the storm drainage system a development amenity.

Stormwater Planning Principles- Ten principles of stormwater drainage management are identified that provide the foundation of the design criteria discussed in this manual. These principles are based on sound engineering practices in combination with other planning considerations that are separate from drainage issues. These principles are summarized below:

1. The primary stormwater planning objective is protection of human health, safety and welfare.
2. A watershed approach for stormwater planning should be adopted because water resources are affected by all who conduct activities within a watershed and, therefore, all parties should be involved in the process to care for its water resources.
3. Stormwater management planning should be compatible with other planning objectives including transportation, open space, recreation, and others.
4. Flood control is primarily an issue of space allocation; if adequate provision is not made for drainage space requirements, stormwater runoff will conflict with other land uses and may result in damage to public and private property.
5. Floodplains should be preserved wherever feasible and practical to maintain naturally occurring stormwater storage.
6. Streams and riparian corridors should be maintained as they naturally occur to the maximum extent practical because buffer areas promote filtering of pollutants from urban runoff before it enters a stream.
7. Every urban area has a minor and a major drainage system, whether or not they are actually planned or designed.
8. Impacts of urbanization should be reduced through the use of Best Management Practices (BMPs).
9. The stormwater drainage system should be designed for sustainability, with careful consideration given to the need for accessibility and maintenance.
10. A stormwater drainage system should be designed beginning with the point of discharge, with careful consideration given to downstream impacts and the effects of off-site flows.

Major Drainage Planning- Major drainageways can consist of open channels or closed conduits. In general, use of open channels is strongly preferred to closed conduits. In cases where major drainageways already exist in a natural condition, they should generally be preserved, except where special measures are necessary. Primary Channels, as defined in Chapter 6 – Open Channel Flow Design of this Manual, will be the foundation of major drainageways. Primary channels must therefore be allotted adequate space for constructing channels to manage planned hydraulic activity and for providing channel maintenance and buffers. When planning new development and redevelopment, the designer must note the drainage patterns on the site and upstream to evaluate the need for implementing a primary channel as a part of the project. Typically, as mentioned earlier, major drainageways already exist in a natural condition. If that is the case on a project then preserving the area near and around the existing major drainageway is required as well as any improvements necessary to compensate for a planned project’s impact to the major drainageway.

Floodplain management and regulation is necessary for a government to exercise its duty to protect the health, safety, and welfare of the public. There are two floodplain management goals: 1) reduce the vulnerability of the residents in the City of Rogers to the danger and damage of floods, and 2) preserve and enhance the natural characteristics of the City’s floodplains. Part of the strategy to manage flood losses involves flood insurance; the City is a participant in the National Flood Insurance Program (NFIP), which is administered by the Federal Emergency Management Agency (FEMA). The planner and engineer should proceed cautiously when planning facilities on lands below the expected elevation of the 100-year flood. Maps that can be referenced to identify flood-prone areas in the City of Rogers include: 1) FEMA National Flood insurance Program Maps, and 2) City Flood Hazard Area maps. Refer to FEMA website (http://www.fema.gov/) and City GIS (http://gis.rogersar.gov/), respectively.

Minor Drainage Planning- The minor drainage system includes features such as street inlets, storm sewers, site drainage, on-site detention and on-site best management practices (BMPs). The objective of the site collection system is to completely collect, control, and convey the required design storm for specific street classifications (see Chapter 4 – Storm Sewer System Design) and protect properties adjacent to streets during runoff from storms up to the 100-year design flow.

The objective of street drainage design is to reasonably minimize inconvenience to the traveling public, provide for safe passage of emergency vehicles during runoff from storms up to a 100-year event, and prevent damage to property and structures due to overflow of runoff from streets onto private property during runoff from storms up to a 100-year event.

Detention for flood control is designed to prevent increases in peak flow from the 1-, 2-, 5-, 10-, 25-, 50- and 100-year storms. Onsite detention shall be located at the low point(s) on the site and discharge to a public right-of-way or drainage easement unless otherwise approved by the City.

Storm water quality BMPs are required on all developments to reduce adverse impacts on downstream water quality and to meet the requirements of the City’s federally-mandated National Pollutant Discharge Elimination System (NPDES) Municipal Separate Storm Sewer (MS4) permit.

Transportation Planning - Developments near major transportation features and facilities, such as highways and railroads, should include a careful evaluation of the effects caused by any storm water conduits or structures related to the transportation facility. Many flooding problems can be created by bottlenecks of conduits under transportation-related structures, particularly by those that are older or inadequate. Conversely, removing such structures may also create downstream flooding problems.

Open Space Planning - Floodplains often serve as excellent locations for community or neighborhood open space, particularly since periodic flooding in these areas makes many types of developments unfeasible. While leaving floodplains open reduces the flood risk to a community, it also serves multiple other purposes, such as enhancement of water quality and habitat, and provides space for the creation of greenway trails and other recreational uses.

In order to encourage developers to not develop all or portions of a floodplain on their project the City has compiled a list of incentives to be considered by the City during rezone or site development applications. The magnitude and combination of how these incentives are used is at the City’s discretion (see Table SWP-1). The list of incentives is as follows:

1. The City could take deed of the undeveloped floodplain. This would move the maintenance and tax burdens attributed to the floodplain off the owner/developer and place that responsibility onto the City. Furthermore, areas to be deeded to the City shall still count towards greenspace requirements.
2. A reduction in the amount of green space required on the site could be allowed. This reduction in green space would in turn provide more useable space to develop.
3. For residential projects, increased density could be provided.
4. A reduction in the amount of road improvements required by City ordinance could be allowed.
5. Requirements established for water quality standards in Chapter 9 – Water Quality could be met by including the undeveloped floodplain area as a water quality BMP (such as vegetated filter strip) and assign credit based on how much and in what manner the floodplain is preserved.

Permitting - Common permits related to stormwater runoff are summarized and include: Site Development Plan, Preliminary Plat (City), Land Disturbance Permit (City), General Permit for Stormwater Discharges Associated with Construction Activity (DEQ), the Section 404 Permit (USACE), and Conditional Letter of Map Revision (CLOMR) and/or Letter of Map Revision (LOMR) (FEMA) as required.

Development Review Process - All Site Development Plans, Subdivision Plans (Preliminary and Final Plats) and any projects that greatly impact the City of Rogers must go through the Development Working Group (DWG) review process. To become familiar with the development approval process, and to understand the development review schedule, refer to the City of Rogers Department of Community Development’s web page that provides the current review schedule. (See link: https://www.rogersar.gov/1158/Community-Development).

Planning of the urban storm drainage system is an integral part of urban design. A well-planned urban drainage system is critical for the overall effectiveness of flood control and water quality measures. Furthermore, the drainage system is a central component of a plan that best utilizes a property and considers the natural drainage.

Planning of urban drainage facilities should be based upon integrating natural waterways, artificial channels, storm sewers, and other drainage works into the layout of a desirable, aesthetic, and environmentally-sensitive urban community. It is imperative that runoff and drainage patterns be considered early in the design process for new developments, before site layout begins, rather than attempting to superimpose drainage works on a development after it is laid out, as is frequently done with water supply and sanitary sewer facilities. A well-planned major drainage system can reduce or eliminate the need for costly underground storm sewers, and it can provide improved protection from property damage, injury, and loss of life caused by flooding.

In addition to involving drainage engineering, planning for the management of urban runoff requires a comprehensive understanding of city planning and the many social, technical, and environmental issues associated with each watershed. Therefore, the drainage engineer should serve as one member of the urban design team and should be included in the earliest stages of the urban planning process.

Planning and development of stormwater drainage systems must be guided by a set of underlying principles that are based on sound engineering practice in combination with other community objectives. Key principles that serve as the foundation of the design criteria provided in this manual are described below.

Major drainageways can consist of open channels or closed conduits. In general, use of open channels is strongly preferred to closed conduits. Primary Channels, as defined in Chapter 6 – Open Channel Flow Design of this Manual, will be the foundation of major drainageways. Open channels can include stabilized natural waterways, modified natural channels, or artificial channels with grass or other lining. Closed conduits include structures such as box culverts and large pipes.

In cases where major drainageways already exist in a natural condition, they should generally be preserved, except where any engineered improvements, such as grade control, erosion protection, or restoration, are needed. The practice of lining, straightening, narrowing, and filling major natural waterways is strongly discouraged, whether the channel is perennial (wet) or ephemeral (dry except for storm runoff). In contrast, the practice of preserving natural waterways is highly encouraged because it generally provides benefits in terms of preserving natural floodplain storage, reduction of channel erosion, water quality enhancement, preservation of habitat, and opportunities for parks, greenway trails, and other recreational uses.

Important planning-level considerations for initial major drainage planning, open channels, and floodplain regulation are discussed in Section 3.1 through Section 3.3, respectively. Detailed design criteria are not provided in this chapter but are provided, where applicable, in other chapters as noted in the text.

In addition to addressing major drainages, effective drainage planning also requires thorough attention to the initial or minor drainage system. The minor drainage system includes features such as street inlets, storm sewers, site drainage, on-site detention and on-site best management practices (BMPs). This section provides planning-level considerations for the minor drainage system and also provides references to chapters in this Manual that have detailed design criteria for specific minor drainage features.

Developments near major transportation features and facilities, such as highways and railroads, should include a careful evaluation of the effects caused by any storm water conduits or structures related to the transportation facility. Many flooding problems can be created by bottlenecks of conduits under transportation-related structures, particularly by those that are older or inadequate. For example, culverts at highway or railroad embankments can cause drainage hazards such as excessive flooding upstream of the culvert or, alternatively, can cause excessive flow velocity and erosion downstream of the culvert.

Many storm drainage problems can be avoided through cooperation and coordination between the developer or transportation agency and the local governing authority over the drainage system. Drainage conditions at transportation facilities should be investigated early in the planning process to determine what limitations exist or what costs might be required to address the situation. Furthermore, it must be shown that any improvements to an existing drainage system won’t create downstream flooding. This situation could occur when replacing historically inadequate crossings with larger crossings, where the original crossing effectively detained upstream runoff and after the improvements the runoff is now allowed to travel downstream more quickly. Proposals for new developments or improvements by transportation agencies should be closely coordinated with the City to identify drainage issues, potential problems, and requirements and incorporation of watershed planning objectives.

Floodplains often serve as excellent locations for community or neighborhood open space, particularly since periodic flooding in these areas makes many types of developments unfeasible. While leaving floodplains open reduces the flood risk to a community, it also serves multiple other purposes, such as enhancement of water quality and habitat, and provides space for the creation of greenway trails and other recreational uses.

The area adjacent to floodplains may be appropriate for a broader riparian and buffer corridor, larger scale recreational uses, or parks. The designer of new developments should consider these options for floodplains and consult the City for any watershed plans that address land use along floodplains or Master Trail plans.

Planning for any new development must consider the need for city, county, state, and federal permits early in the planning process. This is particularly important when the development will involve construction along a major drainageway. Common permits related to stormwater runoff are listed below:

• Site Development Plan, Preliminary Plat – A preliminary plan set designed to meet the requirements of the City of Rogers development ordinances. An approved Preliminary Plat is required prior to obtaining a Land Disturbance Permit (see below).
• Land Disturbance Permit – The City requires any project/site that involves a SDP approval or a Preliminary Plat to obtain a Land Disturbance Permit prior to commencement of earthwork at a project site or before more than 1 acre is disturbed. A Land Disturbance Permit will be issued by the City of Rogers only after approval of the SDP or Preliminary Plat.
• General Permit for stormwater discharges associated with construction activity – The Arkansas Department of Energy & Environment Division of Environmental Quality (DEQ) requires a permit to allow discharges of stormwater from construction sites in cases where those discharges enter surface waters of the State or a municipal separate storm sewer system (MS4) leading to surface waters of the State subject to the conditions set forth in the permit. The general permit that became effective on July 1, 2024 replaces the permit issued in 2019. The reader is encouraged to either contact DEQ or review the permit requirements on the DEQ website (http://www.adeq.state.ar.us/). Careful review of the general permit is necessary to understand which stormwater discharges are allowed under the coverage of the general permit and which are not.
• Section 404 Permit - Section 404 of the Clean Water Act requires approval from the U.S. Army Corps of Engineers (USACE) prior to discharging dredged or fill material into the “Waters of the U.S.” Waters of the U.S. include essentially all surface waters, such as all navigable waters and their tributaries, all interstate waters and their tributaries, all wetlands adjacent to these waters, and all impoundments of these waters. Any waterway with a permanent flow of water is generally considered a Water of the U.S. Some intermittent waterways also may be considered Waters of the U.S.

Wetlands are areas characterized by growth of wetland vegetation (e.g., bulrushes, cattails, rushes, sedges, willows, etc.) where the soil is saturated during a portion of the growing season or the surface is flooded during part of most years. Wetlands generally include swamps, marshes, bogs, and similar areas.

Typical activities within Waters of the U.S. and adjacent wetlands that require Section 404 permits are:

• Site development fill for residential, commercial, or recreational construction
• Construction of in-channel structures
• Placement of riprap
• Construction of roads
• Construction of dams
• Any grading within the channel of Waters of the U.S.

When activities of this type are proposed, the developer should contact the USACE to determine if a Section 404 Permit will be required and to identify major issues involved in obtaining the permit. The City of Rogers is located in the Little Rock District of the USACE.

Because Rogers is located in Benton County, any work considered to be covered under one of the several Nationwide Permits authorized by the USACE still requires the submittal of an “APPLICATION FOR DEPARTMENT OF THE ARMY PERMIT – 33 CFR 325”. Additional requirements needed to complete this permit include, but are not limited to, the following:

• Historic Preservation – evidence must be provided that a project is not going to adversely impact protected historic landmarks. The Arkansas Historic Preservation Program shall be contacted in regards to providing guidance and evidence as to whether a proposed project will or will not adversely impact protected historic landmarks.
• Endangered Species Protection – evidence must be provided that a project is not going to adversely impact protected threatened and endangered species. The US Fish and Wildlife, Arkansas Field Office shall be contacted in regards to providing guidance and evidence as to whether a proposed project will or will not adversely impact threatened or endangered species.

Floodplain Use Permit (if required) – Development requirements and restrictions in Special Flood Hazard Areas in the City of Rogers are described in Chapter 22 of the Code of Ordinances for the City of Rogers. If development is to occur within a FEMA regulatory floodplain, a floodplain use permit must be obtained from the City. In addition, if necessary, additional floodplain requirements, such as a Conditional Letter of Map Revision (CLOMR) or Letter of Map Revision (LOMR) must be obtained through FEMA or a “No Rise Certification” (for floodways) must be obtained through the City.

All Site Development Plans, Subdivision Plans (Preliminary and Final Plats) and any projects that greatly impact the City of Rogers must go through the Development Working Group (DWG) review process. To become familiar with the development approval process in the City of Rogers, and to understand the development review schedule, refer to the City of Rogers Community Development Department’s web page which provides the current review schedule. (See link: https://www.rogersar.gov/1158/Community-Development).

American Society of Civil Engineers and Water Environment Federation. 1992. Design and Construction of Urban Stormwater Management Systems. ASCE Manual and Reports of Engineering Practice No. 77 and WEF Manual of Practice FD-20. Alexandria,VA: Water Environment Federation.

City of Rogers, Arkansas, 2009. Code of Ordinances. http://library.municode.com/index.aspx?clientId=14712&stateId=4&stateName=Arkansas

City of Rogers, Arkansas, 2009. Ordinance No. 08-33. An Ordinance Amending the Code of Ordinances, City of Rogers, Arkansas Concerning Grading, Erosion Control, Stormwater Pollution Prevention, and Tree Preservation.

FEMA 2009. FEMA Map Service Center website: http://msc.fema.gov/webapp/wcs/stores/servlet /FemaWelcomeView?storeId=10001&catalogId=10001&langId=-1.

Leopold, L.B. 1994. A View of the River. Cambridge, MA: Harvard University Press.

Urban Drainage and Flood Control District. 2001. Urban Storm Drainage Criteria Manual, Volume 1. Denver, CO: Urban Drainage and Flood Control District.

Urbonas, B. 1980. Drainageway Erosion in Semi-arid Urbanizing Areas. Flood Hazard News 10:1-2, 8.

Watershed Conservation Resource Center. 2009. Sediment and Nutrient Evaluation of Blossom Way Branch. Rogers, AR: Watershed Conservation Resource Center

Water Environment Federation and American Society of Civil Engineers. 1998. Urban Runoff Quality Management. WEF Manual of Practice No. 23 and ASCE Manual and Report on Engineering Practice No. 87. Alexandria, VA: Water Environment Federation.

This section of the Manual on the determination of storm water runoff was developed using several references including: Urban Storm Drainage Criteria Manual developed by Urban Drainage and Flood Control District in Denver, Colorado; National Engineering Handbook, Section 4 (NEH-4), 1985; NRCS Technical Paper No. 40, 1961; and NRCS Technical Release No. 55, 1986. Detailed information for all references used in this section can be found at the end of this chapter.

For urban watersheds that are not complex and are generally 30 acres or less in size, it is acceptable that the design storm runoff be analyzed by the Rational Method. If properly understood and applied, the Rational Method can produce satisfactory results for the design of urban storm sewers and small on-site detention facilities.

The Soil Conservation Service Technical Release – 55 Synthetic Hydrograph Method (SCS method) is a synthetic hydrograph method developed specifically for use in urbanized and urbanizing areas. This method is useful in analyzing watersheds involving several subareas with complex runoff patterns. The method is most useful in analyzing changes in runoff volume due to development and in the evaluation and design of runoff control measures. The SCS method as described herein shall be used in all cases where the watershed being developed is characterized by complex runoff patterns and site conditions and/or is larger than 30 acres and less than 2000 acres. Complex runoff patterns and site conditions are characterized as areas with continually transitioning surface types, a collection of different flow types, numerous obstructions interfering with the runoff’s direction and flow type, etc. When a watershed is observed to contain two or more distinct interacting sub-basins consistent with the conditions as dictated above then the watershed is considered complex. This method is similar to the Rational Method in that runoff is directly related to rainfall amounts through use of runoff curve numbers (CNs). The SCS method is explained in greater detail in the National Engineering Handbook, Section 4 (NEH-4), “Hydrology” (SCS 1985).

Bedient, Philip B. and Wayne C. Huber. 2002. Hydrology and Floodplain Analysis, Third Edition. Upper Saddle River, NJ ; Prentice Hall

Debo, T.N. and A.J. Reese. 2002. Municipal Storm water Management, Second Edition. Washington, D.C.; Lewis Publishers.

Engman, E.T. 1986. Roughness coefficients for routing surface runoff. Journal of Irrigation and Drainage Engineering 112 (1): 39-53.

Federal Aviation Administration, Department of Transportation. 1970. Airport Drainage. AC No. 150/5320-5B.

Hodge, Scott A. and Gary D. Tasker. 1995. Magnitude and Frequency of Floods in Arkansas. USGS – Water-Resource Investigations Report 95-4224. Little Rock, AR: U.S. Geological Survey (in cooperation with Arkansas State Highway and Transportation Department).

Overton, D.E. and M.E. Meadows. 1976. Storm water modeling. Academic Press. New York, NY. p. 58-88.

United States Army Corps of Engineers (USACE). 2000. Hydrologic Modeling System, HEC-HMS, Technical Reference Manual. Davis, CA: USACE Hydrologic Engineering Center.

United States Department of Agriculture Natural Resource Conservation Service (USDA NRCS). 1961. Rainfall Frequency Atlas of the United States for Duration from 30 Minutes to 24 Hours and Return Periods from 1 to 100 Years. Technical Paper No. 40, Washington, D.C.: USDA.

United States Department of Agriculture Natural Resource Conservation Service (USDA NRCS). 1986. Urban Hydrology for Small Watersheds. Technical Release No. 55, Washington, D.C.: USDA.

United States Department of Agriculture Soil Conservation Service (USDA SCS). 1966. Section 4, Hydrology, National Engineering Handbook. Washington, D.C.: USDA.

United States Department of Agriculture Soil Conservation Service (USDA SCS). 1986. TR-20 Computer Program for Project Formulation Hydrology. Technical Release No. 20, revised by the Hydrology Unit and Technology Development Support Staff, SCS, February 1992 (originally developed 1964). Washington, D.C.: USDA.

Urban Drainage and Flood Control District. 2001 (Revised 2006). Urban Storm Drainage – Criteria Manual (Volume 1). Denver, CO: UDFCD

The stormwater detention requirements outlined in this chapter apply to all new developments and redevelopments.

For sites that are smaller than 1 acre, or for sites that are being redeveloped, the City may allow the property owner to pay a fee in-lieu-of implementing the detention measures described in this chapter. The fee-in-lieu option is discussed further in Section 3.1.

The primary objectives of the City’s stormwater detention requirements are described below:

• Post-project peak flow rates must not exceed pre-project conditions - Onsite detention facilities must be designed so that peak flow rates for post-project conditions are limited to pre-project levels. To maintain peak flow rates at pre-development levels, a multi-frequency outlet design approach is required. The designer must demonstrate that the 1-, 2-, 5- 10-, 25-, 50- and 100-year post-development peak flow rates are limited to the corresponding pre-development flow rates. Surface flows must also be limited to 1 cfs when entering the right-of-way for the design storm unless approved by the City Engineer. If the detention facility is also being used to provide water quality treatment, then the calculated WQCV for the facility (see Chapter 9 – Water Quality) must be added to the 100-year storage volume of the facility.
• Detain and discharge the post-development 100-year runoff – The onsite runoff from the 100-year event must be detained for a period of at least 24 hours following the peak. Wet retention ponds must detain the 100-year event’s runoff for period of at least 12 hours following the peak. All stormwater facilities must be discharged within 72 hours after the peak of the rain event.
• Low-flow orifice - Detention basin designs must include a low-flow orifice designed to discharge at the 1-year peak flow rate. The low-flow orifice must be a minimum of 2 inches in diameter to reduce the potential for plugging.
• Spillways must be designed to convey 100-year runoff - Overflow spillways for detention facilities must permit the passage of the runoff from the 100-year event, based on fully urbanized conditions for the entire tributary watershed with no upstream detention. A freeboard of 1 foot must be provided for the 100-year event design flows. If downstream safety considerations warrant, it may be necessary to size a spillway for greater than a 100-year event.

These criteria for peak flow attenuation apply for onsite facilities unless other rates are recommended in a City-approved master plan. As a result of these requirements, three conditions must be examined for determination of attenuation requirements for onsite facilities:

• Pre-project conditions
• Post-project conditions
• Fully urbanized conditions for the entire tributary watershed with no upstream detention.

American Society of Civil Engineers and the Water Environment Federation (ASCE and WEF). 1992. Design and Construction of Urban Storm water Management Systems. New York: American Society of Civil Engineers and the Water Environment Federation.

Brater, E.F. and King, H.W. 1976. Handbook of Hydraulics for the Solution of Hydraulic Engineering Problems. New York: McGraw-Hill.

Debo, T. N. and A.J. Reese. 2003. Municipal Storm Water Management. Second Edition. New York: Lewis Publishers.

Federal Aviation Administration (FAA). 1966. Airport Drainage. Washington, DC: Federal Aviation Administration.

Guo, J.C.Y. 1999a. Detention Storage Volume for Small Urban Catchments. Journal of Water Resources Planning and Management, 125(6): 380-384.

Guo, J.C.Y. 1999b. Storm Water System Design. Denver, CO: University of Colorado at Denver.

Hwang, N.H.C. and R.J. Houghtalen. 1996. Fundamentals of Hydraulic Engineering Systems. Third Edition. New Jersey: Prentice Hall.

This section is intended to provide the designer with information necessary to perform open channel hydraulic analysis related to channel geometry, channel lining, and flow characteristics. This section includes preliminary design criteria and identifies considerations in selection of channel type.

The purpose of this section is to provide design criteria for open channels, including grass-lined channels, composite channels, concrete-lined channels, riprap-lined channels, bioengineered channels, and natural channels. Open-channel hydraulic principles summarized in Section 2.0 can be applied using these design criteria to determine channel geometry and hydraulics.

American Society of Civil Engineers (ASCE). 1975. Sedimentation Engineering. American Society of Civil Engineers Manuals and Reports on Engineering Practice No. 54. New York: ASCE.

American Society of Civil Engineers and Water Environment Federation (ASCE and WEF). 1992. Design and Construction of Urban Stormwater Management Systems. American Society of Civil Engineers Manuals and Reports of Engineering Practice No. 77 and Water Environment Federation Manual of Practice FD-20. New York: American Society of Civil Engineers.

Barnes, H.H. Jr. 1967. Roughness Characteristics of Natural Channels. Geological Survey Water-Supply Paper 1849. Washington, D.C.: U.S. Government Printing Office.

Bedient, Philip B. and Wayne C. Huber. 2002. Hydrology and Floodplain Analysis, Third Edition. Upper Saddle River, NJ ; Prentice Hall

Biedenharn, D.S., C.M. Elliot, and C.C. Watson. 1997. The WES Stream Investigation and Streambank Stabilization Handbook. Vicksburg, MS: U.S. Army Corps of Engineers, Waterways Experiment Station.

Bohan, J.P. 1970 Erosion and Riprap Requirements at Culverts and Storm Drain Outlets. WES Research Report H-70-2. Vicksburg, MS: U.S. Army Corps of Engineers, Waterways Experiment Station.

Calhoun, C.C., J.R. Compton, and W.E. Strohm. 1971. Performance of Plastic Filter Cloth as Replacement for Granular Filter Materials, Highway Research Record No. 373, pp. 74–85. Washington, D.C.: Highway Research Board.

Chow, V.T. 1959. Open-Channel Hydraulics. New York: McGraw-Hill Book Company.

Daugherty, R.L. and J.B. Franzini. 1977. Fluid Mechanics with Engineering Applications. New York: McGraw Hill.

Denver Regional Council of Governments (DRCOG). 1983 Urban Runoff Quality in the Denver Region. Denver, CO: Denver Regional Council of Governments.

Fletcher, B.P. and J.L. Grace. 1972. Practical Guidance for Estimating and Controlling Erosion at Culvert Outlets, WES Miscellaneous Paper H-72-5. Vicksburg, MS: U.S. Army Corps of Engineers, Waterways Experiment Station.

Fortier, S. and F.C. Scobey. 1926. Permissible Canal Velocities. Transactions of the American Society of Civil Engineers, Volume 89, pp. 940-956.

Guo, J. 1999. Roll Waves in High Gradient Channels. Journal of Water International 24(1)65-69.

Hipschman, R.A. 1970. Erosion Protection for the Outlet of Small and Medium Culverts. Master Thesis, Civil Engineering Department, South Dakota State University.

King, H.W. and E.F. Brater. 1963. Handbook of Hydraulics for the Solution of Hydrostatic and Fluid-Flow Problems. New York: McGraw-Hill.

Lane, E.W. 1952. Progress Report on Results of Studies on Design of Stable Channels. Hyd-352. Denver, CO: Department of Interior, Bureau of Reclamation.

Lane, E.W. 1953. Progress Report on Studies on the Design of Stable Channels by the Bureau of Reclamation. Proceedings of the American Society of Civil Engineers. Volume 79. Separate No. 280, pp. 1-31. 1953.

Lane, E.W. 1955a. Design of Stable Channels. Transactions of the American Society of Civil Engineers, Volume 120, pp. 1234–1260.

Lane, E.W. 1955b. The Importance of Fluvial Morphology in Hydraulic Engineering. Proceedings of the American Society of Civil Engineers.

Leopold, L.B. 1994. A View of the River. Cambridge, MA: Harvard University Press.

Leopold, L.B. and T. Maddock Jr. 1953. The Hydraulic Geometry of Stream Channels and Some Physiographic Implications. Washington, DC: U.S. Geological Survey.

Maricopa County. 2000. Drainage Design Manual for Maricopa County. Phoenix, AZ: Maricopa County, Arizona.

Murphy, T.E. 1971. Control of Scour at Hydraulic Structures, WES Miscellaneous Paper H–71–5. Vicksburg, MS: U.S. Army Corps of Engineers, Waterways Experiment Station.

Posey, C.J. 1960. Flood Erosion Protection for Highway Fills. Bulletin No. 13. Ames, IA: Iowa Highway Research Board.

Rhoads, B.L. 1995. Stream Power: A Unifying Theme for Urban Fluvial Geomorphology. In Stormwater runoff and Receiving Systems, E.E. Herricks, ed. Boca Raton, FL: CRC Press.

Riley, A.L. 1998. Restoring Streams in Cities: A Guide for Planners, Policymakers, and Citizens. Washington, D.C.: Island Press. Washington.

Rosgen, D. 1996. Applied River Morphology. Pagosa Springs, CO: Wildland Hydrology.

Schiechtl, H. 1980. Bioengineering for Land Reclamation and Conservation. Edmonton, Alberta: University of Alberta Press.

Simons, D.B. 1957. Theory and Design of Stable Channels in Alluvial Materials. Ph.D. dissertation, Colorado State University. Fort Collins, CO.

Simons, D.B. and F. Senturk. 1992. Sediment Transport Technology. Littleton, CO: Water Publications.

Smith, C. D. 1975. Cobble Lined Structures. Canadian Journal of Civil Engineering, Volume 2.

Smith, C.D. 1974. Hydraulic Structures. Saskatoon, SK: University of Saskatchewan Printing Services.

Stevens, M.A., D.B. Simons, and F.J. Watts. 1971. Riprapped Basins for Culvert Outfalls. Highway Research Record No. 373. Washington, D.C.: Highway Research Service.

Stevens, M.A., D.B. Simons, and G.L. Lewis. 1976. Safety Factors for Riprap Protection. Journal of the Hydraulics Division 102 (5) 637–655.

Urban Drainage and Flood Control District (District). 1982. Communications between Dr. Michael A. Stevens and District staff.

———. 1984. Guidelines for Development and Maintenance of Natural Vegetation. Denver, CO: Urban Drainage and Flood Control District.

———. 1986. Comparison of Measured Sedimentation With Predicted Sediment Loads. Technical Memorandum in response to District Agreement No. 85–02.078 from WRC Engineering, Inc., to the Urban Drainage and Flood Control District. Denver, CO: Urban Drainage and Flood Control District.

U.S. Army Corps of Engineers (USACE). 1970. Hydraulics Design of Flood Control Channels. USACE Design Manual EM 1110–2–1601. U.S. Army Corps of Engineers.

———. 1991. HEC-2, Water Surface Profiles, User's Manual. Davis, CA: Army Corps of Engineers Hydrologic Engineering Center.

———. 1995. HEC-RAS, River Analysis System, User's Manual. Davis, CA: Army Corps of Engineers Hydrologic Engineering Center.

U.S. Bureau of Reclamation (USBR). 1984. Computing Degradation and Local Scour. Washington, DC: Bureau of Reclamation.

U.S. Environmental Protection Agency (USEPA). 1983. Results of the Nationwide Urban Runoff Program: Final Report. Washington D.C.: U.S. Environmental Protection Agency.

U.S. Federal Interagency Stream Restoration Working Group (USFISRWG). 1998. Stream Corridor Restoration: Principles, Processes, and Practices. Washington, DC: U.S. Federal Interagency Stream Restoration Working Group.

Vallentine, H. R. and B.A. Cornish. 1962. Culverts with Outlet Scour Control. University of New South Report No. 62. Sydney, Australia: Wales, Water Research Laboratory.

Watson, C.C., D.S. Biedenharn, and S.H. Scott. 1999. Channel Rehabilitation: Process, Design, and Implementation. Washington, D.C.: Environmental Protection Agency.

Yang, C.T. 1996. Sediment Transport: Theory and Practice. New York: The McGraw-Hill.

The function of a culvert is to convey surface water under a roadway, railroad, trail, or other embankment. In addition to the hydraulic function, the culvert must carry construction, highway, railroad, or other traffic and earth loads. Therefore, culvert design involves both hydraulic and structural design considerations. The hydraulic aspects of culvert design are set forth in this chapter.

Culverts are available in a variety of sizes, shapes, and materials. These factors, along with several others, affect their capacity and overall performance. Sizes and shapes may vary from small circular corrugated metal pipes to large concrete box sections that are sometimes used in lieu of bridges.

A careful approach to culvert design is essential, both in new land development and retrofit situations, because culverts often significantly influence upstream and downstream flood risks, floodplain management and public safety. Culverts can be designed to provide beneficial upstream and downstream conditions and to simultaneously avoid creating a negative visual impact.

The information and references necessary to design culverts according to the procedure given in this chapter can be found in FHWA’s Hydraulic Design Series, No. 5 (HDS-5 2005 - http://isddc.dot.gov/.../FHWA), Hydraulic Design of Highway Culverts.

This section describes key hydraulic principles that are pertinent to the design of culverts. Application of these principles is presented in Section 3.0 of this chapter.

HDS-5 (FHWA 2005 - http://isddc.dot.gov/.../FHWA) provides valuable guidance for the design and selection of drainage culverts. This particular circular explains inlet and outlet control and the procedure for designing culverts. Culvert design is iterative and consists of the following steps:

1. Determine the flow rate of water the culvert must carry.

2. Select a culvert shape, type, and size with a particular inlet end treatment.

3. Determine a headwater depth from the relevant charts for both inlet and outlet control for the design discharge, the grade and length of culvert, and the depth of water at the outlet (tailwater).

4. Compare the largest depth of headwater (as determined from either inlet or outlet control) to the design criteria. If the design criteria are not met, continue trying other culvert configurations until one or more configurations are found to satisfy the design parameters.

5. Estimate the culvert outlet velocity and determine if there is a need for any special features such as energy dissipators or armoring of the downstream channel.

These steps are described in Sections 3.1 through 3.5 of this chapter.

The capacity of culverts to convey water is limited by the capacity of the inlet. This is frequently overlooked by designers. Culverts and open channels are often carefully designed with full consideration given to slope, cross section, and hydraulic roughness, but without regard to the inlet limitations. Culvert designs based on uniform flow equations rarely can convey their design capacity due to limitations imposed by the inlet.

The design of a culvert, including the inlet and the outlet, requires a balance between hydraulic efficiency, purpose, and topography at the proposed culvert site. Where there is sufficient allowable headwater depth, the choice of inlets may not be critical, but where headwater depth is limited, erosion is a problem, or sedimentation is likely, a more efficient inlet may be required to obtain the necessary discharge capacity for the culvert.

Although the primary purpose of a culvert is to convey flows, a culvert may also be used to restrict flow, such as in cases where a controlled amount of water is discharged while the area upstream from the culvert is used for detention storage to reduce the peak discharge rate. In this case, an inlet with limited capacity may be the appropriate choice.

The inlet types described in this chapter may be selected to fulfill either of the above requirements depending on the topography or conditions imposed by the designer. The entrance coefficient, K_e, as defined for Equation CB-5, is a measure of the hydraulic efficiency at the inlet, with lower valves indicating greater efficiency. Inlet coefficients are given in Table CB-4.

Inlets on culverts, especially on culverts to be installed in live streams, should be evaluated relative to debris control and buoyancy. The following section discusses this further.

Scour at culvert outlets is a common occurrence and must be accounted for. The natural channel flow is usually confined to a lesser width and greater depth as it passes through a culvert barrel. Increased flow velocity typically results with potentially erosive capabilities as it exits the barrel. Turbulence and erosive eddies form as the flow expands to conform to the natural channel. However, the velocity and depth of flow at the culvert outlet and the velocity distribution upon reentering the natural channel are not the only factors that need consideration. Other factors to consider with respect to scour potential include the characteristics of the channel bed and bank material, velocity, and depth of flow in the channel at the culvert outlet, and the amount of sediment and other debris conveyed in the flow. Due to the variation in expected flows and the difficulty in evaluating the variables described above, scour prediction is an inexact science.

Bridges are important roadway hydraulic structures that are vulnerable to failure from flood related causes. In order to minimize the risk of failure, the hydraulic requirements of stream crossings must be recognized and considered in all phases of roadway development, construction and maintenance.

There are extensive manuals on bridges that are available and should be used in bridge hydraulic studies and river stability analysis. Some of the best include:

1. Hydraulics of Bridge Waterways Hydraulic Design Series No. 1 (FHWA 1978). This is a good basic reference.
2. Highway in the River Environment (Richardson 1988 draft with appendices and 1974). This is particularly good for hydraulics, geomorphology, scour, and degradation.
3. Hydraulic Analysis Location and Design of Bridges Volume 7 (AASHTO 1987). This is a good overview document.
4. Technical Advisory on Scour at Bridges (FHWA 1988). This presents information similar to references 2, 3, and 4 above, but in a workbook format, and perhaps oversimplified.

Bridges are required across nearly all open urban channels sooner or later and, therefore, sizing the bridge openings is of paramount importance. Open channels with improperly designed bridges will either have excessive scour or deposition or not be able to carry the design flow.

All structural calculations shall be in compliance with the AASHTO LRFD Bridge Design Specifications (current edition) and stamped by a structural engineer licensed in the State of Arkansas. Trail bridges shall be designed according to the LRFD Guide Specifications for Design of Pedestrian Bridges (current edition) and stamped by a structural engineer licensed in the State of Arkansas. The construction specifications shall be ARDOT’s specifications modified appropriately to reflect Rogers as the owner rather than ARDOT.

The following example problem illustrates the culvert design procedures using the RDM-Culvert spreadsheet application.

American Association of State Highway and Transportation Officials (AASHTO), Highway Subcommittee on Bridges and Structures. 2007. LRFD Bridge Design Specifications, 4^th Ed. Washington, DC: AASHTO

American Association of State Highway and Transportation Officials (AASHTO) Task Force for Roadside Safety, Highway Subcommittee on Design. 1996. Roadside Design Guide. Washington, DC: AASHTO.

———. 1987. Hydraulic Analysis. Location and Design of Bridges. Washington, DC: AASHTO.

American Concrete Pipe Association. 2000. Concrete Pipe Design Manual. Irving, TX: American Concrete Pipe Association.

Arkansas State Highway and Transportation Department (AHTD). 1982. Drainage Manual. Little Rock, AR: AHTD

———. 2000. Standard Drawing PCC-1, Concrete Pipe Culvert Fill Heights and Bedding. Little Rock, AR: Arkansas State Highway and Transportation Department.

Chow, V.T. 1959. Open Channel Hydraulics. New York: McGraw-Hill Book Company, Inc.

Ginsberg, A. 1987. HY8 Culvert Analysis Microcomputer Program Applications Guide. Washington, DC: U.S. Department of Transportation, Federal Highway Administration.

King, H.W. and E.F. Brater 1976. Handbook of Hydraulics. New York: McGraw-Hill Book Company.

New Jersey Department of Transportation (NJDOT). 2008. Soil Erosion and Sediment Control Standards. Trenton, New Jersey: NJDOT

Rhode Island Department of Environmental Management. 1989. Soil Erosion and Sediment Control Handbook. Providence, RI: RI DEM

Richardson, E.V. 1974. Highways in the River Environment. Washington, DC: U. S. Department of Transportation, Federal Highway Administration.

Scourstop.com. 2008. ScourStop. 1 Sep. 2009 .

Urban Drainage and Flood Control District. 2001. Urban Storm Drainage Criteria Manual. Denver, CO: Urban Drainage and Flood Control District.

U.S. Army Corp of Engineers (USACE). 1952. Hydrologic and Hydraulic Analysis, Computation of Backwater Curves in River Channels. Engineering Manual. Washington, DC: Department of the Army.

U.S. Bureau of Reclamation. 1960. Design of Small Dams. Denver, CO: Bureau of Reclamation.

U.S. Federal Highway Administration (FHWA). 1960. Hydraulics of Bridge Waterways. Hydraulic Design Series No.1. Washington, DC: FHWA.

———. 1965. Hydraulic Charts for the Selection of Highway Culverts. Hydraulic Engineering Circular No. 5. Washington, DC: U.S. Department of Transportation, Federal Highway Administration.

———. 1971. Debris Control Structures. Hydraulic Engineering Circular No. 9. Washington, DC: U.S. Department of Transportation, Federal Highway Administration.

———. 1972b. Hydraulic Design of Improved Inlets for Culverts. Hydraulic Engineering Circular No. 13. Washington, DC: U.S. Department of Transportation, Federal Highway Administration.

———. 1978. Hydraulics of Bridge Waterways. Hydraulic Design Series No. 1. Washington, DC: Department of Transportation.

———. 1983. Hydraulic Design of Energy Dissipators for Culverts and Channels. Hydraulic Engineering Circular No.14. Washington, DC: U.S. Department of Transportation, Federal Highway Administration.

———. 1985. Hydraulic Design of Highway Culverts. Hydraulic Design Series No. 5. Washington, DC: U.S. Department of Transportation, Federal Highway Administration.

———. 1988. Highways in the River Environment. Washington, DC: U. S. Department of Transportation, Federal Highway Administration.

———. 1996. Federal Lands Highway Project Development and Design Manual. 1996 Metric Revision. FHWA-DF-88-003. Washington, DC: U.S. Department of Transportation, Federal Highway Administration.

———. 2000. Hydraulic Design of Energy Dissipators for Culverts and Channels. Hydraulic Engineering Circular No. 14M. Washington, DC: U.S. Department of Transportation, Federal Highway Administration.

Virginia Department of Transportation (VDOT). 2002. Drainage Manual. Richmond, Virginia: VDOT

This section of the Manual on construction best management practices was developed using several references including: Urban Storm Drainage Criteria Manual developed by Urban Drainage and Flood Control District in Denver, Colorado; Stormwater Management Manual for Western Washington developed by Washington State Department of Ecology Water Quality Program; and California Stormwater BMP Handbook developed by California Stormwater Quality Association.

Best Management Practices (BMPs) are used to reduce pollutants in stormwater discharges from construction sites and to assure compliance with the terms and conditions of the Construction General Permit.

The impacts to water quality resulting from construction management facilities can be managed by controls on equipment and material storage.

Erosion controls limit the amount and rate of erosion occurring on disturbed areas. They are surface treatments and source controls that stabilize the soil exposed by excavation or grading.

Sediment controls capture soil that has been eroded before it leaves the construction site. They allow soil particles that have been suspended in runoff to be filtered through a porous media or to be deposited by slowing the flow and allowing the natural process of sedimentation to occur.

The planning for the installation of temporary or permanent erosion and sediment controls needs to begin in advance of all major soil disturbance activities on the construction site. Minimizing the area being disturbed at any given time is one of the most effective erosion control measures. This principle needs to be kept in mind whenever developing a SWPPP. All areas of exposed soil will require a control measure to be defined that is dependent on the duration of exposure.

The erosion potential associated with a construction site is reduced when stabilization techniques are employed. Existing vegetation shall be preserved where attainable. Stabilization measures shall be initiated as soon as practicable in portions of the site where construction activities have temporarily or permanently ceased.

Maximizing onsite infiltration will reduce the runoff that will require treatment prior to leaving the site. Sediment basins, detention ponds, grass swales, and sediment traps are BMPs that will encourage onsite infiltration. Infiltration should not be promoted in areas next to building foundations or in soils that are not appropriate.

The erosion and sediment control measures will also be effective in controlling wind erosion. The surface stabilization measures identified for control of precipitation-induced erosion act also to prevent soils from becoming windborne. Although these guidelines were developed to control erosion by rainfall and snowmelt, they are consistent with design principles for wind erosion and will be effective for this purpose. Refer to DEQ Regulation 18: Arkansas Air Pollution Code at www.adeq.state.ar.us

BMP Fact Sheets have been provided for each of the following construction management practices, erosion controls, and sediment controls. They are to be used as guidelines to select the controls needed for each phase of the construction project based on the different demands.

Construction management practices include the BMPs listed in Table CS-1.

At times construction activities must occur within or immediately adjacent to a waterway (drainageway, creek, stream, river, lake, reservoir or wetland). Whenever this occurs, bottom sediment and the soil will be disturbed and sediment movement will occur. The goal is to minimize the movement of sediments resulting from construction activities. This is accomplished by the use of erosion and sediment control practices described in this Manual.

The construction of underground utility lines will be subject to the following criteria:

• Utility agencies shall be responsible for compliance with the requirements of this article.
• The City of Rogers has the right to limit the amount of trench excavated in advance of utility laying. In general, such trenching shall not exceed 400 feet.
• Where consistent with safety and space considerations, excavated material is to be placed on the uphill side of trenches.
• Trench dewatering devices must discharge in a manner that will not adversely affect flowing streams, wetlands, drainage systems, or off-site property. Dewatering that discharges water in a manner that may enter into waters of the State require a Construction General Permit from the Arkansas Department of Energy & Environment Division of Environmental Quality.
• Provide storm sewer inlet protection whenever soil erosion from the excavated material has the potential for entering the storm drainage system.
• Prior to digging or construction of a trench that the fire department shall be notified by email, telephone or internet submission. Further, it is required that all trenches comply with the Occupational and Safety Act of 1980.

Utility agencies shall develop and implement Best Management Practices (BMPs) to prevent the release of sediment and discharge of pollutants from utility construction sites. Disturbed areas shall be minimized and managed. Construction site entrances shall be managed to prevent sediment tracking. Excessive sediment tracked onto public streets shall be removed immediately. The City of Rogers may adopt and impose additional BMPs on utility construction activity.

Prior to entering a construction site or subdivision development, utility agencies shall have obtained from the owner a copy of any SWPPP for the project. Any disturbance to BMPs resulting from utility construction shall be repaired immediately by the utility company in compliance with the SWPPP.

It is the responsibility of the utility agency to obtain necessary permits for the construction of utility lines within Waters of the United States.

All temporary erosion and sediment control measures shall be removed and properly disposed of within thirty (30) days after final stabilization is achieved, after the temporary measures are no longer needed, or as authorized by the City of Rogers. It may be necessary to maintain some of the control measures for an extended period of time, until the upstream areas have been fully stabilized and vegetation has sufficiently matured to provide specified cover.

Trapped sediment and disturbed soil areas resulting from the removal of temporary measures must be returned to final plan grade and permanently stabilized to prevent further soil erosion.

The qualified professional preparing the SWPPP shall submit a schedule of removal dates for the temporary control measures. The schedule should be consistent with key construction phases such as street paving, final stabilization of disturbed areas, or installation of structural stormwater controls.

Permanent post-construction BMPs that were used as sediment controls during construction shall be refurbished to a fully operational form per the design plans and SWPPP. The final site work will not be accepted by the City of Rogers until these permanent post-construction BMPs are in a final and acceptable form.

All temporary and permanent erosion and sediment controls shall be inspected, maintained, and repaired by the owner during the construction phase to assure continued performance of their intended function. Refer to the individual BMP fact sheets for maintenance guidelines.

The qualified professional preparing the SWPPP shall submit a schedule of planned maintenance activities for the temporary and permanent erosion and sediment control measures.

The water quality requirements outlined in this chapter apply to all new developments and redevelopments that add 0.1 acre of impervious area to their site.

For sites that are smaller than 0.5 acre, or for sites that are being redeveloped, the City may allow the property owner to pay a fee in-lieu-of implementing water quality control measures described in this chapter. The fee-in-lieu option is discussed further in Section 3.3.

The primary objectives of the City’s stormwater quality requirements are to:

• Protect drinking water supplies.
• Protect public health and safety related to water resources.
• Maximize the quality of water resources to enhance the quality of life.
• Enable recreational opportunities where feasible and beneficial.
• Meet federal National Pollutant Discharge Elimination System (NPDES) program requirements.

To achieve these objectives, the City requires that new developments incorporate specific design features to improve the quality of stormwater runoff. Specifically, new development must implement one or more of the water quality design principles summarized in Section 3.1 as a means to achieve the specific WQCV design requirement(s) for the site, as discussed in Section 3.2.

Structural BMPs described in this section include vegetated filter strips/grass buffers, grass swales, extended dry detention basins, extended wet detention basins, constructed wetland basins, permeable pavers, porous landscape detention, and proprietary packaged stormwater treatment systems. A brief description of each BMP is provided followed by design procedures and criteria and maintenance considerations. BMPs that capture and treat the WQCV are listed first (Extended Dry Detention Basin, Extended Wet Detention Basin, Constructed Wetland Basin, Porous Landscape Detention, and Permeable Pavers). These are followed by BMPs that do not store the WQCV but that help to reduce the DCIA (Permeable Pavers, Vegetated Filter Strip/Grass Buffer, and Grass Swale) and which can be used to reduce the required WQCV for a site as described in Appendix A.

Experience with many of the BMPs in Rogers is limited as of 2010 (when this manual was initially published). As experience with BMP design, construction, monitoring, and maintenance builds, the criteria listed below may change.

Low Impact Development (LID) is an overall development approach that is designed to mimic a site's predevelopment hydrology. The major components of LID include:

1. Conservation and protection of site features such as streams, wetlands, and valuable habitat areas and avoidance of potential problem areas such as steep slopes.
2. Minimization of site impacts by minimizing clearing and grading, preserving soils with high infiltration capacities (Hydrologic Soil Group A and B soils), limiting lot disturbance, incorporating soil amendments, disconnecting impervious surfaces, and reducing impervious surfaces.
3. Maintaining the natural time of concentration to the extent practicable through using open drainages, incorporating green spaces, flattening slopes, dispersing drainage, lengthening flow paths, using vegetative swales, maintaining natural flow paths, maximizing stream setbacks, and maximizing sheet flow.
4. Implementing LID integrated management practices (IMPs) that address runoff at its source by using design techniques that infiltrate, filter, store, evaporate, and detain runoff close to its source. Instead of conveying and treating stormwater in facilities located at the bottom of drainage areas, LID relies on practices such as open drainage swales, bioretention cells (similar to porous landscape detention), rain gardens, rain barrels, rooftop storage, depression storage, soil amendments, infiltration swales and other similar features. A typical LID site will have multiple dispersed IMPs, rather than a single BMP at the low corner of a development.
5. Implementing pollution prevention practices that focus on maintenance practices and proper use, handling and storage of materials such as pesticides, fertilizers, household hazardous waste, etc.

This rain garden is a low impact development technique that serves as a landscape amenity while also helping to reduce runoff volumes and pollutant loading.

Many of the components of the LID approach have been previously discussed in this chapter. The difference with LID is the overall site design process that incorporates all of the steps described above, resulting in a multi-faceted site design approach.

Because many LID features are natural in appearance and may rely on natural site features (e.g., preservation of soils with high infiltration capacities), it is imperative that the soil structure in these areas not be modified or compacted during construction, thereby reducing the natural infiltration capacity of the soil. This will require careful restriction on the routing of construction equipment, verification that infiltration capacities have been maintained, and possibly the addition of soil amendments.

Another critical requirement for a successful LID site is assuring that regular and proper maintenance is conducted. If the dispersed LID components are not regularly maintained by a qualified landscape professional, the LID site will likely not function as intended. Maintenance costs must be borne by the property owner or POA and maintenance easements must be provided to allow for proper access.

When designing a LID site, it is important to ensure that the landscape practices (such as rain gardens) are attractive and perceived by the property owner as adding value to the property. If these LID practices are viewed as assets, the primary motivation for their long-term maintenance is that of property owners protecting their vested economic interests.

Photograph WQ-13 – Example of a Porous Detention Island.

This porous detention island is designed to reduce runoff rates and volumes and pollutant loading.

Additional design guidance may be incorporated into this Manual in the future regarding LID. In the interim, the Low Impact Development Center website (www.lowimpactdevelopment.org/) provides a good reference for more detailed design guidance, design drawings, and specifications. For example, specifications for engineered soils can be downloaded from the LID website for bioretention cells and swales. LID site designs must be approved by the Department of Planning and Transportation and must be discussed early in the site planning process.

The purpose and objectives of this section are as follows:

1. To maintain and improve the quality of water impacted by the storm drainage system within the City;
2. To prevent the discharge of contaminated stormwater runoff and illicit discharges from industrial, commercial, residential, and construction sites into the storm drainage system within the City;
3. To promote public awareness of the hazards involved in the improper discharge of trash, yard waste, lawn chemicals, pet waste, wastewater, oil, petroleum products, cleaning products, paint products, hazardous waste, sediment, and other pollutants into the storm drainage system;
4. To encourage recycling of used motor oil and safe disposal of other hazardous consumer products;
5. To facilitate compliance with state and federal standards and permits by owners of construction sites within the City; and
6. To enable the City to comply with all federal and state laws and regulations applicable to the National Pollutant Discharge Elimination System (NPDES) permitting requirements for stormwater discharges.

American Society of Civil Engineers and Water Environment Federation. 1992. Design and Construction of Urban Stormwater Management Systems. ASCE Manual and Report on Engineering Practice No. 77 and WEF Manual of Practice FD-20. Alexandria,VA: Water Environment Federation.

Arkansas State Operating Permit Number ARR040041.

Debo, T. and A. Reese. 2002. Municipal Stormwater Management. 2^nd Edition. Boca Raton, FL: Lewis Publishers.

Horner, R.R., J.J. Skupien, E.H. Livingston, and H.E. Shaver. 1994. Fundamentals of Urban Runoff Management: Technical and Intuitional Issues. Washington, DC: Terrene Institute, in cooperation with the U.S. Environmental Protection Agency.

Low Impact Development (LID) Center Website (www.lowimpactdevelopment.org/). (Also see www.lid-stormwater.net/, which can be accessed through this website.)

Schueler, T. and H. Holland. 2000. The Practice of Watershed Protection. Ellicott City, MD: The Center for Watershed Protection.

Urban Drainage and Flood Control District. 1999. Urban Storm Drainage Criteria Manual, Volume 3. Denver, CO: Urban Drainage and Flood Control District.

Water Environment Federation and American Society of Civil Engineers. 1998. Urban Runoff Quality Management. WEF Manual of Practice No. 23 and ASCE Manual and Report on Engineering Practice No. 87. Alexandria, VA: Water Environment Federation.

The required Water Quality Capture Volume (WQCV) for a site can be reduced if measures are implemented to reduce the Directly Connected Impervious Area (DCIA) at the site. A DCIA is an impermeable area that drains directly to the improved storm drainage system without an opportunity to infiltrate into the ground. Minimizing DCIA is a land development design approach that reduces paved areas and directs storm water runoff to landscaped areas, grass buffer strips, and grass-lined swales. The purpose is to slow down the rate of runoff, reduce runoff volumes, attenuate peak flows, and facilitate the infiltration and filtering of storm water. Minimizing DCIA can also reduce pollutant loads to the storm water treatment system because of increased infiltration of runoff near the point where the runoff begins.

To reduce the amount of DCIA, slopes on a site should be designed to direct storm water runoff as sheet flow away from buildings, roads, and parking lots toward grass-covered or other pervious areas prior to reaching the storm water conveyance systems or other BMPs. In areas with high permeability soils (Hydrologic Soil Groups A and B), surface runoff may be successfully infiltrated, whereas areas with less permeable soils may require underdrain systems to reduce surface runoff. Sites with average slopes that exceed 5 percent may not be well suited to implementing some aspects of these BMPs because of the reduced potential for infiltration. Steep sites can be addressed by using terracing or retaining walls.

Minimizing DCIA can be implemented in varying degrees. UDFCD (1999) characterizes two general levels associated with minimizing DCIA as follows:

• Level 1 DCIA – Level 1 DCIA involves minimizing DCIA at the individual site development level. This approach generally involves directing runoff from impervious surfaces to flow over grass-covered areas (e.g., filter strips or swales) and providing sufficient travel time to encourage the removal of suspended solids before runoff leaves the site and enters the City storm water collection system. To gain credit for using Level 1 DCIA, all impervious surfaces must be designed to drain over grass buffer strips or swales before reaching a storm water conveyance system.
• Level 2 DCIA - A more advanced approach for minimizing DCIA involves minimizing DCIA at the subdivision level (in addition to the individual site development level of Level 1). In addition to the measures taken in Level 1, Level 2 involves replacing solid street curb and gutter systems with no curb or slotted curbing and low-velocity grass-lined swales and pervious street shoulders. Conveyance systems and storm sewer inlets are still necessary to collect runoff at downstream intersections and crossings where storm water flow rates exceed the capacity of the swales. Small culverts will be needed at street crossings and at individual driveways unless inlets are provided to convey the flow to a storm sewer. Implementing Level 2 DCIA involves a public street design differing from public improvement standards and will therefore require early planning with City staff and subdivision variances in accordance with subdivision regulations.

Based on the extent of measures used to minimize DCIA (i.e., Level 1 versus Level 2), Figure A-1 can be used to convert the actual impervious area of a site (horizontal scale) to an effective impervious area (vertical scale) for use in calculating the WQCV. The effective impervious area adjustment for Level 1 and Level 2 DCIA is incorporated into the WQCV Worksheet in the BMP spreadsheet.

Figure A-1
Imperviousness Adjustments for Levels 1 and 2 of Minimizing DCIA

(Source: Urban Storm Drainage Criteria Manual, Volume 3, Best Management Practices, UDFCD, 1999)

The City may allow the property owner to pay a fee in-lieu-of implementing water quality control measures. The fee paid in-lieu-of water quality protection measures is acceptable only if the development site disturbs less than one half (0.5) an acre and the site has not been specifically identified by the City as having a significant potential to adversely impact the quality of stormwater runoff. Sites that have an existing regional water quality control facility with adequate capacity, as determined by the City, are exempt of having to pay a fee-in-lieu of water quality protection. Proceeds from fees collected from this option will be used by the City to fund regional stormwater facilities or other measures that will benefit the quality of stormwater in the community.

The following method is used to calculate the fee paid in-lieu-of implementing stormwater BMPs:

Base fee - A base fee is calculated from the impervious surface area.

The base fee is $0.50 per square foot of Impervious Surface Area.

Base fee reductions – The amount of impervious surface area used to calculate the base fee for a site can be reduced if BMPs are implemented to reduce the amount of Directly Connected Impervious Area (DCIA) at the site. The reduction to the impervious area is dependent on the extent of BMPs implemented. Refer to Appendix A, Figure A-1 to determine the adjustment to the impervious area based on the type of BMPs employed (i.e., Level 1 DCIA versus Level 2 DCIA). Multiply the impervious area adjustment by the Impervious Area. The reduced Impervious Area is used to calculate the fee to be paid in-lieu-of implementing water quality BMPs.

Example: The impervious percentage of a 2-acre commercial site is 50%. If Level 2 DCIA measures are employed at the site, using Figure A-1 (see Appendix A), the effective impervious area is 38%. The adjustment factor is 0.38/0.5 = 0.76. Multiplying the total impervious area of the site (2 acres x 43560 sq. ft/acre x 50% impervious area = 43560 sq. ft.) by the adjustment factor (0.76) yields an effective impervious area of 33106 sq. ft., which equates to an ISU for the site of 33.1. Therefore, the adjusted fee for the in-lieu-of payment is $16,553 (based on a rate of $0.50 per square foot of Impervious Surface Area).

Cave Springs Cave is located in the northwest Arkansas community of Cave Springs, near the intersection of Arkansas Highways 264 and 112 in southern Benton County. Cave Springs Cave provides habitat for the largest known population of Ozark Cavefish (Amblyopsis rosae), a federally listed threatened species. In addition to providing habitat for federally protected species, water quality in the cave is an indicator of regional water quality in the shallow aquifer. In 2014, The Nature Conservancy and Ozark Underground Laboratory (OUL) performed an extensive literature review of cave hydrology, biology and water quality1. Based on this study, primary water quality goals for the Cave Springs Recharge Area are to limit discharges of oxygen-depleting contaminants, turbidity/fine sediments, nutrients, and metals to the groundwater system through the use of best management practices (BMPs).

The purpose of this chapter is to provide criteria and guidance for BMPs to protect the unique karst resources of the Cave Springs Recharge Area while allowing for future growth and development. The Cave Springs Recharge Area encompasses lands that are included in the municipalities of Cave Springs, Rogers, Lowell, and Springdale and has a total recharge area of 12,515 acres (19.5 square miles). Exhibit 10-1 shows the Cave Springs Recharge Area, which is comprised of two major areas:

The Direct Recharge Area includes 5,702 acres (8.9 square miles) and provides most of the recharge water for the Cave Springs cave system. This is an area where soils allow for relatively rapid recharge, and there is a direct hydrologic connection between infiltrating runoff and the karst system. The northeastern boundary of the Direct Recharge Area lies roughly parallel to, and west of, Interstate 49 (I- 49).

The Indirect Recharge Area encompasses 6,813 acres (10.6 square miles) and lies to the northeast of the Direct Recharge Area. Groundwater tracing has shown that very little of the water from losing streams in this area reaches the Cave Springs cave system. However, the dye tracing indicates that there is some groundwater movement from the Indirect Recharge Area into the Direct Recharge Area and ultimately to Cave Springs cave. I-49 lies entirely within the Indirect Recharge Area.

Exhibit 10-2 shows losing stream segments, soils mapping, major roads and other features within the Recharge Area. Based on analysis of groundwater elevations and tracing data, a “trough” in the groundwater potentiometric surface is located from Cave Springs to the east. This trough is located roughly parallel to Highway 264 and is shown on Exhibit 10-2. This groundwater trough represents a preferential pathway whereby contaminants can enter the Cave Springs groundwater system. Mapping of the Direct and Indirect Recharge Areas is based on previous studies dating back to the 1970’s and hydrogeologic studies in 2014 presented in the Groundwater Tracing and Recharge Area Delineation Summary Report (OUL 2015).

This chapter has been developed to provide criteria and guidance for compliance with the Cave Springs Area Karst Resource Conservation Regulations (CSK Regulations). This is not a stand-alone chapter and must be used in conjunction with Chapters 1 – 9 of the Drainage Criteria Manual. Chapter 9 - Water Quality provides criteria for many BMPs including swales, buffers, ponds, etc. that can be used to comply with the CSK Regulations with modifications and enhancements as noted in this chapter.

The CSK Regulations and the criteria and guidance in this chapter were developed through a stakeholder process involving representatives from the Cities of Cave Springs, Rogers, Lowell, and Springdale; the United States Fish and Wildlife Service (USFWS); The Nature Conservancy; Northwest Arkansas Regional Planning Commission (NWARPC); the Arkansas Department of Transportation (ARDOT); University of Arkansas; landowners; representatives from the construction and development industry; and others. This chapter and the CSK Regulations apply to the Direct Recharge Area. For the Indirect Recharge Area, the criteria in Chapter 9 – Water Quality apply. ARDOT has already developed plans for BMPs for the I-49 corridor that will be protective of the water quality of the karst system, and this area is not addressed in this chapter.

The CSK Regulations and criteria in this chapter are in addition to other water quality regulations in local codes and ordinances related to stormwater, wastewater, underground storage, etc. The applicant must also comply with Arkansas Department Energy & Environment Division of Environmental Quality (DEQ) permitting requirements and Northwest Arkansas Municipal Separate Storm Sewer System (MS4) requirements, as applicable. Federal regulations including Section 404 and Section 401 may apply on some sites, as well as Federal Emergency Management Agency (FEMA) and local floodplain regulations. To the extent that there are conflicting requirements between various regulations, the more restrictive/protective shall apply. These requirements do not apply to existing development or development projects that have submitted preliminary plats for governmental regulatory review, prior to the effective date of the CSK Regulations as described in Section3.1.3.

A tiered, or zoned, approach has been developed for the Cave Springs Recharge Area using risk-based vulnerability factors. Four risk-based zones are defined based on vulnerability criteria that include the following:

• Location in Direct or Indirect Recharge Area.
• Nature of existing and proposed land use.
• Ability of soils overlying high-permeability and karst layers to treat stormwater.
• Proximity to losing streams and other areas with soils that rapidly recharge the karst system.
• Potentiometric head map including Cave Springs Groundwater Trough. This is an area with high recharge potential to the groundwater system.

Requirements for site characterization, buffers, and best management practices (BMPs) vary by zones.

A Land Disturbance Permit Application, Report, and permit are required for all development projects in the Direct Recharge Area (Zones 1, 2, and 3) under the Cave Springs Area Karst Resource Conservation Regulations. Land Disturbance Permits related to the Cave Springs Indirect Recharge Area (Zone 4) will need to adhere to the regulations in the previous chapters of this manual, including MS4 and State of Arkansas requirements.

The review officer for the Land Disturbance Permit Application shall be the Director of the Department of Community Development or his or her designee unless the proposed development activity requires review by the Planning Commission and/or City Council, in which case the Land Disturbance Permit may be reviewed concurrently with other development applications as is determined appropriate and efficient by the Director of the Department of Community Development or his or her designee.

The Land Disturbance Permit shall be granted as a physical permit to keep on site at a visible and accessible location so long as the permit is active. The Land Disturbance Permit shall include any conditions of approval.

The decision of the Director of the Department of Community Development or his or her designee or Planning Commission may be appealed to the City Council.

The Land Disturbance Permit Application and Report should provide the following information at a minimum:

1. Description of proposed development project including land use, proximity to sensitive features, potential contaminant sources (both surface and subsurface) associated with development (construction and post-construction), proposed plan to disconnect impervious surfaces (as recommended by Drainage Criteria Manual, etc.).
2. A boundary map or diagram separately depicting the boundaries, if any, of the Cave Springs groundwater trough, the boundaries of Zone 1, Zone 2, and Zone 3 vulnerability areas, and depicting the boundary of losing streams as defined on the Cave Springs direct recharge area vulnerability zone map.
3. Mapping should identify karst areas, stream buffers, floodplains, and other vulnerable areas. Filtration practices shall be implemented in conjunction with conventional BMPs to provide a high level of treatment for runoff discharging to these sensitive areas.
4. Preliminary grading, erosion control, and drainage plans shall be provided. The grading and erosion control plans shall utilize soil stabilization measures and practices to minimize the impacts of the proposed disturbance including a time frame for installation of erosion control measures.
5. Preliminary wastewater plans must be provided.
6. Revegetation plans showing quantity and type of plant material to be used for revegetation, time frame for revegetation and proposed soil stabilization measures shall be provided.
7. A slope study map that indicates the areas of less than three percent grade and areas of three percent or greater grade in the inner buffer and outer buffer areas shall be provided.
8. Identification of other sensitive features and risk factors including steep slopes (greater than15%); erosive soils; areas with poor vegetative cover and areas of existing erosion; floodplains; unstable stream reaches; storage areas for hazardous materials, fertilizers, pesticides, etc.; wetlands and waterbodies; sanitary wastewater collection/storage/treatment/pumping facilities; and other potential subsurface sources.
9. Delineation of inner and outer buffers in accordance with buffer widths described by zone herein. In general, each zone will have a restrictive inner buffer where development activities will be strictly limited and an outer buffer that is determined based on site-specific characteristics. The outer buffer may be reduced in some circumstances based upon the use of BMPs that replace functions of the buffer. In these cases, encroachments into the outer buffer may be allowed as long as there are BMPs that would offset or mitigate impacts.
10. Identification of BMPs for stormwater (construction and post-construction), wastewater, industrial source controls, and runoff management practices. In cases where encroachments into the outer buffer are proposed, the applicant shall provide documentation of mitigating factors for buffer width reductions using the Cave Springs Outer Buffer Width Adjustment Worksheet (provided in this chapter).
11. For activities that involve the fill of wetland areas, evidence of acceptance of the plan by the U.S. Army Corp of Engineers shall be provided.
12. Documentation of funding mechanism for maintenance of buffer areas and BMPs and Maintenance Plan describing the nature, frequency and entity responsible for maintenance activities.

An Arkansas-registered Professional Engineer must prepare the Land Disturbance Permit Application and Report, and the final version of the report and supporting drawings must be signed and stamped by an Arkansas-registered Professional Engineer.

The reviewing entity shall use the review criteria in this section for review of Land Disturbance Permits for site development in the direct recharge area. Land Disturbance Permits shall meet all the applicable review criteria. In all cases where an application for a Land Disturbance Permit meets the applicable review criteria, an acceptable disturbance plan is required as a condition of issuance of the Land Disturbance Permit. The review criteria is outlined as follows:

1. The disturbance plan shall comply with standards, criteria and best management practices of Chapter 9 – Water Quality.
2. The minimum requirements for a Land Disturbance Permit Application and Report set forth in this chapter must be met. Any on-going maintenance in the disturbance plan shall be in a legal form that is enforceable by the City of Roger, Arkansas against the property owner or legal entity and shall include provisions for recovery of costs for enforcement against the property owners of record.
3. The Land Disturbance Permit is for a development activity that is permitted in the vulnerability zone. The following uses are prohibited within vulnerability zones 1 and 2:
   1. Excavation greater than 8-feet in depth;
   2. Basements;
   3. Airport or ground transportation terminal facilities;
   4. Heliports;
   5. Major utilities, including but not limited to waste collection, transfer, recycling, and disposal facilities, flood control or drainage facilities, and other minor utilities;
   6. Building materials and services;
   7. Heavy equipment and vehicle maintenance and repair services;
   8. Heavy equipment and vehicle sales and rentals;
   9. Gas stations;
   10. Research and development services;
   11. Vehicle washing;
   12. Vehicle storage;
   13. Food processing;
   14. Linen supply or laundry services;
   15. Salvage yards;
   16. Septic tank services;
   17. Incineration, land fill, mining and processing, and zoos;
   18. Other uses not listed above but present a contamination hazard for the karst recharge area.
4. The proposed disturbance shall avoid any grading or disturbance in the inner buffer area and outer buffer area (if applicable) except those permitted activities and uses as defined in this chapter which cannot be practically avoided if the following additional criteria are met:
   1. The area of disturbance is minimized;
   2. Adequate mitigation and best management practices are proposed in the disturbance plan; and,
   3. Site restoration and revegetation is proposed.

An applicant for a Land Disturbance Permit may apply for a variance from compliance with the review criteria set fourth in this chapter pursuant the review procedures and review criteria established as follows:

1. Applications shall follow the same review procedures and shall provide the same minimum information as required for Land Disturbance Permits. In addition, the application for a variance shall identify those review criteria from which a variance is sought and shall include a narrative and other appropriate descriptive material to describe why the requested variance or variances meet the review criteria set forth. The application shall include any information or soil studies demonstrating that the actual soil types on the subject property are different than the soil types indicated in the vulnerability zones described herein.
2. The review authority shall be the Planning Commission. Decisions of the Planning Commission may be appealed to the City Council.
3. The review authority shall use the following review criteria as the basis for a decision on an application for a variance:
   1. In all cases, conditions or mitigation may be imposed upon a variance to minimize the adverse impacts of the requested variance on the goals and objectives of the CSK regulations or to ensure compliance with approved disturbance plans; and,
   2. At least one of the following criteria must be met:
      1. The variance is needed to relieve hardship caused by the strict and literal interpretation of the Land Disturbance Permit review criteria which hardship is unique to the subject property due to unique characteristics, configuration, access, site conditions, or location of the subject property; or,
      2. The relief from the strict and literal interpretation and enforcement of a specified regulation, criteria, or best management practice is necessary to achieve compatibility and uniformity of treatment among sites in the vicinity or to attain the objectives of these CSK regulations without the grant of special privilege to the subject property; or,
      3. The relief from the strict or literal interpretation and enforcement of a specified regulation, criteria or best management practice is minimized to the extent practical and the goals and objectives of the CSK regulations are otherwise met; or,
      4. Soil studies are submitted that provide evidence the actual soils on the subject property are better than the soil types indicated in the vulnerability zone district designation and that the actual soil types allow for variance from the strict or literal interpretation and enforcement of a specified regulation, criteria or best management practice while still meeting the goals and objectives of these CSK regulations.
4. The review authority shall make the following written findings before granting a variance:
   1. That the granting of the variance will not constitute a grant of special privilege inconsistent with the limitations on other properties classified in the same vulnerability zone;
   2. That the granting of the variance will not be detrimental to the public health, safety or welfare or materially injurious to properties or improvements in the vicinity;
   3. That the variance is warranted for one or more of the following reasons:
      1. The strict, literal interpretation and enforcement of the specified regulation would result in practical difficulty or unnecessary physical hardship inconsistent with the objectives of the development code;
      2. There are several exceptional or extraordinary circumstances or conditions applicable to the site of the variance that do not apply generally to other properties in the same zone; or,
      3. The strict, literal interpretation and enforcement of the specified regulation would deprive the applicant of privileges enjoyed by the owners of other properties in the same zone district.
   4. A variance granted by the review authority may contain limitations as to time or disposition or use of the subject property in order to ensure that the stated purpose of the variance request is realized.
   5. The variance approval expires two years after approval if the Land Disturbance Permit is not commenced, provided that the review authority may approve a longer time period for the variance approval, including a permanent variance approval, as determined appropriate due to the circumstances and nature of the variance application.

The procedures to appeal a decision of the Planning Commission pursuant of these CSK regulations is set forth herein. Only a final decision of Planning Commission may be appealed. Recommendations to a decision making authority are not subject to appeal.

1. An appeal may be submitted by an applicant for a Land Disturbance Permit or by any other party with standing. The appellant must provide a written request for appeal of a decision of the Planning Commission to the City Clerk within 14 days after the date of the decision. The City Council shall conduct a public hearing within 65 days of receipt of a written request for appeal. Written notice of the public hearing date, time and location shall be mailed to the appellant via first class U.S. mail at least 10 days prior to the public hearing, unless the appellant agrees to a shorter time frame and a different notification method.
2. The City Council shall review appeals of decisions of the Director of the Department of Community Development or his or her designee after conducting a public hearing. The City Council shall render the final decision on an appeal.
3. The City Council shall use the applicable review criteria for a Land Disturbance Permit. The City Council shall review decisions de novo.
4. The City Council shall, in writing, confirm, modify, or reverse the decision within 35 days of holding the public hearing on the appeal. Any decision by the City Council that results in action modifying or reversing the decision of a city body or officer shall describe the specific reasons for the modification or reversal. Action of the City Council shall become final immediately. Failure of the City Council to act within the 35 days of holding the public hearing on the appeal shall be deemed action confirming the decision unless the applicant consents to an additional time extension.
5. A decision of the City Council is final. An aggrieved person may appeal a decision of the City Council to the district court or to another Arkansas state court or federal court of competent jurisdiction.

The enforcement of these CSK regulations may necessitate the following actions:

1. In addition to any other criminal penalties that may be prescribed by state law, any development activity which fails to obtain a permit required by these CSK regulations shall be deemed a violation of these CSK regulations.
2. In addition to any other criminal penalties that may be prescribed by state law, any development activity which fails to obtain abide by the terms and conditions of a Land Disturbance Permit issued pursuant to these CSK regulations shall be deemed a violation of these CSK regulations.
3. In addition to any other criminal penalties that may be prescribed by state law, every person violating any provision of these CSK regulations shall be deemed to have committed a violation for each and every day or portion of a day during which any violation is committed, continued, or permitted and shall be subject to the penalties contained in Rogers City Code section 1-5.
4. In addition to any other criminal penalties that may be prescribed by state law, and in addition to other fines and penalties established herein for violations of this CSK regulation, the City of Rogers, Arkansas may seek an injunction requiring complete restoration of any area disturbed in violation of these CSK regulations, or payment in lieu of restoration, and may issue stop work orders, withhold any further permits for site development and cease the processing of any site development applications related to the property, project, or owner that violates the provisions of these CSK regulations.

Maintaining adequate distance between sources of potential contaminants and receiving waters/recharge areas is a fundamental and effective BMP for managing stormwater runoff and minimizing adverse water quality effects. Buffers are used as components of water quality protection strategies in many other karst and non-karst areas in the U.S. and are an important part of the strategy for the Cave Springs Direct Recharge Area. All buffer widths referenced in this chapter and the CSK Regulations are measured from the stream centerline or center of sensitive feature/area. For stream buffers, the buffer width is on both sides of the stream centerline (for example, a 100-foot buffer would actually have a total width of 200 feet, 100 feet on each side of the stream centerline). For “point” features, the buffer width is the radius projected from the center of the feature. Buffer requirements apply only to Zones 1, 2, and 3.

The Cave Springs Outer Buffer Width Adjustment Worksheet provides buffer widths and adjustment factors. The inner buffer width is fixed, but the outer buffer width can be reduced based on site-specific factors and BMPs. The worksheet form and an example are provided below.

BMPs are required in all zones for protection of water quality during both construction and post- construction phases of development and for above ground and below ground potential contaminant sources. This section describes BMP requirements by zone for stormwater (construction and post- construction), wastewater, and others including construction dewatering and industrial discharges.

AASHTO American Association of State Highway and Transportation Officials

ARDOT Arkansas Department of Transportation

ASCE American Society of Civil Engineers

ASTM American Society for Testing and Materials

ASWCC Arkansas Soil and Water Conservation Commission

BMP Best Management Practice

CMP Corrugated Metal Pipe

CN Curve Number

CPP Corrugated Polyethylene Pipe

CWB Constructed Wetland Basin

DCIA Directly Connected Impervious Area

DEQ Arkansas Department of Energy & Environment Division of Environmental Quality

EDB Extended Dry Detention Basin

EOR Engineer of Record

EWDB Extended Wet Detention Basin

GB Grass Buffer

GS Grass Swale

IMP Integrated Management Practice

LID Low Impact Development

MBP Modular Block Porous Pavement

MEP Maximum Extent Practicable

MS4 Municipal Separate Storm Sewer System

NPDES National Pollutant Discharge Elimination System

POA Property Owners Association

PLD Porous Landscape Detention

PVC Polyvinylchloride

RCB Reinforced Concrete Box

RCHEP Reinforced Concrete Horizontal Elliptical Pipe

RCP Reinforced Concrete Pipe

SCS Soil Conservation Service

SLCCP Smooth Lined Corrugated Polyethylene Pipe

SLCMP Smooth Lined Corrugated Metal Pipe

STS Storm Sewer

TMDL Total Maximum Daily Load

TRM Turf Reinforcement Mat

UDC Unified Development Code

UDFCD Urban Drainage and Flood Control District

USACE United States Army Corps of Engineers

USDCM Urban Storm Drainage Criteria Manual

USFWS United States Fish and Wildlife Service

USEPA United States Environmental Protection Agency

WEF Water Environment Federation

WCRS Watershed Conservation Resource Center

WQCV Water Quality Capture Volume

Basin: A hydrologic unit consisting of a part of the surface of the earth covered by a drainage system consisting of a surface stream or body of impounded surface water plus all tributaries.

Best Management Practices (BMPs): A wide range of structural treatment processes, pollution prevention practices, schedules of activities, prohibitions on practices, and other management practices. Nonstructural BMPs, such as preventative maintenance and preserving natural vegetation, are mainly definitions of operational or managerial techniques. Structural BMPs include physical processes ranging from diversion structures to silt fences to retention ponds.

Clean Water Act: Legislation that provides statutory authority for the National Pollutant Discharge Elimination System (NPDES) program; Public law 92-500; 33 U.S.C. 1251 et seq. Also known as the Federal Water Pollution Control Act.

Culvert: A short, closed (covered) conduit or pipe that passes stormwater runoff under an embankment, usually a roadway.

Design Storm: A rainfall event of specific depth, duration, intensity, and return frequency (e.g., the 1-year storm) that is used to calculate runoff volume and peak discharge rate.

Detention: The storage and slow release of stormwater from an excavated pond, enclosed depression, or tank. Detention is used for pollutant removal, stormwater storage, and peak flow attenuation. Both wet (permanent pool) and dry (completely drained between runoff events) detention methods can be applied.

Drainage Facility: Systems including but not limited to watercourses, constructed channels, natural channels, storm drains, culverts, and detention/retention facilities that are used for conveyance and/or storage of stormwater runoff. See also Stormwater Facility.

Erosion: When land is diminished or worn away due to wind, water, or glacial ice. Often the eroded debris (silt or sediment) becomes a pollutant via stormwater runoff. Erosion occurs naturally, but can be intensified by land clearing activities that remove established vegetation such as farming, development, road building, and timber harvesting.

Grading: Stripping, excavating, filling and/or stockpiling soil to shape land area for development or other purposes.

Grass Buffer: Uniformly graded and densely vegetated area of turf grass. This BMP requires sheet flow to promote filtration, infiltration, and settling to reduce runoff pollutants.

Grass Swale: Vegetated drainageway with low-pitched side slopes that collects and slowly conveys runoff. Design of longitudinal slope and cross-section size forces the flow to be slow and shallow, thereby facilitating sedimentation and promoting infiltration while limiting erosion.

Hydrologic Soil Group: Soils are classified by the Natural Resource Conservation Service into four Hydrologic Soil Groups based on the soil's runoff potential. The four Hydrologic Soils Groups are A, B, C and D. Where A's generally have the smallest runoff potential and Ds the greatest.

Hydrology: The science addressing the properties, distribution, and circulation of water across the landscape, through the ground, and in the atmosphere.

Inlet: An entrance into a ditch, storm culvert, or other conveyance.

Jointing and Bedding Aggregate: ASTM C-33 sand or another aggregate material for the use of in-fill between paver blocks as well as a leveling course for the blocks as specified by the paver blocks’ manufacturer’s specs.

National Pollutant Discharge Elimination System (NPDES): The national program under Section 402 of the Clean Water Act for regulation of discharges of pollutants from point sources to waters of the United States. Discharges are illegal unless authorized by an NPDES permit.

Nonstructural BMPs: Stormwater runoff treatment techniques that use natural measures to reduce pollution levels and do not require extensive construction efforts and/or promote pollutant reduction by eliminating the pollutant source.

Outfall: The point where wastewater or drainage discharges from a sewer pipe, ditch, or other conveyance to a receiving body of water.

Peak Flow: The maximum instantaneous discharge of a stream or river at a given location. It usually occurs at or near the time of maximum stage.

Peak Runoff Rate: The highest actual or predicted flow rate (measured in cubic feet per second) for runoff from a site for a given frequency event.

Receiving Waters: Natural or manmade water systems into which materials are discharged.

Retention Pond: A BMP consisting of a permanent pool of water designed to treat runoff by detaining water long enough for settling, filtering, and biological uptake. Retention (aka wet) ponds may also be designed to have an aesthetic and/or recreational value. These BMPs have a permanent pool of water that is replaced with stormwater, in part or in total, during storm runoff events. In addition, a temporary extended detention volume is provided above this permanent pool to capture storm runoff and enhance sedimentation. It requires a perennial supply of water to maintain the pool. Retention ponds are more common in larger catchments.

Runoff: Water from rain, melted snow, or irrigation that flows over the land surface.

Runoff Coefficient: A value ranging from 0.0 to 1.0 representing the fraction of precipitation volume that becomes runoff. The runoff coefficient, C, is used in the Rational Formula to calculate a peak flow rate (cfs) by multiplying the runoff coefficient by the rainfall intensity, I (inches/hour), and the tributary drainage area, A (acres). Q = CIA.

Sediment: Soil, sand, and materials washed from land into water, usually after rain. Sediment can destroy fish-nesting areas, clog animal habitats, and cloud water so that sunlight does not reach aquatic plants.

Slope: Angle of land measured in horizontal distance necessary for the land to fall or rise one foot, expressed by horizontal distance in feet to one vertical foot. Slope may also be expressed as a percent or decimal as the quotient of vertical elevation change divided by the horizontal distance over which the change occurs.

Stormwater Facilities: Systems including but not limited to watercourses, constructed channels, natural channels, storm drains, culverts, and detention/retention facilities that are used for conveyance and/or storage of stormwater runoff. See also Drainage Facility.

Stormwater Management: Functions associated with planning, designing, constructing, maintaining, financing, and regulating the facilities (both constructed and natural) that collect, store, control, and/or convey stormwater.

Stormwater: Precipitation that accumulates in natural and/or constructed storage and stormwater systems during and immediately following a storm event.

Structural BMPs: Devices that are constructed to provide temporary storage and/or treatment of stormwater runoff. Examples of structural BMPs used on construction sites include sediment basins, silt fence, and inlet protection.

Surface Water: Water that remains on the surface of the ground, including rivers, lakes, reservoirs, streams, wetlands, impoundments, seas, estuaries, etc.

Suspended Sediment: Soil particles that remain in suspension in water for a considerable period of time without contact with the solid fluid boundary at or near the bottom. They are maintained in suspension by the upward components of turbulent currents.

Traffic Areas: Any area used by vehicular traffic (cars, trucks, buses, etc.) to travel to destinations or gain access to such destinations; essentially any area where vehicular traffic could likely be anticipated to operate. Including but not limited to: paved and unpaved streets; residential, commercial, and industrial driveways; parking lots; etc.

Waters of the State: “Waters of the state” means all streams, lakes, marshes, ponds, watercourses, waterways, wells, springs, irrigation systems, drainage systems, and all other bodies or accumulations of water, surface and underground, natural or artificial, public or private, which are contained within, flow through, or border upon this state or any portion of the state.

Watershed: That geographical area that drains to a specified point on a watercourse, usually a confluence of streams or rivers (also known as drainage area, catchment, or river basin).

Wetlands: Areas that are inundated or saturated by surface or groundwater at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs, and similar areas. Classification of an area as a wetland depends on hydrology, soils and vegetation. The USACE has jurisdiction for determining areas that are or are not jurisdictional wetlands.

The purpose of this Submittal Requirements chapter is to provide a means to standardize the plans and drainage reports for proposed improvements submitted to the City for review.

The following items shall be included in the Drainage Report that accompanies each proposed improvement plan set submitted to the City.

• Project title and date.
• Project location – include the street address and a vicinity map.
• Project description – a brief description of the site drainage for the proposed project, including structural best management practices for water quality management.
• Description of best management practices, including location, surface footprint area, depth and material of section layers, and connection to the downstream stormwater system, as applicable. Project owner’s name, address and telephone number.
• Site area – to the nearest 0.1 acre.
• Site drainage – a brief description of the site drainage for the proposed project.
• Area drainage problems – provide a description of any know on-site, downstream or upstream drainage/flooding problems. Drainage problems may include, but not be limited to, infrastructure adjacent to or downstream/upstream from the site not adequately sized to handle runoff, stormwater facilities experiencing erosion and needing remediation, and known sources of flooding. If the development is found to exacerbate or contribute to existing known drainage issues or create additional issues upstream or downstream of the site, off-site improvements to adjacent drainage facilities may be required.
• Upstream and downstream drainage – pre- and post-developed drainage area maps as well as inlet area maps with the time of concentration flow paths and proposed and existing topography shown as appropriate.
• Summary of runoff – provide a table with the 1, 2, 5, 10, 25, 50 and 100-year storm flows for existing and proposed conditions (with and without detention if shown) and the proposed difference in flows.
• Calculations – provide copies of all calculations performed, including:
  • Runoff flow calculations for the 1,2, 5, 10, 25, 50 and 100-year storm events (existing and proposed conditions),
  • Coefficients or runoff curve numbers,
  • Inlet calculations,
  • Pipe or culvert calculations,
  • Open-channel calculations including any flumes,
  • Hydraulic grade line calculations,
  • Water Quality Capture Volume (WQCV) calculations,
  • Detention calculations including
    • Basin sizing calculations
    • Outlet structure design with release rates computations for the 1, 2, 5, 10, 25, 50 and 100-year storm events,
  • Best management practices’ storage calculations or stage-storage capacity of the WQCV within onsite stormwater best management practices, as applicable.
• Operations and Maintenance manual, as applicable.
• Operations and Maintenance checklist and agreement letter signed by the project owner indicating perpetual maintenance, as applicable.
• Recommendations/Summary – description of any assumptions made in the calculations, drainage improvements to be made to the site or downstream/upstream of the site, and the expected effects of the project.
• Certification – all drainage reports shall be signed, sealed and dated by an engineer registered in the State of Arkansas and shall include the following certification:
  
  I ___________________, Registered Professional Engineer No.___________ in the State of Arkansas, hereby certify that the drainage designs and specifications contained in this Report have been prepared by me, or under my responsible supervision, in accordance with the regulations of the City of Rogers, Arkansas, the Professional Engineers Registration Act of the State of Arkansas, and reflect the application of generally accepted standards of engineering practice. I further certify that the improvements outlined in this Report will not have any adverse effects to life or downstream properties. I understand that review of these plans is limited to general compliance with the City codes and regulations and does not warrant the engineer’s design or imply any liability to the City of Rogers for the designs contained herein.
  
  _________________________________________
  Signed and Sealed by Professional Engineer

• Should a development alongside a primary channel require a hydraulic and hydrologic analysis to model the impacts of the development on the overall watershed and the channel itself, a peer review of the model analysis from a qualified Professional Engineer may be required before final approval is granted from the City. The reviewer will be chosen by the City and a flat rate fee of five thousand dollars ($5,000.00) will be required to be paid to the City of Rogers to cover the cost of the peer review. The payment of the peer review fee will be received and filed with the Department of Community Development prior to Final Plat for a subdivision or issuance of the Certificate of Occupancy for a Site Development. Computer modeling software utilized must be found on the most up to date list of FEMA approved Hydrologic & Hydraulic Numerical Models Meeting the Minimum Requirement of National Flood Insurance Program.

Final as-built plans and a certification letter shall be submitted to the City’s Planning Office upon completion of all work for the drainage improvements. The certification letter shall be signed and sealed by the engineer of record affirming that all improvements have been constructed as shown in the as-built plans which shall conform to the approved construction plans except for modifications approved through the City. All improvements must be in place and as-built plans, certifications, one-year maintenance bond for 100% of the cost of drainage improvements and easements provided to the City Planner prior to Final Plat for a subdivision or issuance of the Certificate of Occupancy for a Site Development. Drainage improvements that are incomplete or fail an inspection must be completed and pass a reinspection and may not be bonded as a performance bond. As-built plans shall be based on surveyed data of the constructed improvements. As-builts will be submitted on:

• An AutoCAD .dwg file formatted to AutoCAD 2013 or earlier
• One PDF copy of as-built plans and drainage report
• Shapefile information per the Engineering Manual

If drainage planning is incorporated after other decisions have been made related to the layout of a new project, costly drainage and urban space allocation problems may result that are difficult to correct. In contrast, if drainage planning is incorporated into the initial stages of an urban design, the benefits that result from a well-planned storm drainage system are numerous and include the following:

Improved functionality of drainage system

• Minimized increases in peak flow rates, diversions, improper discharges, and other actions that can potentially harm neighboring properties
• Minimized constrictions to flow conveyance and storage
• Improved water quality
• Protection and enhancement of environmentally sensitive areas
• Improved public health, safety and welfare

Reduced development costs

• Reduced storm drainage system construction and maintenance costs
• Reduced excavation, fill, and grading costs
• Reduced street construction and maintenance costs
• Reduced costs for open space and parks

Improved building sites and land use

• Improved building sites for residential and commercial development
• Improved aesthetics of overall development and increased opportunities to make the storm drainage system a development amenity
• Increased recreational opportunities

Watershed plans identify requirements for flood control, detention, and water quality management throughout a watershed. As watershed plans are completed and made available to the public, developments can be designed in accordance with the plans, which provide a basis for the proper location and sizing of inlets, pipes, detention basins, and Best Management Practices (BMPs) that are necessary to effectively control downstream flooding and meet water quality requirements. These factors will have a direct bearing on the layout of a new development.

During the master planning phase, major decisions are made related to drainage that address factors such as design velocities, locations of structures, open space allocation for drainages, and integration of drainage features with recreational uses. Potential alternate uses for stormwater facilities, such as parks or open space, are identified for open channels, detention facilities, and water quality facilities. In addition, the master planning phase involves making decisions whether to use downstream or upstream detention storage, and the use of either off-stream or in-channel ponds or reservoirs. It is noted that off-channel detention is preferred and on-line detention requires approval by the City staff during the conceptual phase of the development process.

Major Drainage System - The major drainage system frequently consists of open channels, as either stabilized natural waterways, modified natural channels, or artificial channels with grass or other lining; alternatively, the major drainage system may also include closed conduits such as box culverts or large pipes. When well-planned, the major system can reduce or eliminate the need for underground storm sewers, and can protect an urban area from extensive property damage, injury, and loss of life from flooding.

The major drainage system exists in a community regardless of whether it has been planned and regardless of where development is located. The planning process can best serve the community by ensuring that natural drainageways are maintained along major drainage routes. Floodplain delineation and zoning are tools that should be used freely to designate major drainageways. Small waterways and valleys lend themselves to floodplain regulations in the same manner as larger creeks.

Minor Drainage System - The minor drainage system, or initial system, consists of grass and paved swales, streets and gutters, storm sewers, and smaller open channels. If properly planned and designed, the minor drainage system can eliminate many "complaint" calls to the city. A well planned minor drainage system provides convenient drainage, reduces costs of streets and storm sewers, and has a direct effect on the orderliness of an urban area during runoff events.

Planning of urban drainage features should proceed on a well-organized basis with a defined set of drainage policies that have the backing of suitable ordinances. The policies presented in this Manual provide a basis upon which additional localized and specific policies can be built.

The primary objective of stormwater drainage design is the protection of public health, safety, and welfare. Stormwater systems should be designed to minimize the potential for health risks associated with stormwater systems and runoff and should minimize the risk of damage to both public and private property, including minimizing the risk of structure inundation. Streets and the minor drainage system should be designed for the safe and efficient movement of traffic to the maximum extent practicable. Consideration should also be given to the public health and welfare benefits that result from the protection of water quality and other environmental characteristics of a watershed.

The water resources of a watershed are affected by all who conduct activities within it and, therefore, all should be a part of the process to care for its water resources. Stormwater drainage is independent of government boundaries and, hence, stormwater system planning and implementation should include coordination with all affected agencies, communities, and neighborhoods within the watershed, regardless of government boundaries. The watershed approach to stormwater drainage and management has been embraced by the U.S. Environmental Protection Agency (USEPA) and many other agencies and communities across the country.

In addition to protecting public health, safety and welfare, the stormwater drainage system must consider other urban planning objectives. Stormwater system planning and design for any new development must be compatible with watershed master plans and objectives and be coordinated with plans for land use, open space, transportation, and other community objectives. Watershed master plans must consistently address both stormwater quantity and quality issues in the context of the local and regional drainage basins.

Flood control is primarily an issue of space allocation. The amount of stormwater runoff present at any time in an urban watershed cannot be compressed or diminished. Open and enclosed storm systems serve both conveyance and storage functions. If adequate provision is not made for drainage space requirements, stormwater runoff may conflict with other land uses and result in damage to public and private property and the impairment or disruption of other urban systems. In urban watersheds that have been developed without adequate stormwater planning, there is generally inadequate space available to construct detention storage facilities to reduce peak flows significantly along major waterways. Creation of adequate space to construct such storage facilities frequently requires the removal of valuable existing buildings or other facilities and is often not economically or socially feasible.

Floodplains should be preserved wherever feasible and practical to maintain naturally occurring stormwater storage. Floodplains serve as natural outfall areas for urban drainage, riparian corridors, and habitat for diverse ecological systems. Encroachment into floodplains should be avoided and should occur only after careful planning and engineering have been conducted so that the effects are fully recognized and minimized. Preservation of urban floodplains provides value to communities through flood hazard reduction, water quality enhancement, stream protection, preservation of plant and animal habitat, creation of open spaces and linear parks, and provision of recreational opportunities. When determining the width of a floodplain to preserve, consideration should be given to the intended use of the floodplain and the dynamic nature of stream channels.

As discussed in the Chapter Summary, the City has compiled a list of incentives to be considered during rezone or site development applications to encourage developers to not develop all or portions of a floodplain on their project. A list of these incentives along with additional detail describing suggested criteria to be used during negotiations is provided in Table SWP-1. The magnitude and combination of how these incentives are used is at the City’s discretion. The City recommends that the owner/developer meet with City staff to determine the total incentives that will be allowed.

Streams and riparian corridors should be maintained as they naturally occur to the maximum extent practical. Providing buffers between valuable riparian corridors and urban development promotes filtering of pollutants from urban runoff before it enters a stream. Each site’s development plan should include careful consideration to preserve and enhance natural features, including riparian corridors, to the maximum extent practicable. Consideration should be given to environmentally sensitive stream stabilization in areas where urbanization, altered hydrology, or soil characteristics result in unstable natural channel conditions. In certain cases, urban hydrologic conditions will require structural stabilization of streams to avoid degradation. These improvements should be completed in an aesthetic and environmentally sensitive manner.

In December 2009 the Watershed Conservation Resource Center (WCRC) submitted its final report on “Sediment and Nutrient Evaluation of Blossom Way Branch” to the City of Rogers. This report and study consisted of “1) Conducting a land use analysis of Blossom Way Watershed; 2) Developing streambank erosion prediction curves for the Osage Creek basin and evaluating streambank erosion occurring on Blossom Way tributary; and 3) Assessing watershed conditions for sediment and nutrients.” (WCRC 2009). A copy of the final report and related materials can be viewed at the City’s Engineering and Planning office. Additionally, the City has incorporated the information obtained and prepared from this study into the City’s GIS (http://gis.rogersar.gov). The information displayed on the GIS shows BEHI and NBS Ratings which are indications of how badly and likely the streambank in certain locations within the stream will erode.

The intent of discussing and providing information about this study is to make designers aware that information does exist about the condition of local streambanks within the City. Designers should review the information and maps provided in WCRC’s report whenever they have a project in the vicinity of a stream that was studied in this report. It is the responsibility of the design engineer to make sure that their project and its discharge amounts and discharge locations do no additional harm to streambanks in vicinity of the project.

Every urban area has a minor and a major drainage system, whether or not they are actually planned or designed. Generally, the minor and major drainage systems have distinctly different design criteria based on public health, safety and welfare, and economic considerations. The minor drainage system is typically designed to accommodate moderate flooding. For minor drainage systems, local street flooding resulting from extreme, less frequent rainfall events may be permissible for short periods, provided that public health, safety, and welfare are protected, and structures are protected from inundation. The major system will generally have a higher design standard to minimize the impacts of flooding from more severe, less frequent floods. This approach is used because of the greater potential threat to public health, safety, and welfare that generally exists along major waterways.

Impacts of urbanization should be reduced through the use of Best Management Practices (BMPs). In general, urbanization tends to increase downstream peak flows, runoff volumes, and runoff velocities, which can cause the capacity of inadequately designed downstream systems to be exceeded and can disrupt natural waterways. The impacts of new urbanization must be reduced through the use of structural and non-structural BMPs that typically include stormwater detention to limit peak flow rates to predevelopment rates. Detention facilities may be constructed either on-site or as regional facilities. Regional facilities developed by the City will be constructed and evaluated as the need arises. It will be up to the City to determine the need and location of any regional detention they see as a cost effective and useful tool for controlling stormwater runoff in nuisance/flooding prone areas of the city. Other BMPs include hydraulically disconnecting impervious areas to the extent practicable to achieve maximum contact between runoff and vegetation, thereby maximizing infiltration and filtering of pollutants. While it is generally not practical to maintain predevelopment runoff volumes, accepted stormwater BMPs should be used to the maximum extent practicable to minimize runoff volume. For redevelopment projects, consideration should be given to retrofitting the existing stormwater controls as necessary, given the size of the redevelopment project and its location within the watershed.

The stormwater drainage system should be designed for sustainability, with careful consideration given to the need for accessibility and maintenance to sustain adequate function, whether the facilities will be publicly or privately maintained. The major drainage system is more likely to be maintained by a public entity, whereas the minor system is more often maintained by a private entity. Parts of the major system that serve specific functions for private entities, should be maintained by those private entities. Failure to provide proper maintenance reduces both the hydraulic capacity and the pollutant removal efficiency of the drainage system. Planning and design of drainage facilities should include consideration of the funding necessary to provide proper maintenance.

A stormwater drainage system should be designed beginning with the point of discharge, with careful consideration given to downstream impacts and the effects of off-site flows. The location and method of discharge from a development site must be carefully determined to avoid causing harm to properties located either downstream or adjacent to the site. The engineer should evaluate the conveyance system downstream of each point of discharge from a new development to ensure that it has sufficient capacity for design discharges without adverse backwater or downstream impacts such as flooding, stream bank erosion, and sediment deposition. If the development is found to exacerbate or contribute to existing known drainage issues or create additional issues upstream or downstream of the site, off-site improvements to adjacent drainage facilities may be required. In addition, great care must also be taken to determine the method of receiving, conveying, and discharging stormwater runoff that originates from off-site.

When planning a new development, a variety of drainage concepts should be evaluated prior to determination of the location of streets and lot layout. Decisions made at this point in the development process have the greatest impact regarding the cost and performance of the drainage facilities.

Developments should be designed around the existing natural drainage patterns and topography to achieve the most efficient drainage system. The designer should begin by determining the location and width of existing waterways and floodplains. A preliminary estimate of the design flow rate is necessary to approximate the capacity and size of a channel or conduit (See Chapter 3 - Determination of Stormwater Runoff).

Streets and lots should be laid out in a manner that preserves the existing drainage system to the greatest extent practical. Constructed channels should only be used when it is not practical or feasible to use existing waterways. Proposals to modify major natural waterways should be submitted to the City for approval prior to detailed design. In such cases, it must be shown why it is not feasible to preserve the natural major drainageway.

The use of open channels for major drainageways can provide significant advantages, compared with closed conduits, in terms of cost, capacity, potential for recreational uses, aesthetics, environmental protection/enhancement, and detention storage. Disadvantages of open channels compared with closed conduits include increased space and right-of-way requirements and additional maintenance needs associated with channel instability.

Open channels in new developments typically fall in one of the following categories:

Existing natural channels

• Existing natural channels that are stable and are expected to remain stable and are being preserved in a natural state.
• Existing natural channels that are unstable or are not expected to remain stable because of changes in the watershed and are being stabilized with bioengineering methods to maintain the natural character of the channel.

Existing or proposed semi-improved channels

• Existing or proposed semi-improved channels where some modifications are made, such as grading, but the channel appears to be natural and is lined with vegetation such as grass and trees.

Existing or proposed improved channels

• Existing or proposed improved channels with a natural lining, such as a trapezoidal grass channel that is mowed on a regular basis. An improved channel may include a small, concrete low-flow channel to reduce erosion and allow the grade to be maintained.
• Existing or proposed improved channels where a hard lining such as concrete, rock or other hard armor material makes up a significant part of the channel. Examples include rectangular or trapezoidal channels lined with riprap or concrete.

The volume of storm runoff, peak discharge rate, and frequency of bank-full discharges from an urban area are often larger than under historic, undeveloped conditions, depending on the nature of the development (Leopold 1994; Urbonas 1980; ASCE and WEF 1992; WEF and ASCE 1998). When natural channels begin to carry storm runoff from a newly urbanized area, the changed runoff regime may result in new and increased erosional tendencies.

Careful hydraulic analysis of natural channels must be made to assess and address these potential impacts. Some modification of the channel is frequently required to create a more stabilized condition to withstand changes to surface runoff created by urbanization. Channel modifications should not be undertaken unless they are found to be absolutely necessary. The objective is to avoid excessive and extensive channel disturbance and the subsequent negative impacts on erosion, sediment deposition, and water quality.

Factors to consider when choosing between using the existing channel or making improvements to the channel include:

• Required channel capacity for flood control compared with the existing channel capacity
• Space availability within the development
• Recent and expected changes in upstream runoff from the contributing watershed
• Physical characteristics of the natural channel such as slope, soil characteristics, and vegetative condition

Measures to stabilize a natural channel frequently include construction of grade controls or drop structures at regular intervals to decrease the longitudinal slope of the thalweg (channel invert), thereby controlling erosion. Bank and bottom stabilization measures may also be necessary.

If site conditions are conducive, channels should be left in a condition that resembles the natural state to the extent feasible, provided it can be demonstrated that the channel is stable during the 25-year event. It is preferred that natural channels be preserved or stabilized through bioengineering methods. If bioengineering methods are not feasible, improved grass channels are generally preferred to channels with a hard lining, except where armoring is necessary because of the physical or hydrologic characteristics of the site. Benefits of a stabilized natural channel can include:

• Lower flow velocities
• Longer concentration times and lower downstream peak flows
• Channel and adjacent floodplain storage that tends to decrease peak flows
• Protection of riparian and aquatic habitat
• Greenbelt and recreational area that adds significant social benefits

Specific design criteria along major drainageways are provided in Chapter 6 – Open Channel Flow Design.

Floodplain management and regulation is necessary for a government to exercise its duty to protect the health, safety, and welfare of the public. The concept of the existence of a natural floodway fringe for the storage and passage of floodwaters is fundamental to the assumption of regulatory powers in a definable flood zone. Floodplain regulation must define the boundary of the natural floodway fringe and must delineate easement occupancy that will be consistent with public interests.

The initial collection system within a development may include curbs, gutters, inlets, swales, pipes, flumes, channels, open waterways, detention, and water quality BMPs. The collection system is critical to the protection of adjacent streets and properties from flooding. The objective of the site collection system is to completely collect, control, and convey the required design storm for specific street classifications (see Chapter 4 – Storm Sewer System Design) and protect properties adjacent to streets during runoff from storms up to the 100-year design flow.

Discharges from the site must connect directly to the existing drainage system where possible, as opposed to discharging to the street. Provision must be made to protect streets and sidewalks from flooding. Discharges to the street should not exceed the street design criteria and discharges across a sidewalk must protect the sidewalk from inundation up to the 2-year flow.

Streets serve as part of the initial collection system in an overall drainage system. The objective of street drainage design is to reasonably minimize inconvenience to the traveling public, provide for safe passage of emergency vehicles during runoff from storms up to a 100-year event, and prevent the overflow of runoff from streets onto private property (unless in an easement) during runoff from storms up to a 100-year event. Well-planned street location and preliminary design can greatly reduce street drainage improvement construction costs.

Inlets must be properly selected and designed to minimize the possibility of clogging and to limit spread based on the street classification. Typical inlet types include curb opening inlets, open-side drop inlets and grated inlets. (See Chapter 4 - Storm Sewer System Design, for detailed design criteria.) Site storm sewer pipes and box culverts must be designed to convey flow from the design storm frequency associated with site specific infrastructure as described in Chapter 4 – Storm Sewer System Design and Chapter 7 – Culvert and Bridge Hydraulic Design.

Any development that increases runoff must address runoff through construction of onsite detention or other compensatory measure approved by the City. Detention for flood control is designed to prevent increases in peak flow from the 1-, 2-, 5-, 10-, 25-, 50- and 100-year storms. Onsite detention should be located at the low point(s) on the site and shall discharge to a public right-of-way or drainage easement.

Detention basins should be planned to match existing topography to minimize cut and fill, land disturbance, and environmental impacts. Aesthetics should also be considered during design so that the facility complements surrounding land uses. In all cases, opportunities should be sought to create amenities with detention basins by utilizing permanent pools, gentle slopes, landscaping, and trees and incorporating multi-purpose uses, such as recreation. Design criteria for detention basins are provided in Chapter 5 - Detention Design.

In-line detention that collects offsite runoff should be avoided, particularly when the volume of runoff from offsite is greater than the volume from onsite. Larger offsite areas draining through a detention basin cause increased requirements for volume and control structure size, resulting in higher basin construction costs. In addition, in-line detention basins along major drainageways may require a U.S. Army Corps of Engineers (USACE) Section 404 Permit. Therefore, it is preferred to have off-line detention with the waterway preserved in a more natural state. The use of in-line detention as a means to control stormwater runoff requires City approval prior to implementation.

As an alternative to constructing onsite detention, a payment in lieu of constructing detention may be acceptable by the City, but only if an existing regional detention facility with adequate capacity, as determined by the City, exists downstream from the proposed development or as determined by the City. The funds collected from fee-in-lieu payments will be used by the City for regional stormwater facilities or other measures that will benefit the stormwater management in the community.

Permanent pool detention basins are encouraged because they provide added benefits with respect to water quality, aesthetics and habitat. When designed and constructed properly, permanent pool detention basins can be an amenity to both the development and the community. Detailed design criteria for permanent pool detention areas are provided in Chapter 5 - Detention Design.

Detention basins sited on or near the upstream portion of a site to reduce offsite peak runoff may be considered as an option to compensate for increased peak runoff from the site in cases where the low point of the site is not conducive to detention facilities. It must be shown that the total peak runoff rates for the design storms at locations downstream of the site are no greater than pre-development conditions. Careful attention must be given to the timing of peak runoff; a conservative design may be appropriate to assure that peak flow rates are not increased because of inaccurate modeling of the peak timing.

Stormwater quality and quantity (rate and volume) are closely related and should be planned and designed concurrently. Stormwater quality BMPs are required on new developments to reduce adverse impacts on downstream water quality and to meet the requirements of the City’s federally-mandated National Pollutant Discharge Elimination System (NPDES) Municipal Separate Storm Sewer (MS4) permit. Planning for a new development should include determination of the BMPs to be used, which commonly include extended or wet detention basins, disconnecting impervious areas, and utilizing grass buffer strips, swales, and channels.

BMPs should also include open channel designs that both filter runoff and maintain long-term stability, thereby reducing pollutants and sediment. Detailed design criteria for several common water quality BMPs are provided in Chapter 9 - Water Quality. Design criteria for open channels that provide stable channel linings and reduce the amount of impervious area are provided in Chapter 6 - Open Channel Flow Design.

Submittal requirements for subdivision development in the City of Rogers are specified in Chapter 14 of the Code of Ordinances for the City. Early planning for a new subdivision should include meeting with the Department of Community Development to develop an acceptable stormwater management plan that will be less likely to experience problems in the review process and will result in a more efficient and optimum storm water design. Major conceptual storm water issues can be identified to help with development of a design that can maximize flood control and water quality protection and minimize project costs and future conflicts and construction difficulties.

Major design features that should be identified first are the preservation of major drainageways, the location and configuration of detention basins and water quality controls, and the location and configuration of streets and lots. Any watershed plans affecting the development should be identified so that compliance approach can be incorporated early in the design process. The developer should obtain a copy of the Preliminary Plat checklist from the Department of Community Development , to begin preparation of acceptable stormwater drainage plans and plat layout.

Submittal requirements for a Site Development Plan (SDP) in the City of Rogers are specified in the Unified Development Code for the City. In accordance with the ordinance, storm drainage design for an SDP must meet the minimum drainage requirements as defined by city ordinance. Drainage improvements must be indicated on the plans and a drainage report must accompany the plans. An engineer's certified calculations must be provided for all improvements. Improvements must be completed and certified by the engineer of record prior to the issuance of a certificate of occupancy.

Developments within a floodplain or floodway must provide floodplain data certified by an engineer or architect and must meet all FEMA requirements for new construction in floodplains or floodways.

The intent of this chapter of the Manual is to provide reasonably dependable and consistent methods of approximating the characteristics of runoff in urban and nonurban areas within the City of Rogers, Arkansas. This chapter will guide the designer in how to choose the proper method for calculating runoff, based on the conditions present at a site as well as the necessary information/calculations the City requires for their review prior to development of the site.

The City allows the use of three different methods for calculating urban runoff: (1) The Rational Method, (2) the Soil Conservation Service Technical Release – 55 Synthetic Hydrograph Method (SCS method), and (3) Computer models such as HEC-HMS, TR-20, or equivalent. It is the responsibility of the design engineer to properly choose which method(s) to implement for drainage design of a site and then to properly execute that design methodology.

This chapter of the Manual should be utilized in conjunction with other universally accepted articles and engineering references and studies. NRCS Technical Release 55 is referenced extensively throughout this chapter as it is an excellent resource for urban hydrology design and methodology. It is important for the individual using this section of the manual to already have a firm understanding of the information provided in this document prior to implementing the recommendations outlined in this Manual.

The summary below outlines some of the most critical design criteria essential to design engineers for calculating stormwater runoff according to City of Rogers requirements. The information below contains exact numerical criteria as well as general guidelines that must be adhered to during the design process. This section is meant to be a summary of critical design criteria for this section; however, the engineer is responsible for all information in this chapter. It should be noted that any design engineer who is not familiar with Rogers’ drainage manual and its accepted design techniques and methodology should review the entirety of this chapter.

• Refer to Section 2.0 for more detailed information/explanation
• Rational Method Formula: Q = k_i*C*I*A
• Refer to Table RO-2, Table RO-3, and Table RO-4 for more detailed information

Use the included Weighted C spreadsheet for all composite analysis.

• Refer to Section 2.6 for more detailed information/explanation
• Refer to Table RO-5 for Rainfall Intensity-Duration-Frequency (IDF) Chart

• Refer to Section 2.8 for more detailed information/explanation
• Minimum t_c = 5-minutes for an urban watershed and 10-minutes for a non-urban watershed
• Time of Concentration equation: t_c= t_o + t_s + t_t
  • 
    • t_o = overland flow time (minutes)
    • n = Manning’s roughness coefficient (Table RO-6)
    • L = length of overland flow in feet (300-ft maximum in non-urban areas; 100-ft maximum in urban areas)
    • P₂= 2-year, 24-hour rainfall (inches) calculated from Table RO-5 (or obtained from Table RO-9)
    • S = average basin slope (ft-per-ft) expressed as a decimal

• 
  • t_s = shallow concentrated flow time (minutes)
  • (Paved Areas)
  • (Unpaved Areas)
• where V is calculated from Manning’s equation (use Table RO-7)
  • t_t = channel flow time (minutes)

• Refer to Section 3.0 for more detailed information/explanation
• SCS Method equation: where,
  • Q = runoff (inches)
  • P = rainfall depth from Table RO-9
  • S = potential maximum retention after runoff begins (inches)
    • where,
  • I_a = initial abstraction (inches)
    • where,
  • CN = runoff curve numbers (see Table RO-10 and Table RO-11 for urban and non-urban areas; also included on the next two pages of this Summary)
• For those models which require it, the Type II rainfall distribution shall be used within the City of Rogers planning boundary.

Determining the peak flow rate and volume of storm water runoff generated in a watershed for a given storm event is an essential step in evaluating drainage design. The size of rainfall event, type of flow condition, and flow rate of the runoff all play a major role in the sizing, configuration, and operation of storm drainage and flood control systems. Numerous methods for calculating runoff have been developed and studied as engineering design options but only a few are accepted by the City of Rogers, based on the climate and natural environment.

There are a number of different methods and procedures for computing runoff on which the design of storm drainage and flood control systems are based. The three methods the City accepts are:

1. The Rational Method
2. The Soil Conservation Service Technical Release – 55 Synthetic Hydrograph Method (SCS method)
3. USGS Regional Regression Equations. This third method will not be discussed in detail in this Manual, but can be examined and further studied in Magnitude and Frequency of Floods in Arkansas (USGS – WRIR 95-4224, 1995).

The two main drainage methods described in this Manual are: (1) the Rational Method and (2) SCS method. The Rational Method is generally used for smaller watersheds when only the peak flow rate or the total volume of runoff is needed at a design point or points (e.g., storm sewer sizing or simple detention basin sizing). The SCS method is used for larger watersheds and when a hydrograph of the storm event is needed (e.g., sizing large detention facilities). The watershed size limits and/or ranges for each analysis method are shown in Table RO-1.

The Rational Method is based on the Rational Formula which is expressed as:

Q = k_i*C*I*A(Equation RO-1)

in which:

Q = peak rate of runoff (cubic feet per second [cfs]). Q is actually in units of acre-inches per hour (ac-in/hr), but conversion of the results to cubic-feet per second (cfs) differs by less than 1 percent. Since the difference is so small, the Q value calculated by the equation is accepted as cubic feet per second (cfs).

k_i = adjustment multiplier for design storm recurrence interval
(see Table RO-4)

C = runoff coefficient - represented in the ratio of the amount of runoff to the amount of rainfall (see Section 2.5).

I = average intensity of rainfall (inches per hour [in/hr]) for a period of time equal to the critical time of full contribution of the drainage area under consideration (see Section 2.6). This critical time for full contribution is commonly referred to as "time of concentration," t_c (see Section 2.8)

A = area (acres) that contributes to runoff at the point of design or the point under consideration (see Section 2.7).

The general procedure for Rational Method calculations for a single watershed is as follows:

1. Delineate the watershed boundary and measure its area in acres.
2. Define the flow path from the hydraulically most distant point of the watershed to the design point. This flow path should be divided into reaches of similar flow type [i.e. overland flow (sheet flow), shallow concentrated flow (swales, shallow ditches, etc.)], and channelized flow (gutters, storm sewers, open channels, etc.). The length and slope of each reach should be measured.
3. Determine the time of concentration, t_c, for the watershed. Refer to Section 2.8 of this chapter for additional information on calculating t_c.
4. Find the rainfall intensity, I, for the design storm using the calculated t_c and the rainfall intensity-duration-frequency information (see Table RO-5). Use arithmetic interpolation to calculate rainfall intensity for t_c not displayed in the table.
5. Determine the runoff coefficient, C, (see Table RO-2 and/or Table RO-3) for the watershed boundary and its resulting subareas.
6. Calculate the peak flow rate from the watershed using Equation RO-1.

Calculations for the Rational Method shall be carried out using the spreadsheets or other software aides discussed in Section 4.0 of this chapter.

Basic assumptions associated with use of the Rational Method are as follows:

1. The computed peak rate of runoff to the design point is a function of the average rainfall rate during the time of concentration for the watershed.
2. The time of concentration is the critical value in determining the design rainfall intensity and is equal to the time required for water to flow from the hydraulically most distant point in the watershed to the point of design.
3. The runoff coefficient, C, is uniform during the entire duration of the storm event.
4. The rate of rainfall or rainfall intensity, I, is uniform for the entire duration of the storm event and is uniformly distributed over the entire watershed area.
5. The depth of rainfall used is that which occurs from the start of the storm to the time of concentration. The design rainfall depth during that time period is converted to the average rainfall intensity for that period in inches per hour (in/hr).
6. The maximum runoff rate occurs when the entire area is contributing flow. However, this assumption has to be modified when a more intensely developed portion of the watershed with a shorter time of concentration produces a higher rate of maximum runoff than the entire watershed with a longer time of concentration.

The Rational Method is an adequate method for approximating the peak rate of runoff from a design rainstorm in a given watershed area. The greatest drawback to the Rational Method is that it normally provides only one point on the runoff hydrograph. When the areas become complex and where sub-watersheds come together, the Rational Method will tend to overestimate the actual flow, which results in oversizing of drainage facilities. The Rational Method provides no direct information needed to route hydrographs through the drainage facilities. One reason the Rational Method is limited to small areas is that good design practice requires the routing of hydrographs for larger watersheds to achieve an economic design.

Another disadvantage of the Rational Method is that in the typical design procedure one normally assumes that all of the design flow is collected at the design point and that no water bypasses or runs overland to the next design point. However, this is not a limitation of the Rational Method but of the design procedure. The Rational Method must be modified, or another type of analysis used, when analyzing an existing system that is under-designed or when analyzing the effects of a major storm on a system designed for the minor storm.

The runoff coefficient, C, represents the integrated effects of infiltration, detention storage, evaporation, retention, flow routing, and interception, all of which affect the time of distribution and peak rate of runoff. The proportion of the total rainfall that runs off depends on the relative porosity or imperviousness of the ground surface, the surface slope, and the ponding character of the surface. Impervious surfaces, such as asphalt pavements and roofs of buildings, will be subject to nearly 100 percent runoff, regardless of the slope, after the surfaces have become thoroughly wet. On-site inspections and aerial photographs are valuable in determining the types of surfaces within the drainage area and are essential when assessing the runoff coefficient, C.

Rainfall intensity, I, is the design rainfall rate in inches-per-hour (in/hr) for a particular drainage basin or subbasin of a watershed. The rainfall intensity, I, is obtained from an intensity-duration-frequency (IDF) chart for a specified return period under the assumption that the duration is equal to the time of concentration for the watershed being evaluated. Once the time of concentration is known, the design intensity of rainfall may be interpolated from Table RO-5. The frequency of recurrence interval is a statistical variable which is established by City standards.

The drainage area is measured in acres when using the Rational Method. Drainage areas should be calculated using planimetric-topographic maps, supplemented by field surveys where topographic data has changed or where the contour interval is too great to distinguish the exact direction of overland flows. Field surveys are also useful for verifying flows through culverts or other drainage structures. Recent topographic aerial surveys of the City of Rogers can be viewed using the City’s GIS web browser at http://gis.rogersar.gov and downloadable aerial images can be obtained by logging in at http://www.geostor.arkansas.gov/. City topography is available for use in designating off-site drainage or preliminary designs. An actual site survey will be required for all site developments and subdivisions.

The time of concentration, t_c, is best defined as the time required for water to flow from the hydraulically most distant point of a watershed to the design point at which peak runoff is desired. The critical time of concentration is the time to the peak of the runoff hydrograph at the location of the design point. Runoff from a watershed usually reaches a peak at the time when the entire watershed area is contributing to flow. The critical time of concentration, therefore, is assumed to be the flow time measured from the most remote part of the watershed to the design point. A trial and error procedure should be used to select the most remote point of a watershed since type of flow, ground slopes, soil types, surface treatments and improved conveyances all affect flow velocity and time of flow.

Water moves through a watershed as overland flow (sheet flow), shallow concentrated flow (swales, shallow ditches, etc.), channelized flow (gutters, storm sewers, open channels, etc.) or some combination of these. The type that occurs is a function of the conveyance system and is best determined by field inspection.

The time of concentration, t_c, is represented by Equation RO-2 for both urban and non-urban areas:

t_c= t_o + t_s + t_t(Equation RO-2)

in which:

t_c = time of concentration (minutes)

t_o = overland flow time (minutes)

t_s = shallow concentrated flow time (minutes)

t_t = channelized flow time (minutes)

Urban areas are characterized as densely populated areas, where the collection of streets, parking lots, and rooftops in close proximity to one another create a situation where the collective runoff area is more impervious than not. Non-urban areas are characterized as less populated and more agricultural, where the majority of the area is farmland, open pastures, woodlands. This combination of agricultural land creates the situation where the collective runoff area is more pervious than not.

A convenient tool for calculating the time of concentration (as outlined in Equation RO-2) is located in the T_c and PeakQtab within the RDM-Rational Method spreadsheet. This tab allows for the calculation of the total time of concentration for a watershed based on the collective equations presented in this section of the Manual for calculating overland flow time (t_o), shallow concentrated flow time (t_s), and channelized flow time (t_t). All time of concentration calculations shall be performed on this spreadsheet and included in the drainage report.

Runoff, Q, for the SCS method is represented by Equation RO-9:

(Equation RO-9)

in which:

Q = runoff (inches)

P = rainfall depth for design storm (inches)

S = potential maximum retention after runoff begins (inches)

I_a = initial abstraction (inches)

Initial abstraction, I_a , is all losses before runoff begins. It includes water retained in surface depressions, water intercepted by vegetation, evaporation, and infiltration. I_a is highly variable but generally is correlated with soil and cover parameters. A relationship between I_a and S was developed by USDA NRCS through studies of many small agricultural watersheds. The empirical relationship used in the SCS runoff formula is:

(Equation RO-10)

Substituting Equation RO-10 into Equation RO-9 gives:

(Equation RO-11)

S is related to the soil and cover conditions of the watershed through the CN. CN has a range of 0 to 100, and S is related to CN by:

(Equation RO-12)

Figure RO-1 and Table RO-8 solve Equation RO-11 and Equation RO-12 for a range of CNs and rainfall. Refer to Section 3.3 for explanations and direction in determining proper CNs for use in Equation RO-12.

The SCS method is based on 24-hour rainfall amounts for various design storm recurrence intervals (e.g., 1-year, 10-year, or 100-year storm events). These rainfall amounts are taken from the U.S. Weather Bureau Technical Paper No. 40 for Rogers and are as follows: 3.32 inches for the 1-year frequency rainfall; 4.08 inches for the 2-year frequency rainfall; 5.28 inches for the 5-year frequency rainfall; 6.00 inches for the 10-year frequency rainfall; 6.96 inches for the 25-year frequency; 7.92 inches for the 50-year frequency; and 8.64 inches for the 100-year frequency. Table RO-9 provides rainfall data derived from several sources for storm durations other than the 24-hour event and for a range of storm return frequencies, if needed for further detailed analysis.

The runoff curve number (CN) determines the amount of runoff that will occur given a specified rainfall amount. The determination of the CN value for a watershed is a function of the hydrologic soil group (HSG), cover type and hydrologic condition, and antecedent moisture condition (AMC). Another factor considered is whether impervious areas outlet directly to the drainage system (connected) or whether the flow spreads over pervious areas before entering the drainage system (unconnected).

CN values in Table RO-10 and Table RO-11 represent average antecedent moisture conditions for undeveloped and developed lands. For watersheds with multiple soil types or land uses, an area-weighted CN should be calculated. When significant differences in land use or natural control points exist, the watershed shall be broken into smaller drainage areas for modeling purposes. Curve Numbers presented in Table RO-10 and Table RO-11 are based on the assumption that impervious areas are directly connected. The following sections provide details on the factors governing the determination of CN values and their relationship to runoff.

• Do not use the SCS method when large changes in CN values occur among watershed subareas and when runoff volumes are less than about 1-1/2 -inches for CN values less than 60.
• The CN procedure is less accurate when runoff is less than 1/2-inch. As a check, use another procedure to determine runoff when this occurs.
• Do not use the SCS method for watersheds that have several subareas with times of concentration below six minutes. In these cases, subareas should be combined to produce a time of concentration of at least six minutes (0.10 hours) for the combined areas.
• Curve numbers describe average conditions that are useful for design purposes. If the rainfall event used is a historical storm, the modeling accuracy decreases.
• Use the runoff curve number equation with caution when re-creating specific features of an actual storm. The equation does not contain an expression for time and, therefore, does not account for rainfall duration or intensity.
• The initial abstraction relationship, I_a = 0.2*S, which consists of interception, initial infiltration, surface depression storage, evapotranspiration, and other factors, is based on data obtained by the USDA NRCS from agricultural watersheds (where S is the potential maximum retention after runoff begins). In reality not all watersheds (urban conditions and non-urban conditions) share the same I_a because of differing combinations of impervious and pervious areas along with differing storage features. However, for this Manual I_a will be related the same for all watershed conditions.
• Runoff from snowmelt or rain on frozen ground cannot be estimated using these procedures.
• The SCS method procedures apply only to direct surface runoff. Do not overlook large sources of subsurface flow or high ground water levels that contribute to runoff. These conditions are often related to HSG A soils and forest areas that have been assigned relatively low CNs in Table RO-10 and Table RO-11. Good judgment and experience based on stream gage records are needed to adjust CNs as conditions warrant.
• When the weighted CN is less than 40, use another procedure to determine runoff.

Due to the large number of computations involved in runoff calculations and routing, use of modern computer models by experienced engineers is allowed by the City for the drainage calculations/methods outlined above. The U.S. Army Corps of Engineers (USACE) Hydrologic Engineering Center (HEC) has developed computer programs that can be downloaded online at the USACE hydrologic website (http://www.hec.usace.army.mil/) that can be applied to some of the drainage methods. HEC-HMS is one such program available from USACE. Additionally, versions of TR-20 and TR-55 are available through the NRCS, which allow user input of rainfall distributions and perform acceptable detention and channel routing routines. The Type II rainfall distribution type shall be used within the City of Rogers planning boundary, refer to Figure RO-4. The HEC-HMS, TR-55, and TR-20 models are available free of charge from the agencies that developed them. Table RO-12 provides additional information on the computer models as well as a link for downloading the available software.

Commercial software, such as StormCAD, Hydraflow, PondPack, etc., is also an acceptable method for evaluating the drainage methods mentioned in this chapter. It is the responsibility of the design engineer to understand the methods employed within the commercial software used and ensure that the software’s results will match and correspond with the methodology outlined in this chapter of the Manual.

The intent of this chapter of the Manual is to give concise, practical guidelines for the design of urban storm water collection and conveyance systems. Procedures and equations are presented for the hydraulic design of storm sewer systems, locating inlets and determining capture capacity and efficiency, and sizing storm sewers. In addition, examples are provided to illustrate the hydraulic design process. Spreadsheet solutions accompany the hand calculations for most example problems.

Proper sizing and placement of stormwater capture and conveyance structures is pivotal in the handling of stormwater runoff in urban areas. The primary function of stormwater collection and conveyance systems is to collect excess stormwater from street gutters, convey the excess stormwater through storm sewers and along the street right-of-way or drainage easements, and discharge it into a detention basin, water quality best management practice (BMP) or the nearest receiving water body (FHWA 1996). The main premise of urban stormwater systems is to minimize disruption to the natural drainage system, promote safe passage of vehicular traffic during minor storm events, maintain public safety and manage flooding during major storm events, preserve and protect the urban stream environment, and minimize capital and maintenance costs of the stormwater collection system. To ensure these measures are met, consistent and strategic use of accepted and proven design methodology for sizing and placing stormwater capture and conveyance structures is required. Within this section of the Manual the City of Rogers addresses specific stormwater system design methods and system requirements that have been deemed acceptable and compatible with the type of transportation system and stormwater system characteristic within the City.

Urban stormwater collection and conveyance systems are comprised of three primary components: (1) street gutters and roadside swales, (2) stormwater inlets, and (3) storm sewers (and appurtenances like manholes, junctions, bends and transitions, etc.). Street gutters and roadside swales collect runoff from the street (and adjacent areas) and convey the runoff to a stormwater inlet while maintaining the street’s level-of-service.

Inlets collect stormwater from streets and other land surfaces, transition the flow into storm sewers, and often provide maintenance access to the storm sewer system. Storm sewers convey stormwater in excess of a street’s or a swale’s capacity along the right-of-way and discharge it into a stormwater management facility or a nearby receiving water body. All of these components must be designed properly to achieve the stormwater collection and conveyance system’s objectives. This chapter of the Manual spells out the steps involved in the design and evaluation of the three primary components mentioned above.

The design procedures presented in this chapter are based upon fundamental hydrologic and hydraulic design concepts. The design equations provided are well accepted and widely used. They are presented without derivations or detailed explanation, but are properly referenced if the reader wishes to study their background. Therefore, it is assumed the reader has a fundamental understanding of basic hydrology and hydraulics. A working knowledge of the Rational Equation (Chapter 3 – Determination of Stormwater Runoff) and open channel hydraulics (Chapter 6 – Open Channel Flow Design) is particularly helpful.

The summary below outlines some of the most critical design criteria essential to design engineers for proper drainage design of streets, inlets, and storm sewers according to City of Rogers’ requirements. The information below contains exact numerical criteria as well as general guidelines that must be adhered to during the design process. This section is meant to be a summary of critical design criteria for this section; however, the engineer is responsible for all information in this chapter. It should be noted that any design engineer who is not familiar with Rogers’ Drainage Criteria Manual and its accepted design techniques and methodology should review the entirety of this chapter. If additional specific information is required, it will be necessary to review the appropriate section as needed.

The primary function of a street or roadway is to provide for the safe passage of vehicular traffic at a specified level of service. If stormwater collection and conveyance systems are not designed properly, this primary function can be impaired when streets flood due to surcharge in storm sewers and street encroachment. To make sure this does not happen, streets are classified for drainage purposes based on their traffic volume, parking practices, and other criteria (Wright-McLaughlin Engineers 1969). The five street classifications for the City of Rogers are:

• Minor: low-speed traffic for residential or industrial area access.
• Collector: low/moderate-speed traffic providing service between local streets and arterials.
• Minor Arterial: moderate/high-speed traffic moving through urban areas.
• Major Arterial: moderate/high-speed traffic moving through urban areas.
• Boulevard: moderate/high-speed traffic moving through urban areas.

For drainage design, the classification shown on the Rogers Master Street Plan shall be used unless a higher standard is deemed appropriate by the Engineer of Record or City. Refer to Article 6 – Engineering Manual - for layout design standards and criteria for the street classifications mentioned above (http://library.municode.com).

Streets serve another important function other than traffic flow. They contain the first component in the urban stormwater collection and conveyance system. That component is the street gutter or adjacent swale, which collects excess stormwater from the street and adjacent areas and conveys it to a stormwater inlet. Proper street drainage is essential to:

• Maintain the street’s level-of-service.
• Reduce skid potential.
• Minimize the potential for cars to hydroplane.
• Maintain good visibility for drivers by reducing splash and spray.
• Minimize inconvenience/danger to pedestrians during storm events (FHWA 1984).

Stormwater which flows in a street will flow in the gutters of the street until it reaches an overflow point or some other outlet/inlet. During its travel time the top width (or spread) of the stormwater flowing in the gutter widens as more stormwater is collected. Certain design considerations must be taken into account in order to meet the drainage objectives of a street to handle the stormwater flowing in the gutter. The primary design objective is to maintain permissible values of spread (encroachment) for minor storm (10-yr frequency) events. If the width and depth of the flow becomes great enough, the street loses its effectiveness as a traffic-carrier and travel becomes hazardous. Based on this, the City has established encroachment standards for the minor storm event. These encroachment standards are shown in Table ST-1.

Table ST-1 — Pavement Encroachment and Curb Depth Standards for the Minor Storm, 10-yr Return Frequency

Additional design objectives are required for major storm (100-yr frequency) events and resulting gutter flows and street cross flows. The main factor to be considered when evaluating the major storm event is to determine the potential for flooding and public safety. Cross-street/intersection flows also need to be regulated for traffic flow and public safety. The City has established street inundation standards during the major storm event and allowable cross-street/intersection flow standards. These standards are shown in Table ST-2 and Table ST-3.

Table ST-2 — Street Inundation Standards for the Major Storm, 100-yr Return Frequency

Table ST-3 — Allowable Cross-Street/Intersection Flows

Hydraulic computations are performed to determine the capacity of roadside swales and street gutters and the encroachment of stormwater onto the street. The design discharge is usually determined using the Rational Method (covered later in this chapter). Stormwater runoff ends up in swales, roadside ditches and street gutters.

Once the design flow spread (encroachment) has been established for the minor storm, the placement of inlets can be determined. The primary function of stormwater inlets is to intercept excess surface runoff and deposit it in storm sewers, thereby reducing the possibility of surface flooding.

The location of storm drain inlets along a road is influenced by the roadway’s geometry as well as adjacent land features. As a rule, inlets are placed at all low points in the gutter grade, median breaks, intersections, and at or near crosswalks. Along with adhering to the geometric controls outlined above, storm drain inlet spacing shall be such that the gutter spread under the design storm (10-yr frequency) conditions will not exceed the allowable encroachment for the type of street class under consideration. (Table ST-1)

There are five major types of storm drain inlets: grate, curb opening, combination, slotted and area. Figure ST-5 depicts the major types of inlets along with some associated geometric variables. Table ST-5 provides general information on the appropriate application of the different inlet types along with basic advantages and disadvantages of each.

Figure ST-5 –– Types of Storm Drain Inlets (FHWA – HEC-22 2001)

Table ST-5 — Applicable Settings for Various Inlet Types

Stormwater inlet design takes two forms: inlet placement location and inlet hydraulic capacity. As previously mentioned, inlets must be placed in sumps to prevent ponding of excess stormwater. On streets with continuous grades, inlets are required periodically to keep the gutter flow from exceeding the encroachment limitations. In both cases, the size and type of inlets need to be designed based upon their hydraulic capacity.

Inlets placed on continuous grades rarely intercept all of the gutter flow during the minor (design) storm. The effectiveness of the inlet is expressed as an efficiency, E, which is defined as:

(Equation ST-15)

in which:

E= inlet efficiency

Q_i= intercepted flow rate (cfs)

Q= total gutter flow rate (cfs)

Bypass (or carryover) flow is not intercepted by the inlet. By definition,

(Equation ST-16)

in which:

Q_b= bypass (or carryover) flow rate (cfs)

The ability of an inlet to intercept flow (i.e., hydraulic capacity) on a continuous grade generally increases with increasing gutter flow, but the capture efficiency decreases. In other words, even though more stormwater is captured, a smaller percentage of the gutter flow is captured. In general, the inlet capacity depends upon the following factors:

• Inlet type and geometry (length, width, etc.).
• Flow rate (depth and spread of water).
• Cross (transverse) slope (of road and gutter).
• Longitudinal slope.

As a general rule, an effective way to achieve an economic design and spacing for storm drain inlets is to allow 20- to 40-percent of gutter flow reaching the inlet to carry over to the next inlet downstream, provided that water flowing in the gutter does not exceed the allowable encroachment.

Inlets in sumps operate as weirs for shallow pond depths, but eventually will operate as orifices as the depth increases. A transition region exists between weir flow and orifice flow, much like a culvert. Grate inlets and slotted inlets tend to clog with debris, especially in sump conditions, so calculations shall take that into account. Curb opening inlets tend to be more dependable in sumps for this reason.

The hydraulic capacity of an inlet is dependent on the type of inlet (grate, curb opening, combination, or slotted) and the location (on a continuous grade or in a sump). The methodology for determination of hydraulic capacity of the various inlet types is described in the following sections:

1. grate inlets on a continuous grade (Section 2.3.1)
2. curb opening inlets on a continuous grade (Section 2.3.2)
3. combination inlets on a continuous grade (Section 2.3.3)
4. slotted inlets on a continuous grade (Section 2.3.4)
5. inlets located in sumps (Section 2.3.5).

Once stormwater runoff is collected from the street surface and local watershed areas and captured by an inlet, the water is conveyed through the storm sewer system. The storm sewer system is comprised of inlets, manholes, pipes, bends, outlets, and other appurtenances. The stormwater passes through these components and is discharged into a stormwater management device for mitigation purposes, such as a detention pond or wetland, or discharged directly to an open channel or other waterbody. This section addresses the combination of storm sewer features and how they interrelate to convey stormwater to an outlet.

The design of a storm sewer system requires the collection and evaluation of multiple pieces of information concerning the existing conditions of the study area. Required information includes topography, drainage/watershed boundaries, soil types, impervious surface areas, and locations of any existing storm sewers, inlets, and junction boxes and their sizes. In addition, it is necessary to identify the type and location of existing utilities. With the information described above it is possible to accurately examine proposed layouts of a new storm sewer system or adjustments to an existing system.

When looking at proposed layouts for a storm sewer system each conceptual layout plan shall show inlet and manhole locations, drainage boundaries serviced by each inlet, storm sewer locations, flow directions, and outlet locations. Emphasis should be placed on how the proposed layout interfaces with the existing right-of-way and site topography as these two factors greatly affect the cost of any new storm sewer construction or renovations of an existing system.

Once a final layout is chosen, storm sewers are sized using hydrologic techniques (to determine peak flows generated by the watershed) and hydraulic analysis (to determine pipe capacities). The constraints discussed below and the following design methods shall be used to evaluate the design requirements of a proposed storm sewer system with respect to the design storm.

The purpose of this chapter is to provide guidance for designing facilities to detain stormwater runoff from new developments and redevelopments. The intent of the detention facilities is to protect downstream channels and property from adverse impacts caused by increased peak flow rates and runoff volumes that can result if stormwater control measures are not implemented when areas are developed.

Urbanization results in increased levels of imperviousness which frequently causes increased peak flow rates and increased runoff volumes from developed sites. Hence, development can result in adverse impacts such as flooding of downstream properties, widening and instability of downstream channels and ecosystem disruption unless measures are taken to detain the runoff and control the rate of discharge off of newly developed sites.

The City requirements for stormwater detention described in this chapter apply to all new developments and redevelopments.

For sites that are 1 acre in size or smaller, for sites that are being redeveloped, or for sites of any size that are adjacent to a primary channel, the City may allow the property owner to pay a fee in-lieu-of implementing the stormwater detention measures described in this section.

There are two basic approaches to designing storage facilities: 1) Onsite or private facilities – facilities that are planned on an individual site basis and 2) Common or regional facilities - facilities that are planned to serve multiple lots, a subdivision, or larger area. These facilities can be constructed either on-line (in the drainageway) or off-channel, though off-channel facilities are preferred by the City and on-line facilities must be approved during the concept phase of the development.

The specific type of detention basin used falls into one of three design categories: 1) Dry detention basin (including underground detention chambers) – drains within 1 to 2 days, for flood control only, 2) Extended detention basin– drains over 1 to 3 days,for pollutant removal and flood control, and 3) Wet basin - contains a permanent pool of water and is designed for pollutant removal, flood control, and often aesthetics.Permeable paver systems must also drain the open graded aggregate base course over 1 to 3 days for pollutant removal and flood control.

Two methods are described for detention basin sizing: 1) The Rational formula-based Modified FAA Method – for additional impervious area of 2 acres or less, and 2) Hydrograph Methods – for any size of additional impervious area (these include the Hydrograph Volumetric Method for estimating the required detention volume and the Modified Puls routing method for designing detention facilities).

For the basin outlet works, design guidance is provided for orifices and weirs (including rectangular sharp-crested weirs, broad-crested weirs, and broad-crested slot and v-notch weirs). Design guidance for pipe outlet control is addressed in the culvert section of this Manual. Other design considerations for detention basins are also described, including factors such as public safety, layout, grading, lining materials, vegetation, access, and maintenance.

Plantings within detention facilities will need to be provided as part of a site’s planting plan, but will be in addition to other landscaping requirements, so as to provide a natural landscape that supplements the hydrologic capacity of the detention facility. Landscaping considerations outlined herein include minimizing the need for herbicides, fertilizers, pesticides or soil amendments, minimizing the need for mowing, pruning and irrigation, and not impeding the primary function of the detention facility.

Design examples are provided for: 1) the Modified FAA method for sizing smaller basins, 2) the Hydrograph Volumetric Method for initial sizing of larger basins, and 3) the Modified Puls routing method for the design of larger basins.

To comply with the City requirements for detention of stormwater, new developments and redevelopments must satisfy the applicable criteria in this chapter.

Urbanization results in increased levels of imperviousness which frequently causes increased peak flow rates and increased runoff volumes from developed sites. Historically, the traditional approach for stormwater management was to move runoff away from structures and transportation systems as quickly and efficiently as possible. However, this approach resulted in impacts such as:

• Flooding of downstream properties.
• Widening and instability of downstream channels.
• Habitat damage and ecosystem disruption, resulting in streambed and bank erosion and associated sediment and pollutant transport.

These types of adverse impacts will occur unless measures are taken to detain the runoff and control the rate of discharge off of newly developed sites.

For sites that are 1-acre in size or smaller, for sites that are being redeveloped, or for sites of any size that are adjacent to a primary channel (see primary channel description in Section 2.5 of Chapter 6 – Open Channel Flow Design), the City may allow a developer/property owner to make a monetary payment or some other form of valuable consideration in lieu of implementing the stormwater detention measures described in this chapter. The City shall make the determination of whether fee-in-lieu of detention will be allowed or required on a case by case basis based upon capacity of the receiving stormwater drainage system and whether regional detention facilities are either proposed or in place and the increase in flow rates to these downstream conditions will not adversely affect downstream property owners. The amount of the fee shall be based on the number of square feet of impervious area on the property post-development. The developer/owner shall provide the City calculations of the number of square feet of impervious area and the City shall prepare a bill for payment in-lieu of detention. The fee shall be paid at the time the final plat is approved by the City Council or before issuance of a Certificate of Occupancy. When these fees are collected, they shall be deposited into a stormwater capital improvements fund, which will be used for future or ongoing stormwater improvement and regional detention projects that will benefit stormwater management in the community. The methodology for calculating the fee-in-lieu is described in Appendix A at the end of this chapter.

In addition to the design considerations above, the following factors shall be considered when selecting and designing a detention facility for a site:

• Public Safety – Detention facilities shall be evaluated in terms of public safety and the risks or liabilities that occur during implementation. Public safety is always one of the most important design considerations. Wet detention ponds must have side-slopes that are no steeper than 3:1 (H:V) and must incorporate either a safety bench or fencing into the design (see Section 6.11).
• Public Acceptability - The detention facility shall consider the expected response from the public, particularly neighboring residential properties, if any.
• Agency Acceptability – Selection of a detention facility for a site shall consider the expected response of agencies that will oversee the facility and their relationship to regulatory requirements.
• Mosquito Control – A specific component of public health and safety related to the design of detention facilities is the issue of mosquito control. The potential for mosquito breeding and the spread of mosquito-borne illnesses in detention facilities must be addressed. In general, the biggest concern is the creation of areas with shallow stagnant water and low dissolved oxygen that creates prime mosquito habitat. Studies indicate that pools of deep water (≥ 5 feet) and pools with residence times less than 72 hours are less likely to breed mosquitoes. Therefore, dry detention basins must have outlets designed to drain in 24 to 48 hours. Careful design and proper management and maintenance of systems can effectively control mosquito breeding.
• Reliability/Maintenance/Sustainability – The detention facility shall be effective over an extended time and be able to be properly operated and maintained over time. This may involve requiring subdivision covenants and designating individuals responsible for the operation and maintenance of detention facilities.

There are two basic approaches to designing storage facilities, which vary depending on the type of development:

• Onsite or private facilities – Detention facilities that are planned on an individual site basis.
• Common or regional facilities – Detention facilities that are planned to serve multiple lots, a subdivision, or larger area.

Depending on the type of development, requirements for detention basins may vary, as described below:

• Residential or Commercial Subdivision - These are developments that involve the subdivision of property. One or more detention basins may be required depending on the natural drainage patterns of the development.
• Single Lot Commercial - Generally, these are developments on lots that are not part of a subdivision. Basins shall be designed for full development of the lot based on zoning unless land use restrictions dictate less land is available for development.
• Multiple Properties - Multiple properties or developments may be served by a regional basin that is not within the boundary of the development.

Generally, the type of detention is determined by the required design objectives and the appearance and function desired by the developer. Detention basins fall into one of the following three design categories:

• Dry detention basin - Designed for several different frequency rainfalls for flood control only. Dry basins drain over 1 to 2 days. The outlet is typically composed of orifices and/or weirs.
• Extended detention basin - Designed for pollutant removal and potentially for flood control. Extended detention basins drain over an extended period of time, typically 1 to 3 days. The outlet is typically composed of a filtered control as well as orifices and/or weirs.
• Wet basin – Awet basin, also referred to as a retention basin, contains a permanent pool of water and is designed for pollutant removal, flood control, and often aesthetics. Wet basins may be designed to drain down to the permanent pool level over a short or long period of time.

Unplanned(or non-engineered)storage may also be present in features such as sinkholes and the upstream side of railroad and highway embankments. When planning a development along a major waterway, such non-engineered storage should be accounted for when calculating existing flow rates but generally should not be accounted for when calculating ultimate future peak flow rates.

In developments where an offsite area drains across the property, the developer must consider whether to: 1) construct an off-line detention basin to capture only the local site runoff and bypass the offsite runoff around the basin, or 2) construct an in-line basin with offsite runoff directed through the basin. In-line and off-line storage are defined below:

• Off-Line Storage: A facility located off-line from the drainageway that receives runoff from a smaller drainage area or from a particular site. These facilities often are smaller and may store water less frequently than in-line facilities. This is the approach preferred by the City for cases where an offsite area drains across a property.
• In-Line Storage: A facility located in-line with the drainageway that captures and routes the entire flood volume. A disadvantage with in-line storage is that it must be large enough to store and convey the total flood volume of the entire tributary catchment, including offsite runoff, if it exists. A U.S. Army Corps of Engineers (USACE) Section 404 permit for dredge and fill activities within the waters of the United States and a Section 401 Water Quality Certification from the Arkansas Department of Environmental Quality (ADEQ) are typically required for in-line storage. In-line storage is only allowed by the City if it can be demonstrated that off-line storage is not practicable.

For all types of basins, the designer should consider safety, aesthetics, and multipurpose uses during both wet and dry conditions. The use of other specialists such as landscape architects, biologists, and planners is encouraged to achieve these objectives.

Two design methods that are acceptable for use in detention design are summarized in Table DET-1. The appropriate method is dependent on the detention volume required and the impervious area added by the development. When determining which method is acceptable, the calculated volume takes precedence over the impervious area added.

To maintain peak flow rates at pre-development levels, a multi-frequency outlet design approach is required. The designer must demonstrate that the 1-, 2-, 5-, 10-, 25-, 50- and 100-year post-development peak flow rates are limited to the corresponding pre-development flow rates. The outlet design must be compatible with the calculated volume and volume design for each design event to ensure peak discharges do not exceed pre-development rates for each design event. For example, for the water surface elevation corresponding to the volume calculated for the 10-year event, the outlet should be designed to discharge no greater than the 10-year pre-development peak flow rate. If the facility is also providing water quality treatment, then the detention volume and outlet design must also incorporate the WQCV (See Chapter 9 – Water Quality).

The hydraulic capacity of the various components of the outlet works (i.e., pipes, orifices, weirs) can be determined using standard hydraulic equations described below. (Note: Because the discharge pipe of an outlet works functions as a culvert, the reader is directed to Chapter 7 – Culvert & Bridge Hydraulic Design, for guidance regarding the calculation of the hydraulic capacity of outlet pipes).

To create a rating curve for an entire outlet, a composite total outlet rating curve can be developed based on the rating curves developed for each of the components of the outlet and then selecting the most restrictive element that controls the release at a given stage.

When designing a detention facility, multi-purpose uses, such as active or passive recreation and wildlife habitat, are encouraged in addition to providing the required storage volume. Facilities used for recreation should be designed to inundate no more frequently than every two years.

Detention basins should be located at the natural low point of the site and must discharge to the natural drainage location to minimize downstream impacts.

Detention basin grading shall conform to the natural topography of the site to the maximum extent practical. Developments should be laid out around the existing waterways and proposed detention basin (see Figure DET-3). Layouts conforming to existing topography often reduce construction costs, land disturbance and maintenance costs, and increase aesthetic quality. Existing slopes should be used to the maximum extent practical. If slopes are modified, the maximum allowable slope is 3H:1V. Exceptions to these criteria must be justified through engineering studies and are subject to City approval. Significant modifications to existing topography may require geologic impact studies and geotechnical analysis, particularly where shallow bedrock or karst topography is believed to be present.

The geometry of a detention facility depends on specific site conditions such as adjoining land uses, topography, geology, existing natural features, volume requirements, etc. A cross-section of the proposed detention facility shall be provided in the plans showing at a minimum the basin profile, normal water surface elevation (WSE) if applicable, the 100 year WSE and the outlet works. The following criteria apply to the geometry of detention facilities:

• Pond side slopes - Pond side slopes of 3:1 (H:V) are the maximum permissible; slopes between 5H:1V and 10H:1V are encouraged. If slopes steeper than 3H:1V are desired, the engineer must demonstrate why 3H:1V slopes are not feasible and provide an explanation regarding how the steeper slopes will be maintained and how safety concerns will be addressed. Steeper slopes are subject to City approval. For all wet detention facilities, a safety bench sloped at 10:1 and 15-feet wide shall be provided starting at the normal water surface elevation unless a safety fence is provided (see Section 6.11)
• Pond bottom slopes – For dry detention ponds, the pond bottom slopes must be a minimum of 1 percent to ensure drainage.
• Pond shape - The water quality portion of a facility (if present) should be shaped with a gradual expansion from the inlet and a gradual contraction toward the outlet, thereby minimizing short-circuiting. The minimum length:width ratio shall be 2:1. Storage facility geometry and layout are best developed with input from a land planner/landscape architect.
• Low-flow channel - A 5-ft wide concrete low-flow channel may be required. However, for water quality basins or wetlands, concrete low-flow channels are not desirable, in which case alternative materials, as described below, should be discussed with and approved by City staff.
• Materials - Between the 1- and 10-year design flows, hard armor/grass composites may be considered, provided that velocities are low enough to ensure stability. Above the 10-year water surface, sod, turf reinforcement mat or other composite designs may be used, provided that they are appropriate for design velocities. Sod is acceptable for velocities less than 5 ft/s. Turf reinforcement mat or other composite materials are acceptable for velocities less than 8 ft/s. For velocities of 8 ft/s or more, a manufactured hard lining, or other suitable armor material is necessary (see Chapter 6 – Open Channel Flow Design).

If the detention storage structure is a jurisdictional facility, meaning it is subject to regulation by the Arkansas Soil and Water Conservation Commission (ASWCC), the embankment shall be designed, constructed, and maintained to meet most current ASWCC criteria for jurisdictional structures. The design for an embankment of a storm water detention or retention storage facility shall be based upon a site-specific engineering evaluation. The embankment shall be designed to prevent catastrophic failure during the 100-year and larger storms. The following criteria frequently apply (ASCE and WEF 1992):

• Side Slopes—For ease of maintenance, side slopes of the embankment shall not be steeper than 3:1 (H:V). The embankment’s side slopes shall be well vegetated, and riprap protection (or the equivalent) may be necessary to protect it from wave action on the upstream face, especially in retention ponds.
• Emergency Spillway—An emergency spillway is required to convey the 100-year flow if the primary outlet becomes clogged or for storm events larger than the 100-year event. The spillway shall be designed to accommodate the 100-year flow from the fully developed watershed assuming no upstream detention.
• Freeboard—The elevation of the top of the embankment shall be a minimum of 1 foot above the water surface elevation when the emergency spillway is conveying the maximum design or emergency flow. When relevant, all Arkansas Natural Resources Commission dam safety criteria must be carefully considered when determining the freeboard capacity of an impoundment.
• Settlement—The design height of the embankment shall be increased by roughly 5 percent to account for settlement. All earth fill shall be free from unsuitable materials and all organic materials such as grass, turf, brush, roots, and other material subject to decomposition. The fill material in all earth dams and embankments shall be compacted to at least 95 percent of the maximum density obtained from compaction tests performed by the Modified Proctor method in ASTM D698.
• Embankment—A geotechnical engineer shall provide a stamped report for any embankment over 10-feet tall. The City reserves the right to require a report for any embankment between 5 and 10-feet as well. (See Section 6.14)
• Vegetation—No trees shall be planted or allowed to grow on a detention facility embankment. Vegetation will be required elsewhere within the detention facility per Section 6.10 of this Chapter.

Detention facilities may require an impermeable clay or synthetic liner for a number of reasons. Storm water detention and retention facilities have the potential to raise the groundwater level in the vicinity of the basin. If the basin is close to structures or other facilities that could be damaged by raising the groundwater level, consideration should be given to lining the pond. An impermeable liner may also be warranted in a retention basin where the designer seeks to limit seepage from a permanent pond. Alternatively, there are situations where the designer may seek to encourage seepage of storm water into the ground. In this situation, a layer of permeable material may be warranted.

Inlets to the facility should incorporate energy dissipation to limit erosion and should be designed in accordance with drop structure criteria in Chapter 6 – Open Channel Flow Design, or using other approved energy dissipation techniques. In addition, forebays or sediment traps should be incorporated at inflow points to storage facilities to settle sediment being delivered by stormwater to the facility.

A forebay, while optional, should be considered when the design volume exceeds 20,000 ft³ or a large sediment, trash, or debris load is anticipated due to upstream land use. A forebay provides an opportunity for larger particles to settle out in the inlet area, which has a solid surface bottom to facilitate mechanical sediment removal. The forebay volume for the extended dry detention basin should be between 3 and 5 percent of the design volume. Forebays will need regular maintenance to reduce the sediment being transported and deposited on the storage basin’s bottom.

Outlet works shall be sized and structurally designed to release at the specified flow rates without structural or hydraulic failure. Design guidance for outlet works used for water quality purposes is included in Chapter 9 – Water Quality. A staff gauge shall be installed on all outlet works. The staff gauge shall be a porcelain-coated metal USGS Type C gauge.

Trash racks are required and shall be sized so as not to interfere with the hydraulic capacity of the outlet and must be designed in a manner that is protective of public health, safety and welfare. See Chapter 9 – Water Quality for minimum trash rack sizes.

The type of vegetation specified for a newly constructed storage facility is a function of several factors, including:

• The frequency and duration of inundation of the area
• Soil types
• The desire for native versus non-native vegetation
• Other potential uses of the area (e.g., park, open space, etc.)
• All plants within required stormwater facility areas shall be appropriate species approved by the City Engineer. The developer must plant a variety of tree and shrub species so that no single species comprises more than 25% of the plantings.
• The design for plantings shall minimize the need for herbicides, fertilizers, pesticides or soil amendments at any time before, during and after construction and for a long-term basis
• Plantings should be designed to minimize the need for mowing, pruning and irrigation. Grass or wildflower seed shall be applied at the rates specified by the suppliers. If plant establishment cannot be achieved with seeding by the time of substantial completion of the stormwater facility portion of the project, the contractor shall plant the area with wildflower sod, plus, container plants or some other means to complete the specified plantings and protect against erosion as approved by the City Engineer
• Plantings shall not impede the primary function of the stormwater facility. Should plantings be proposed that call into question the ability of the stormwater facility to operate to the satisfaction of the City Engineer, the developer shall provide sufficient information (calculations, etc.) for review, at the time of submittal
• Dry detention basins shall be sodded up to the top of bank. Wet detention basins shall be sodded from the top of bank to the normal water surface elevation.
• A planting plan should be developed for new facilities to meet their intended use and setting in the urban landscape. The stormwater facility area for a dry detention or wet retention facility is defined to be equivalent to the area of the detention basin, including the bottom and the side slopes, plus a 10-foot buffer around the detention basin. The developer must install minimum plant material quantities per 3,000 square feet of the dry detention facility area as follows:
  • 1 tree from the canopy tree list in the UDC
  • 2 trees from the understory tree list in the UDC
  • 2 large shrubs
  • 6 shrubs/large grass-like plants, 1-gallon containers or equivalent
  • Ground cover plants, 1 per 12 inches on center with triangular spacing, unless sod is specified and installed
  • At least 50% percent of the facility must be planted with grasses or grass-like plants
  • Wildflowers, native grasses and ground covers shall be designed to require mowing no more than twice annually.
• The developer must install minimum plant material quantities per 1,500 square feet of a wet retention facility area as follows:
  • 1 tree from the canopy tree list in the UDC
  • 2 trees from the understory tree list in the UDC
  • Trees planted within the stormwater facility area for wet retention facilities must be planted in a manner so as to not obscure sight of the pond or on the top of the constructed berms. Plantings must be within the slopes of the pond.

For retention ponds (i.e., a pond that typically has a permanent pool), the pond must either have a safety bench or be surrounded by a minimum 48-inch tall wrought iron fence or equivalent, as approved by City.

For detention ponds (i.e., a pond that is generally dry), and especially if children are apt to play in the vicinity of the impoundment, use of relatively flat side slopes along the banks is advisable. In addition, installation of landscaping that will discourage entry, such as thick, thorny shrubs, is suggested for locations along the periphery, near the inlets and at steeper embankment sections.

The use of thin steel plates as sharp-crested weirs should be avoided because of potential accidents, especially with children. Trash racks must protect all outlets, especially ones made of a thin plate.

If the impoundment is situated adjacent to and at the same or a lower grade than a street or highway, installation of a guardrail between the roadway and the pond is required.

Consideration shall be given for safety at outlet structures. The City reserves the right to require safety appurtenances at outlet structures.

Maintenance considerations during design include the following (ASCE and WEF 1992):

1. Maintenance access - The facility shall be accessible to maintenance equipment for removal of silt and debris and for repair of damages that may occur over time. An access easement and/or right-of-way is required to allow access to the impoundment by the owner or agency responsible for maintenance. The access shall have a maximum grade of 10 percent and have a solid driving surface of gravel, rock, concrete, or reinforced turf on a stabilized bed designed to support vehicle loads.
2. Sediment removal considerations - Permanent ponds shall have provisions for complete drainage for sediment removal or other maintenance. The frequency of sediment removal will vary among facilities, depending on the original volume set aside for sediment, the rate of accumulation, rate of growth of vegetation, drainage area erosion control measures, and the desired aesthetic appearance of the pond. Sediment should be removed when its depth accumulates to 6 inches. A depth gauge is required at the outlet to facilitate determining when sediment removal is necessary as well as the pond depth. Also, appearance may dictate more frequent cleaning. Detention facilities shall be designed with sufficient depth to allow accumulation of sediment for several years prior to its removal. A general guideline is to oversize the storage capacity of a detention facility by 20 percent of the WQCV (see Chapter 9 – Water Quality) to allow for sediment storage.
3. Sediment concerns - Secondary uses that are incompatible with sediment deposits should not be planned unless a high level of maintenance will be provided. French drains or the equivalent are almost impossible to maintain and should be used with discretion where sediment loads are apt to be high.
4. Dissolved oxygen concentrations in pond - Adequate dissolved oxygen supply in permanent ponds (to minimize odors and other nuisances) shall be maintained by artificial aeration. Use of fertilizer and pesticides adjacent to the permanent pool pond should be carefully controlled.
5. Underground tank maintenance - Underground tanks or conduits designed for detention shall be sized and designed to permit pumping. Multiple entrance points shall be provided to remove accumulated sediment and trash.
6. Permanent pool depth - Permanent pools shall have a minimum depth of 6 feet to discourage excessive aquatic vegetation on the bottom of the basin, unless the vegetation is specifically provided for water quality purposes.
7. Aesthetics and landscaping - Trash racks and/or fences are often used to minimize hazards. These may become eyesores, trap debris, impede flows, hinder maintenance, and, ironically, fail to prevent access to the outlet. On the other hand, desirable conditions can be achieved through careful design and positioning of the structure, as well as through landscaping that will discourage access. Creative designs, integrated with innovative landscaping, can be safe and can also enhance the appearance of the outlet and pond. In addition, bank slopes, bank protection, and vegetation types are important design considerations for site aesthetics and maintainability.
8. Avoid moving parts - To reduce maintenance and avoid operational problems, outlet structures should be designed with no moving parts (i.e., use only pipes, orifices, and weirs). Manually and/or electrically operated gates should be avoided and must be approved by City staff during the design concept stage of development.
9. Outlet openings - To reduce maintenance, outlets should be designed with openings as large as possible, be compatible with the depth-outflow relationships desired, and be designed with water quality, safety, and aesthetic objectives in mind.
10. Resistant to vandalism - Outlets should be robustly designed to lessen the chances of damage from debris or vandalism.
11. Maintenance of forebays and sediment traps - Clean out all forebays and sediment traps on a regular basis or when routine inspection shows them to be ¾ full.

See Chapter 9 – Water Quality, for additional recommendations regarding operation and maintenance of water quality related facilities, some of which also apply to detention facilities designed to meet other objectives.

All-weather, stable access to the bottom, inflow, forebay, and outlet works areas shall be provided for maintenance vehicles. Maximum grades should be 10 percent, and a solid driving surface of gravel, rock, concrete, or reinforced turf on a stabilized bed designed to support vehicle loads.

The designer must account for the geotechnical conditions of the site. These considerations may include issues related to embankment stability, geologic hazards, seepage, and other site-specific issues such as karst topography. It may be necessary to confer with a qualified geotechnical engineer during both design and construction, especially for larger detention and retention storage facilities.

A geotechnical engineer shall provide a stamped design for any dam 10 feet or more in height. This design shall include, but may not be limited to, minimum factors of safety for stability (including global stability). The City may require a design for dams 5-10 feet in height. Unless otherwise shown, dam embankments shall be compacted at 95% standard proctor within ± 2% of optimum moisture content.

The designer must account for environmental considerations surrounding the facility and the site during its selection, design and construction. These can include regulatory questions such as: 1) Will the facility be located in a jurisdictional wetland?, or 2) Will the facility be located on a waterway regulated by the USACE as a “Water of the U.S.,” and 3) Are there threatened and endangered species or habitat in the area? See Chapter 1 – Stormwater Submittal Requirements for more information on regulatory and permitting requirements.

Other non-regulatory environmental issues should also be taken into account. Detention facilities can become breeding grounds for mosquitoes unless they are properly designed, constructed and maintained. Area residents may object to facilities that impact riparian habitat or wetlands. Considerations of this kind must be carefully accounted for, and early discussions with relevant federal, state and local regulators are recommended.

Use the Rational Formula-Based Modified FAA Procedure (described in Section 5.1.1) to determine the required detention volume for the 100-year storm event for a 40-acre watershed, based on single-family land use. The watershed has a 100-year runoff coefficient of 0.56 and a time of concentration of 25 minutes. The post-development 100-year, undetained peak flow rate from the watershed is 157 cfs. The pre-project 100-year peak flow rate for the site is 90 cfs.

Given the information above, the following variables are known:

A = 40 acres

C= 0.56

Q_po = 90 cfs

t_c = 25 minutes

Following the methodology outlined in Section 5.1.1, Table DET-3 can be created to determine the required detention volume.

The required detention volume is determined from the maximum storage volume (see column 7 in Table DET-3). For this example, the required detention volume is 110,832 ft³ or 2.5 acre-feet (see shaded cell in Table DET-3). Because this volume exceeds the 20,000-ft³ threshold for applicability of the FAA method for final detention sizing, this should be treated as an initial estimate, and a hydrograph-based method should be used to determine detention storage requirements.

Use the Hydrograph Volumetric method (described in Section 5.1.2.1) to determine the preliminary detention volume required, given an inflow hydrograph for a 20-acre commercial site (calculated according to guidelines in Chapter 3 – Determination of Stormwater Runoff) and a maximum allowable release rate of 30 cfs.

The tabular format for use with the inflow hydrograph method is shown in Table DET-4 below. The time and flow ordinates of the inflow hydrograph are entered in columns 1 and 2. Based on the inflow hydrograph, the allowable release rate of 30 cfs is matched on the falling limb at a time between 102 and 108 minutes, so 108 minutes is used as an estimate for T_p.

Use the Modified Puls Method (described in Section 5.1.2.2) to determine the outflow hydrograph for a proposed detention facility. Given the inflow hydrograph from the example in 7.2 for a 20-acre commercial site, a detention basin with the stage-storage relationship in Table DET-5 is proposed.

For sites that are 1-acre in size or smaller, for sites that are being redeveloped, or for sites of any size that are adjacent to a primary channel (see primary channel description in Section 2.5 of Chapter 6 – Open Channel Flow Design), the City may allow a developer/property owner to make a monetary payment or some other form of valuable consideration in lieu of implementing the stormwater detention measures described in this chapter. The City shall make the determination of whether fee-in-lieu of detention will be allowed or required on a case by case basis based upon capacity of the receiving stormwater drainage system and whether regional detention facilities are either proposed or in place and the increase in flow rates to these downstream conditions will not adversely affect downstream property owners. The fee shall be paid at the time the final plat is approved by City Council or prior to issuance of any Certificate of Occupancy for a Site Development. When these fees are collected, they shall be deposited into a stormwater capital improvements fund, which will be used for future or ongoing stormwater improvement and regional detention projects that will benefit stormwater management in the community.

The fee in-lieu of detention rate is set at $0.20/ft² of impervious area on developments that are approved for fee in-lieu.

The purpose of this chapter is to provide guidance for designing facilities to convey stormwater runoff in open channels. The goal of open channels is to convey stormwater runoff from and through urban drainage areas without damage to adjacent properties/developments, to the open channel, or to the storm drainage system connected to it. Specifically, this chapter provides information on physical channel criteria and design methodology necessary to design open channels according to City requirements.

Once stormwater runoff has been collected in a storm drainage system it continues to combine with other sections of the storm drainage system until, typically, culminating into open channels. Except for roadside ditches and swales, open channels are nearly always a component of the major drainage system. There are a number of factors which must be considered in determining whether to specify an open drainageway as opposed to an underground storm drain: material and installation cost, maintenance costs and problems, acceptability to the developer or home buyer, public safety, water quality, appearance, etc. Effective planning and design of open drainageways can significantly reduce the cost of storm drainage facilities, while enhancing the quality of the development.

In planning a development, the designer should begin by determining the location and the width of existing drainageways. Streets and lots should be laid out in a manner to preserve the existing drainage system to the greatest degree practical. Constructed channels should be used only when it is not practical or feasible to utilize existing drainageways.

This section covers the evaluation of capacity and stability of natural drainage channels, and design of constructed drainage channels.

To comply with the City requirements for open channel flows, channels must be planned and designed to address the applicable criteria outlined below:

• Layout and Structure
  • Safety of the general public and preventing damage to private property are the most important considerations in the selection of the cross-sectional geometry and type of open channel. Channel shape, type, and alignment should be selected to ensure that velocities and depths do not exceed those specified in Section 2.0 and Section 3.0 of this chapter. The range of design channel discharges should be selected by the designer based on flood hazard risks and local site conditions.
  • Channels must be designed with long-term stability in mind. Following the guidelines and design criteria presented in this chapter for designing open channels provides reasonable parameters that when met provide adequate channel stability. Regular channel maintenance will be a necessary part of maintaining channel stability as well. The design of open channels must consider the frequency and types of maintenance expected and make allowance for maintenance access along and within the channel.
• Environmental and Regulatory
  • Environmental and regulatory criteria as mentioned herein are not discussed in detail in this Manual. Local, state, and federal regulations must be reviewed and addressed for the appropriate agency having jurisdiction over impacted areas.
  • Environmental impacts of channel modifications, including disturbance of fish habitat, wetlands and channel stability, should be assessed and if needed remediation planned within the overall drainage design for such impacted areas.
  • Channel designs that impact existing open channels shall satisfy the policies of the Federal Emergency Management Agency (FEMA) applicable to floodplain management and regulation. Wherever possible, disturbance of natural channels/streams shall be avoided and encroachment onto flood plains shall be minimized to the fullest extent practical.
  • Coordination with other Federal, State and local agencies (US Army Corp of Engineers, US Fish and Wildlife Service, Arkansas Department of Energy & Environment Division of Environmental Quality, Arkansas Historic Preservation Program, etc.) concerned with water resources planning must be carried out as part of the design of open channels to ensure all laws and regulations are adhered to in a design.

The summary below outlines some of the most critical design criteria essential to design engineers for proper drainage design of open channels according to City of Rogers’ requirements. The information below contains exact numerical criteria as well as general guidelines that must be adhered to during the design process. This section is meant to be a summary of critical design criteria for this section; however, the engineer is responsible for all information in this chapter. It should be noted that any design engineer who is not familiar with Rogers’ Drainage Criteria Manual and its accepted design techniques and methodology should review the entirety of this chapter.

More detailed information can be found in Section 2.5 and Table OC-7a

Refer to the City of Rogers’ GIS site for Streambank Inventory-BEHI (Bank Erosion Hazard Index) for areas of special design considerations (http://gis.rogersar.gov).

In addition to the design criteria outlined in the following tables in this chapter, primary channels will also need to incorporate natural channel design elements, unless otherwise approved by the Director of Community Development and the City Engineer. Natural channel design elements would include riparian zones, floodplains, alluvial landscapes, natural weirs and vanes, toe wood along channel banks, etc.

Major drainage is the cornerstone of an urban storm runoff system. The major drainage system will exist whether or not it has been planned and designed, and whether or not urban development is wisely located in respect to it. Thus, major drainage must be given high priority when considering drainage improvements.

A core component of any major drainage system is open channels. Open channels are the most common major drainage system component used to transport all of the stormwater runoff collected in drainage systems. Open channels are versatile and come in several different types and consist of several different channel components. Open channels are in effect the final instrument within a drainage system for handling stormwater and as such have the final interaction with stormwater before it flows into a major river or other large body of water.

While the primary function of open channels is conveyance of runoff, many design decisions contribute to the role of channels in the urban environment in terms of stability, multiple use benefits, social acceptance, aesthetics, resource management, and maintenance. It is important for the engineer to be involved from the very start of a land development project, so that the criteria in this Manual have bearing on the critical planning decisions involved in route selection for open channels within the major drainage system. The importance of route selection cannot be overstated since the route selected will influence every element of the major drainage project from the cost, to the type of channel to use, to the benefits derived to the community.

Secondary and primary open channels shall be placed in Drainage and Recreation Easements.

The types of major drainage channels available to the designer are numerous. Section 2.3.1 describes in detail the types of channels engineers can consider as potential major open channels in urban areas and then select the one that addresses the hydraulic requirements, environmental considerations, sociological/community impact and needs, permitting limitations the best. Table OC–6 lists the types of channels discussed within this chapter along with the City’s attitude toward each channel type.

This chapter addresses the major topics related to the design of open channels, beginning with essential background on the issues of open channel planning and engineering (Section 1.4) and fluvial geomorphology (Section 1.5). General open channel hydraulics and preliminary design criteria are presented in Section 2.0. It is the responsibility of the designer to be knowledgeable of open channel hydraulics, and, therefore, the key principles and equations are reviewed without extensive background of the subject matter, theoretical considerations, etc. Section 3.0 contains specific design criteria for a variety of channel types and includes example calculations, typical cross sections, and other representative design details.

The most fundamental function of open channels is conveyance of the major storm runoff event, and an important characteristic is their stability during major and minor storms. Stability must be examined in the context of the future urbanized condition, in terms of both runoff events and altered base flow hydrology. Base flow within a channel is flow that is not caused by rainfall events, but rather aquifer seepage resulting from a variety of causes. Some of the most common base flow sources are yard irrigation, artesian groundwater, and other constant flow sources. Urbanization in the City of Rogers commonly causes base flows to increase, and the planner and engineer must anticipate and design for this increase.

In addition to stability issues, there are many planning and engineering decisions that contribute to the role of open channels in the urban environment, in terms of multiple use benefits, social acceptance, aesthetics, and resource management. The choices of the type and layout of open channels are of prime importance.

Open channels for transporting major storm runoff are the most desirable type of major drainageway because they offer many opportunities for creation of multiple use benefits such as incorporation of parks and greenbelts along the channel and other aesthetic and recreational uses that closed-conveyance drainageway designs preclude. Open channels are also usually less costly and they provide a higher degree of flood routing storage. Where opportunities are presented for establishing primary or secondary channels, or enhancing the functionality of already existing open channels, deviating from utilizing an open channel would first need to be approved by the City Engineer.

The choice of the type of open channel is a critical decision in planning and design of major drainageways. The preferred channel is a stable natural one carved by nature over a long period of time that can remain stable after urbanization. Generally, the closer an artificial channel’s character can be made to that of a natural channel, the more functional and attractive the artificial channel will be. In an urban area, however, it is rarely feasible to leave a natural channel untouched since urbanization alters the hydrology of the watershed. Consequently, some level of stabilization is usually necessary to prevent the channel from degrading and eroding. Channel type evaluation should be done in ascending order as shown in Table OC-6.

A drainage system within a watershed involves flowing water or movement of water, thus the term fluvial. When flowing water develops a drainage pattern or surface forms, the process is identified as fluvial geomorphology. Surface form characteristics represented by open channels (natural and manmade) behave in a complex manner dependent on watershed factors such as geology, soils, ground cover, land use, topography, and hydrologic conditions. These same watershed factors contribute to the sediment eroded from the watershed and transported by the stream channel. The sediments moved by the flowing water also influence channel hydraulic characteristics. The natural-like channel and stabilization systems recommended in this Manual are based on fluvial geomorphology principles. The remainder of this section will provide the reader with a basic understanding of the workings and evolution of open channels within an urban watershed.

When performing open channel design, hydraulic analyses must be completed to evaluate flow characteristics including flow regime, water surface elevations, velocities, depths, and hydraulic transitions for multiple flow conditions. Hydraulic grade lines and energy grade lines shall be prepared on all design projects.

The purpose of this section is to provide the designer with an overview of open channel flow hydraulics principles and equations relevant to the design of open channels. The reader should already be familiar with the open channel flow principles discussed in this section. Water surface profile computations are not addressed herein, and the reader is referred to other references [such as Chow (1959), Daugherty and Franzini (1977), and King and Brater (1963)] for discussion of this topic.

Open channels must be able to convey the flow from a fully urbanized watershed for the design considerations outlined here. Methods for calculating the flow from a fully urbanized watershed are described in Chapter 3 – Determination of Stormwater Runoff. A channel’s lining, geometry (depth, width, alignment, etc.), and freeboard characteristics shall be designed in relation to the channel’s maintenance classification as defined in Section 2.5 of this chapter. Channels shall be designed according to the following design storm frequencies as follows:

• Primary Channel – 100-year design storm with ≥2-foot of freeboard
• Secondary Channel – 100-year design storm with ≥1-foot of freeboard
• Tertiary Channel – 10-year design storm and pass 100-year design storm between structures

Furthermore, open channels, including residual floodplain, must be able to convey the flow from a fully urbanized watershed, assuming no upstream detention, for the event with a 100-year recurrence interval without significant damage to the system. In addition to the capacity consideration of the 100-year event, the designer must also consider events of lesser magnitudes. For the low-flow channel in any type, 5-year storm peak discharge for fully developed conditions, assuming no upstream detention, is to be used for its design. Base flow must also be assessed, especially for grassed channels, channels with wetland bottoms, and bioengineered channels. Base flows are best estimated by examining already-urbanized watersheds that are similar to the planned urban area in terms of imperviousness, land use, and hydrology.

In order for open channels to function according to their original design, channels require periodic maintenance and repair. Maintenance and repair includes removal of debris and litter from the channel, regular mowing of grass-lined and composite channels to maintain expected channel roughness, repair and stabilization of eroded channel banks/bottoms, repair/replacement of any erosion control structures (including but not limited to channel drop structures, armored channel lining, etc.), and any other necessary upkeep work within the established open channel boundaries that don’t reflect the channels intended purpose.

Being that open channels provide a benefit to a number of different users the City has established certain physical and operational criteria that designate channels within the city limits as either primary, secondary, or tertiary. The definitions below along with Table OC-7a describe the use, maintenance/repair responsibilities, and designation criteria of each of the channels.

• Primary Channel – a major open channel that serves as a primary waterway to conduct runoff generated in a large composite area (typically ≥ 30-acres). More so, any channel that has a flood zone (floodway, floodplain, etc.) as determined/studied by the City and/or FEMA is to be considered a primary channel. Runoff conducted by primary channels is collected in the channel from discharges of a watershed, closed storm sewer systems, secondary and tertiary channels, and from the convergence of other primary channels. These types of channels are to be maintained by the City, POA, developer of the subdivision or other responsible entity for a development and shall be placed in a Drainage and Recreation Easement. Designate extent of 100-year water surface elevation on grading plan and as a Drainage and Recreation Easement.
• Secondary Channel – a moderate open channel that collects runoff from storm sewer systems, tertiary and other secondary channels, and feeds the runoff into primary channels. Drainage areas for secondary channels typically range from ≥ 2-acres and ≤ 30-acres. These types of channels are to be maintained by a POA, developer of the subdivision, or other responsible entity for a development and shall be placed in a Drainage and Recreation Easement. Designate extents of 100-year water surface elevation on grading plan and as a Drainage and Recreation Easement.
• Tertiary Channel – a small minor channel that serves as a conduit to channel runoff (typically ≤ 2-acres). These types of channels are to be maintained by the owners of the property which the channel serves. Maintenance responsibilities for the property owner end at the furthest point upstream and/or downstream the channel exists within the property’s legal recorded boundaries. These channels are not typically placed in a drainage easement.

Grass-lined channels are considered by the City the most desirable type of artificial channels for new development where natural channels are absent or have limited environmental value. Channel storage, lower velocities, and aesthetic and recreational benefits create advantages over other channel types.

When the trickle channel flow capacity limits, as discussed in Section 3.1.4, are exceeded the use of a composite channel is required, namely a channel with a stabilized low-flow section and an overflow section above it to carry major flow. Composite channels are, in essence, grass-lined channels in which more dense vegetation (including wetland-type) is encouraged to grow on the bottom and sides of the low-flow channel. Hence they are sometimes known as “wetland bottom” channels. Under certain circumstances, such as when existing wetland areas are affected or natural channels are modified, the USACE’s Section 404 permitting process may mandate the use of composite channels that will have wetland vegetation in their bottoms. In other cases, a composite channel with a wetland bottom low-flow channel may better suit individual site needs if used to mitigate wetland damages elsewhere or if used to enhance urban stormwater runoff quality. Composite channels can be closely related to bioengineered and natural channels. Composite channels can provide aesthetic benefits, habitat for aquatic, terrestrial and avian wildlife and water quality enhancement as base flows come in contact with vegetation.

Wetland bottom vegetation within a composite channel will trap sediment and, thereby, reduce the low-flow channel’s flood carrying capacity over time. To compensate for this the channel roughness factor used for design must be higher than for a grass-lined channel. As a result, more right-of-way is required for composite channels that have the potential for developing wetlands in their bottom.

The use of concrete-lined channels is subject to City approval. Although not recommended for general use because of safety water quality and aesthetic reasons; hydraulic, topographic, or right-of-way constraints may necessitate the use of a concrete-lined channel in some instances. A common constraint requiring a concrete-lined channel is the need to convey high velocity, sometimes supercritical, flow. Whether the flow will be supercritical or subcritical, the concrete lining must be designed to withstand the various forces and actions that cause overtopping of the bank, damage to the lining, and erosion of unlined areas.

Concrete-lined channels can be used for conveyance of both subcritical and supercritical flows. In general, however, other types of channels such as grass-lined channels or channels with wetland bottoms shall be used for subcritical flows. The use of a concrete-lined channel for subcritical flows shall not be used except in unusual circumstances where a narrow right-of-way exists.

The use of riprap-lined channels is discouraged and subject to City approval. Channel linings constructed from riprap (grouted or partially grouted), soil riprap, grouted boulders, or wire-encased rock (gabion) to control channel erosion may be considered on a case-by-case basis for the following situations:

1. Where major flows such as the 100-year flood are found to produce channel velocities in excess of allowable non-eroding values (5-ft/sec) or when main channel depth is greater than 5 feet.
2. Where channel side slopes must be steeper than 3H:1V.
3. For low-flow channels.
4. Where rapid changes in channel geometry occur such as channel bends and transitions.

Design criteria applicable to these situations are presented in this section. Riprap-lined channels shall only be used for subcritical flow conditions where the Froude number is 0.8 or less. Loose stones serving as a protective blanket will not be accepted for riprap lining. Instead riprap shall either receive a full grout matrix or be partially grouted. The type of grouting, full or partial, a riprap lining is to receive will be as directed by the City. The grout for riprap receiving a full grout matrix shall adhere to the methods and specifications outlined in Table OC-12 of this Manual. Furthermore, requirements for riprap that is grouted that aren’t covered in this Manual shall adhere to ARDOT’s Standard Specification for Highway Construction – Section 816 – Filter Blanket and Riprap for Dumped Riprap (Grouted). Partially grouted riprap shall be designed, specified, and constructed according to the criteria presented in FHWA’s HEC 23 (2001) and other trusted sources on the subject. Furthermore, when used, it is required that all riprap outside frequent flow zones have the voids filled with soil, the top of the rock covered with topsoil, and the surface revegetated with native grasses. This combination of riprap, soil, and vegetation is considered soil riprap.

Bioengineered channels emphasize the use of vegetative components in combination with structural measures to stabilize and protect stream banks from erosion. The City advocates the integration of bioengineering techniques into drainage planning, design, and construction when the use of such channels is consistent with the City’s policies concerning flow carrying capacity, stability, maintenance, and enhancement of the urban environment and wildlife habitat. The following discussion on bioengineered channels interfaces closely with Section 3.2, Composite (Wetland Bottom) Channels, and Section 3.6, Natural Channels; designers are encouraged to read Section 3.2, Section 3.5, and Section 3.6, concurrently. In addition, because bioengineered channels require some structural assistance to maintain stability in urban settings, the designer should be familiar with the design of drop structures as discussed in FHWA’s Hydraulic Engineering Circular No. 14, 3^rd Edition (HEC-14 2006).

Natural waterways in the City of Rogers are sometimes in the form of steep, almost vertical stream banks, which have eroding banks and bottoms. On the other hand, many natural waterways exist in urbanized and to-be-urbanized areas, which have mild slopes, are reasonably stable, and are not currently degrading. If the channel will be used to carry storm runoff from an urbanized area, it can be assumed that the changes in the runoff regime will increase channel erosion and instability. Careful hydraulic analysis is needed to address this projected erosion. In most cases, stabilization of the channel will be required. Stabilization using bioengineering techniques, described in Section 3.5 of this chapter, has the advantage of preserving and even enhancing the natural character and functions of the channel. Some structural stabilization measures will also be required in combination with the bioengineered stabilization measures.

In the Rogers area, most natural waterways will need drops and/or erosion cutoff check structures to maintain a mild channel slope and to control channel erosion. Typically, these grade control structures are spaced to limit channel degradation to what is expected to be the final stable longitudinal slope after full urbanization of the tributary watershed. In the Rogers area, this slope, depending on watershed size and channel soils, has been observed to range from 0.30% to 1.5%, with the Illinois River itself approaching a slope of 0.06% to 0.10% within Benton County. Whenever feasible, natural channels shall be kept in as near a natural condition as possible by limiting modifications to those necessary to protect against the destabilizing hydrologic forces caused by urbanization.

Investigations needed to ensure that the channel is stable will differ for each waterway; however, generally, it will be necessary to measure existing cross sections, investigate the bed and bank material, determine soil particle size distribution, and study the stability of the channel under future conditions of flow. At a minimum, the designer should consider the concept of the stable channel balance discussed in Section 1.5.2 of this chapter, complete tractive force analysis, and apply the Leopold equations to evaluate channel stability and changes in channel geometry. Oftentimes, more sophisticated analysis will be required. When performing stability and hydraulic analyses, keep in mind that supercritical flow normally does not exist in natural-earth channels. During backwater computations, check to ensure that the computations do not reflect the presence of consistent supercritical flow (Posey 1960). Because of the many advantages of natural channels to the community (e.g., preservation of riparian habitat, diversity of vegetation, passive recreation, flood control and aesthetics), the designer should consult with experts in related fields as to method of development. Nowhere in urban hydrology is it more important to convene an environmental design team to develop the best means for using a natural waterway. It may be concluded that park and greenbelt areas should be incorporated into the channel design. In these cases, the usual rules of freeboard, depth, curvature, and other rules applicable to artificial channels often will need to be modified to better suit the multipurpose objectives. For instance, there are advantages that may accrue if the formal channel is designed to overtop, resulting in localized flooding of adjacent floodplain areas that are laid out for the purpose of being inundated during larger (i.e., > 10-year) flood events. See the Chapter 5 – Detention Design.

The following design criteria are required when evaluating natural channels:

1. The channel and overbank floodplain shall have adequate capacity for the 100-year flood.
2. A water surface profile shall be defined in order to identify the 100-year floodplain, to control earthwork, and to build structures in a manner consistent with Roger’s floodplain regulations and ordinances.
3. Use roughness factors (n) representative of un-maintained channel conditions for analysis of water surface profiles. Roughness factors for a variety of natural channel types are presented in Table OC-7.
4. Use roughness factors (n) representative of maintained channel conditions to analyze effects of velocities on channel stability. Roughness factors for a variety of natural channel types are presented in Table OC-7.
5. Prepare plan and profile drawings of the channel and floodplain.
6. Provide erosion-control structures, such as drop structures or grade-control checks, to control channel erosion and/or degradation as the tributary watershed urbanizes.
7. Outfalls into natural channels shall be 2 feet above the channel invert to account for vegetation and sediment accumulation. The engineer should visit the site of any outfalls into natural drainageways to examine the actual ground surface condition.

The purpose of this chapter is to provide guidance for culvert and bridge hydraulic design. The primary objective of a culvert or bridge is to convey stormwater flows, based on a design flow rate, through embankments or under roadways without causing damage to adjacent properties and developments, the roadway, or to the drainage structure. Specifically, this chapter provides information on the criteria and methodology necessary to design culverts and bridges according to City requirements.

The function of culverts and bridges is to convey surface water under a highway, city street, railroad, recreation trail, or other embankment. In addition to the hydraulic function, the culverts and bridges must carry construction, highway, railroad, or other traffic and earth loads. Therefore, culvert and bridge design involves both hydraulic and structural design considerations. The hydraulic aspects and design loading criteria of culvert and bridge design are set forth in this chapter.

The summary below outlines some of the most critical design criteria essential to design engineers for proper drainage design of streets, inlets, and storm sewers according to City of Rogers requirements. The information below contains exact numerical criteria as well as general guidelines that must be adhered to during the design process. This section is meant to be a summary of critical design criteria for this section; however, the engineer is responsible for all information in this chapter. It should be noted that any design engineer who is not familiar with Rogers’ Drainage Criteria Manual and its accepted design techniques and methodology should review the entirety of this chapter. If additional specific information is required, it will be necessary to review the appropriate section as needed.

The hydraulic design of a culvert consists of an analysis of the required performance of the culvert to convey flow from one side of an embankment to the other. The designer must select a design flood frequency, estimate the design discharge for that frequency, and set an allowable headwater elevation based on the selected design flood and headwater considerations. These criteria are dictated by the City of Rogers. The culvert size and type can be selected after the design discharge, controlling design headwater, slope, tailwater, and allowable outlet velocity have been determined.

The design of a culvert requires that the following be determined:

• Impacts of various culvert sizes and dimensions on upstream and downstream flood risks, including the implications of embankment overtopping.

• How will the proposed culvert/embankment fit into the relevant major drainageway master plan, and are there multipurpose objectives that should be satisfied?

• Alignment, grade, and length of culvert.

• Size, type, end treatment, headwater, and outlet velocity.

• Amount and type of cover.

• Public safety issues, including the key question of whether or not to include a safety/debris rack.

• Pipe material.

• Need for protective measures against abrasion and corrosion.

• Need for specially designed inlets or outlets.

• Structural and geotechnical considerations, which are beyond the scope of this chapter. The City may require a structural or geotechnical analysis.

For purposes of the following review, it is assumed that the reader has a basic working knowledge of hydraulics and is familiar with the Manning’s, continuity and energy equations, which are presented in Chapter 6 – Open Channel Flow Design:

(Equation CB-1)

where:

Q = Flow rate or discharge (ft³/sec)

n = Manning’s Roughness Coefficient

A = Flow Area (ft²)

R = Hydraulic Radius (ft)

S = Channel Slope (ft/ft)

(Equation CB-2)

where:

Q = Flow rate or discharge (ft³/sec)

v = Velocity (ft/sec)

A = Flow Area (ft²)

constant(Equation CB-3)

where:

v = Velocity (ft/sec)

g = Gravity (32.2 ft/sec²)

p = Pressure (lb/ft²)

γ = Specific weight of water (62.4 lb/ft³)

z = Height above datum (ft)

In short conduits, such as culverts, the losses caused by the entrance can be as important as the friction losses through the conduit. The losses that must be evaluated to determine the carrying capacity of the culverts consist of inlet (or entrance) losses, friction losses along the length of the culvert and outlet (or exit) losses. These losses are described in Sections 2.2.1 through 2.2.3 of this chapter, respectively.

The first step to consider in the hydraulic design of a culvert is the determination of the flow rate that the culvert must convey. There is no single method for determining peak discharge that is applicable to all watersheds. The method chosen should be a function of drainage area size, availability of data, and the degree of accuracy desired.

The following methods described in Chapter 3 – Determination of Stormwater Runoff, shall be used to generate peak discharge:

Rational Method – used for drainage areas less than 30 acres.

Soil Conservation Method – used for drainage areas between 30 and 2000 acres.

Although nomographs can still be used for design, the majority of engineers currently design culverts using computer applications. Among these applications are the FHWA’s HY8 Culvert Analysis (Ginsberg 1987) and numerous proprietary applications such as CulvertMaster. FHWA’s HY8 Culvert Analysis (Version 7.2) is located FHWA’s webpage (http://www.fhwa.dot.gov) for download.

In addition, the City of Rogers has developed spreadsheets to aid in the sizing and design of culverts. Use of the RDM-Culvert spreadsheet application is required when sizing and designing culverts in the City of Rogers.

The actual design of a culvert installation is more complex than the simple process of sizing culverts because of problems arising from topography and other considerations. Since the problems encountered are too varied and too numerous to be generalized, the information in the design procedure presented below is only a guide to design. Several combinations of entrance types, invert elevations, and pipe diameters should be evaluated to determine the most economic design that will meet the conditions imposed by topography and engineering. Descriptions of different variables that must be evaluated are presented in Sections 3.3.1 through 3.3.2 of this chapter.

The outlet velocity must be checked to determine if significant scour will occur downstream during the major storm. If scour is indicated (which is normally the case), refer to Section 6.0 of this chapter for guidance on outlet protection. The maximum allowable discharge velocities from culverts for particular downstream conditions are listed in Table CB-3:

To minimize sediment deposition in the culvert, the culvert slope must be equal to or greater than the slope required to maintain a minimum velocity of 3-ft/sec flowing full as recommended in FHWA HEC-22. The slope should be checked for each design, and if the proper minimum velocity is not obtained, the pipe diameter may be decreased, the slope steepened, a smoother pipe used, or a combination of these measures implemented.

(FHWA HDS-5, 2005)

Projecting inlets vary greatly in hydraulic efficiency and adaptability to requirements with the type of pipe material used. Figure CB-7 illustrates this type of inlet.

Headwalls may be used for a variety of reasons, including increasing the efficiency of the inlet, providing embankment stability, and providing embankment protection against erosion. The relative efficiency of the inlet varies with the pipe material used. The range of inlet coefficients for different headwall configurations is summarized in Table CB-4. Different configurations of pipe with headwalls are described in Sections 4.2.1 through 4.2.4 of this chapter. Figure CB-8 illustrates a headwall with wingwalls.

In addition to the common inlets described above, a large variety of other special inlet types exist. Among these are special end-sections, which serve as both outlets and inlets and are available for both corrugated metal pipe and concrete pipe. Because of the difference in requirements due to pipe materials, the special end-sections are addressed according to pipe material. Mitered inlets are discussed in Section 4.3.3 of this chapter.

Accumulation of debris at a culvert inlet can result in the culvert not performing as designed. This can result in damage caused by overtopping of the roadway and/or inundation of the upstream property. Three main options exist to address the debris problem:

1. Retain the debris upstream of the culvert.
2. Attempt to pass the debris through the culvert.
3. Install a bridge.

If the debris is to be retained by an upstream structure or at the culvert inlet, frequent maintenance may be required. The design of a debris control structure shall include a thorough study of the debris problem. Factors to be considered in a debris study include the following:

• Type of debris

• Quantity of debris

• Expected changes in type and quantity of debris due to future land use

• Stream flow velocity in the vicinity of culvert entrance

• Maintenance access requirements

• Availability of storage

• Maintenance plan for debris removal

• Assessment of damage due to debris clogging, if protection is not provided

FHWA’s Hydraulic Engineering Circular, No. 9 (HEC-9 2005 - http://www.fhwa.dot.gov/engineering/), Debris Control Structures, shall be referenced when designing debris control structures.

When a culvert is functioning with inlet control, an air pocket forms, just inside the inlet, that creates a buoyant effect when the inlet is submerged. The buoyancy forces increase with an increase in headwater depth under inlet control conditions. These forces, along with vortexes and eddy currents, can cause scour, undermine culvert inlets, and erode embankment slopes, thereby making the inlet vulnerable to failure, especially with deep headwater.

The large unequal pressures resulting from inlet constriction, which are accentuated when the capacity of the culvert is impaired by debris or damage, are in effect buoyant forces that can cause entrance failures, particularly on corrugated metal pipe with mitered, skewed, or projecting ends. The failure potential will increase with steepness of the culvert slope, depth of the potential headwater, flatness of the fill slope over the upstream end of the culvert, and the depth of the fill over the pipe.

Anchorage at the culvert entrance helps to protect against these failures by increasing the dead load on the end of the culvert, protecting against bending damage, and by protecting the fill slope from the scouring action of the flow. When inlet control conditions are present a standard concrete headwall or endwall will be provided unless otherwise approved by the City to counteract the hydrostatic uplift and to prevent failure due to buoyancy.

Because of a combination of high head on the outside of the inlet and the large region of low pressure on the inside of the inlet due to separation, a large bending moment is exerted on the end of the culvert, which may result in failure. This problem has been noted in the case of culverts under high fills, on steep slopes, and with projecting inlets. In cases where upstream detention storage requires headwater depth in excess of 20 feet, reducing the culvert size is required to limit the discharge rate rather than using an inefficient projecting inlet.

Protection against scour at culvert outlets varies from limited riprap placement to complex and expensive energy dissipation devices. At some locations, use of a rougher culvert material may alleviate the need for a special outlet protection device. Pre-formed scour holes (approximating the configuration of naturally formed holes) dissipate energy while providing a protective lining to the streambed. Methods for predicting scour hole dimensions are provided in FHWA’s Hydraulic Engineering Circular, No. 14 (HEC-14 , 2006), Hydraulic Design of Energy Dissipators for Culverts and Channels.

Riprap-armored channel expansions and concrete aprons protect the channel and redistribute or spread the flow. Barrel outlet expansions operate in a similar manner. Headwalls and cutoff walls protect the integrity of the fill. When outlet velocities are high enough to create excessive downstream problems, consideration should be given to more complex energy dissipation devices. Design information for the general types of energy dissipators can be found in HEC-14 (FHWA 2006).

Four examples of energy dissipators and erosion control are given below: Drop Structures, Turf Reinforcement Mats, Hydraulic Jump Energy Dissipators and Riprap (requires City approval).

Culvert location is an integral part of the total design. The main purpose of a culvert is to convey storm water drainage across the roadway section expeditiously and effectively. The designer should identify all live stream crossings, springs, low areas, gullies, and impoundment areas created by the new roadway embankment for possible culvert locations. Note that environmental permitting constraints will often apply for new culverts or retrofits, such as a Section 404 permit that regulates construction activities in jurisdictional wetlands and “Waters of the United States.”

Culverts shall be located on existing stream alignments and aligned to give the stream a direct entrance and a direct exit. Abrupt changes in direction at either end may retard the flow and make a larger structure necessary. If necessary, a direct inlet and outlet may be obtained by means of a channel change, skewing the culvert, or a combination of these. The choice of alignment should be based on environmental concerns, hydraulic performance, and/or maintenance considerations.

If possible, a culvert shall have the same alignment as its channel. Often this is not practical and where the water must be turned into the culvert, headwalls, wingwalls, and aprons with configurations similar to those in Figure CB-9 shall be used as protection against scour and to provide an efficient inlet.

Deposits usually occur within the culvert barrels at flow rates smaller than the design flow. The deposits may be removed during larger floods depending upon the relative transport capacity of flow in the stream and in the culvert, compaction and composition of the deposits, flow duration, ponding depth above the culvert, and other factors.

Culvert location in both plan and profile is of particular importance to the maintenance of sediment-free culvert barrels. Deposits occur in culverts because the sediment transport capacity of flow within the culvert is often less than in the stream.

Deposits in culverts may also occur because of the following conditions:

• At moderate flow rates the culvert cross section is larger than that of the stream, so the flow depth and sediment transport capacity is reduced within the culvert compared to the stream.
• Point bars form on the inside of stream bends. Culvert inlets placed at bends in the stream will be subject to deposition in the same manner. This effect is most pronounced in multiple-barrel culverts with the barrel on the inside of the curve often becoming almost totally plugged with sediment deposits.
• Abrupt changes to a flatter grade in the culvert or in the channel adjacent to the culvert will induce sedimentation. Gravel and cobble deposits are common downstream from the break in grade because of the reduced transport capacity in the flatter section.

Entrances to open channels often require the same careful planning and design as is needed for culverts and long conduits if the necessary hydraulic balance is to be achieved. The energy grade line shall be analyzed by the designer to provide proper balanced energy conversion, velocity control, energy loss, and other factors that control the downstream flow. Channel confluences, in particular, require careful hydraulic design to eliminate scour, reduce oscillating waves, and minimize upstream backwater effects.

Transitions from pipe flow to open channels, between different rigid channels, and from slow flow to supercritical flow must be designed using the concepts of conservation of energy and open channel hydraulics. Primarily, a transition is necessary to change the shape or cross section of flowing water.

Normally, the designer will have as an objective the avoidance of excessive energy losses, cross waves, and turbulence. It is also necessary to provide against scour and overtopping.

Supercritical flow transitions must receive more attention than is generally provided to subcritical flow transitions. Care must be taken to prevent unwanted hydraulic jumps or velocities that cause critical depth. Froude numbers between 0.8 and 1.2 must be avoided.

In general, the rate at which the flow prism may be changed shall not exceed perhaps 5 to 12½ degrees, depending upon velocity. Sharp angles shall be avoided. The water surface hydraulic grade line shall normally be smooth. More information on transitions is available in HEC-14 (FHWA 2006).

When installing or replacing a culvert, careful consideration should be taken to ensure that upstream and downstream property owners are not adversely affected by the new hydraulic conditions. The potential upstream flooding impacts associated with the backwater from the calculated headwater depth must be considered and the determination of the available headwater should take into account the area inundated at the projected water surface elevation. If a culvert is replaced by one with more capacity, the downstream effects of the additional flow must be factored into the analysis. Assuring consistency with existing major drainageway master plans and/or outfall studies is important.

Culverts are frequently located at the base of steep slopes. Large box culverts, in particular, can create conditions where there is a significant drop, which poses risk to the public. In such cases, handrail or fencing (or a guardrail configuration) is required for public safety. A handrail or fence shall be placed to provide a barrier between adjacent pedestrian areas and culvert openings when the culvert height/drop is ≥ 30-inches and ≤ 10-feet from the edge of the closest travel way.

Typical culvert inlets consist of concrete headwalls and wingwalls for larger structures and beveled-end sections for smaller pipes that may represent an obstacle to motorists who run off the road. This type of design may result in either a fixed object protruding above an otherwise traversable embankment or an opening into which a vehicle can drop causing an abrupt stop. The options available to a design engineer to minimize these obstacles are: use a traversable design, extend the structures so it is less likely to be hit, shield the structures (guardrail, concrete barrier wall, etc.), or delineate the structure if the other alternatives are not appropriate. Guidance for when to use which option is located in Section 3.4.2 Cross-Drainage Structures of the AASHTO Roadside Design Guide (2002).

Refer to Chapter 5 for pipe and

The minimum cover for reinforced concrete pipe shall be one-foot and shall meet minimum ASTM Class III specifications. The minimum cover for metal pipe is two-feet. Minimum cover less than these values shall be fully justified in writing and approved by the City Engineer prior to proceeding with final plans. Maximum fill heights and bedding descriptions for pipes are shown on Tables CB-6 and CB-7.

Box culverts shall be structurally designed to accommodate earth and live load to be imposed upon the culvert. Refer to the Arkansas State Highway and Transportation Department’s Reinforced Concrete Box Culvert Standard Drawings. When installed within public right of way, all culverts shall be capable of withstanding minimum HL-93 loading.

When culverts under railroad facilities are necessary, the designer shall obtain approval from the affected railroad.

Numerous local, State and Federal agencies have vested interests in surface waters. These agencies represent interests in water rights, flood control, drainage, conservation, navigation and maintenance of navigation channels, recreation, floodplain management and safety of floodplain occupancy, fish and wildlife, preservation of wetlands, and regulation of construction for the protection of environmental values. Other local, State and Federal agencies have vested interest in historic bridge structures and archeological resources. Early coordination with other agencies will reveal areas of mutual interest and offer opportunities to conserve public funds by resolving conflicts between roadway plans and water resources plans.

Bridge openings shall be designed to have as little effect on the flow characteristics as reasonable, consistent with good bridge design and economics. However, with respect to supercritical flow with a lined channel, the bridge shall not affect the flow at all—that is, there shall be no projections into the design water prism that could create a hydraulic jump or flow instability in the form of reflecting and standing waves.

The hydraulic analysis procedures described below are suitable, although the use of HEC-RAS is preferred.

The design of a bridge opening generally determines the overall length of the bridge. The length affects the final cost of the bridge. The hydraulic engineering in the design of bridges has more impact on the bridge cost than does the structural design.

The reader is referred to Hydraulics of Bridge Waterways (U.S. Bureau of Public Roads 1978) for more guidance on the preliminary hydraulic assessment approach described below. In working with bridge openings, the designer may use the designation shown in Figure CB-17.

The following is a brief step-by-step outline for determination of backwater produced by a bridge constriction:

1. Determine the magnitude and frequency of the discharge for which the bridge is to be designed.
2. Determine the stage of the stream at the bridge site for the design discharge.
3. Plot a representative cross section of the stream for design discharge at Section 1, if not already done under Step 2. If the stream channel is essentially straight and the cross section substantially uniform in the vicinity of the bridge, the natural cross section of the stream at the bridge site may be used for this purpose.
4. Subdivide the above cross section according to marked changes in depth of flow and roughness. Assign values of Manning's roughness coefficient, n, to each subsection. Careful judgment is necessary in selecting these values. Refer to Table OC-7 in Chapter 6 – Open Channel Flow Design.
5. Compute conveyance and then discharge in each subsection.
6. Determine the value of the kinetic energy coefficient.
7. Plot the natural cross section under the proposed bridge based on normal water surface for design discharge and compute the gross water area (including area occupied by piers).
8. Compute the bridge opening ratio, M, observing modified procedure for skewed crossings.
9. Obtain the value of K_b from the appropriate base curve.
10. If piers are involved, compute the value of J and obtain the incremental coefficient, ΔK_p.
11. If eccentricity is severe, compute the value of eccentricity and obtain the incremental coefficient, ΔK_e (FHWA, HDS-1 1978).
12. If a skewed crossing is involved, observe proper procedure in previous steps, and then obtain the incremental coefficient, ΔK_s, for proper abutment type.
13. Determine the total backwater coefficient, K^*, by adding incremental coefficients to the base curve coefficient, K_b.
14. Compute the backwater by Equation CB-14.
15. Determine the distance upstream to where the backwater effect is negligible.

Detailed steps illustrated by examples are presented in Hydraulics of Bridge Waterways (FHWA, HDS-1 1978).

The engineer will often encounter existing bridges and culverts that have been designed for storms having return periods less than 100 years. In addition, bridges will be encountered which have been improperly designed. Often the use of the orifice formula will provide a quick determination of the adequacy or inadequacy of a bridge opening:

(Equation CB-17)

or

(Equation CB-18)

in which:

Q_m = The major storm discharge (ft³/s)

C_b = The bridge opening coefficient (0.6 assumed in Equation CB-17)

A_b = The area of the bridge opening (ft²)

g = Acceleration of gravity (32.2 ft/s²)

H_br = The head, that is the vertical distance from the bridge opening center point to the upstream water surface about 10H upstream from the bridge, where H is the height of the bridge, in feet. It is approximately the difference between the upstream and downstream water surfaces where the lower end of the bridge is submerged.

These expressions are valid when the water surface is above the top of the bridge opening.

Given: Q_5-yr = 20 cfs, Q_100-yr = 35 cfs, L = 95 feet

The maximum allowable headwater elevation is 5288.5. The natural channel invert elevations are 5283.5 at the inlet and 5281.5 at the outlet. The tailwater depth is computed as 2.5 feet for the 5-year storm, and 3.0 feet for the 100-year storm.

Solution:

The purpose of this chapter of the Manual is to provide technical guidance for erosion, sediment, and runoff control for construction activity along with the implementation of Best Management Practices (BMPs) for the period of time from initial earth disturbance until the final landscaping and permanent stormwater measures are accepted by the City of Rogers and coverage under the Arkansas Department of Energy & Environment Division of Environmental Quality (DEQ) Construction General Permit has been terminated.

The City of Rogers requires that a Stormwater Pollution Prevention Plan (SWPPP) be developed for construction sites in accordance with the DEQ Construction General Permit prior to obtaining a Land Disturbance Permit. The City of Rogers has the right under the Federal Clean Water Act and the Arkansas Water and Air Pollution Control Act to require that BMPs for erosion, sediment, and runoff control be implemented at construction sites. The owner of a site of construction activity shall be responsible for compliance with the requirements of this article.

Two copies of the SWPPP shall be submitted for review and approval to the City of Rogers for sites with disturbed areas of five (5) acres or more.

For sites with disturbed areas greater than or equal to one (1) acre and less than five (5) acres, the City of Rogers will review the proposed BMPs based on the submitted erosion control plans. The SWPPP must be submitted to the City of Rogers.

Surface runoff controls for construction sites and activities in Arkansas are mandated by the Clean Water Act of the Federal Government and the Arkansas Water and Air Pollution Control Act. All sites where construction will disturb soil or remove vegetation on one (1.0) or more acres of land in total for all phases of work during the life of the construction project must be covered under the DEQ Construction General Permit. This Construction General Permit provides authorization to discharge stormwater associated with construction activity under the National Pollutant Discharge Elimination System (NPDES) to all Arkansas receiving waters in accordance with effluent limitations, monitoring requirements, and other conditions set forth in the Construction General Permit. Coverage under the Construction General Permit does not relieve the site owner or operator from addressing and obtaining, as needed, other local, State and Federal permits (e.g., permit for work in a floodplain, Corps of Engineers 404 permit, building permit, local Land Disturbance Permit, etc.).

A Stormwater Pollution Prevention Plan (SWPPP) must be developed for construction sites in accordance with the Construction General Permit. The SWPPP shall be prepared in accordance with good engineering practices and shall identify potential sources of pollution which may reasonably be expected to affect the quality of stormwater discharges from the construction site. In addition, the SWPPP shall describe and ensure the implementation of Best Management Practices (BMPs) which are to be used to reduce pollutants in stormwater discharges and to assure compliance with the terms and conditions of the Construction General Permit. The initially developed SWPPP has to be viewed as a starting point that will be modified as the work progresses and its effectiveness is tested in the field.

The DEQ identifies two construction project sizes for the City of Rogers including: large construction sites (disturbance of five (5) or more acres of total land area) and small construction sites (greater than or equal to one (1) acre and less than five (5) acres of total land area). The owner or operator of large construction sites must submit a Notice of Intent (NOI) and permit fee to DEQ to be covered under the Construction General Permit. In addition, for large construction sites a copy of the SWPPP must be submitted to DEQ. For small construction sites, an NOI and a DEQ permit fee is not required; however, a Land Disturbance Permit fee is required through the City. Rather than an NOI, the owner or operator must complete and sign a Construction Site Notice and post it at the construction site. For small construction sites, a SWPPP must be developed but does not need to be submitted to DEQ unless requested.

DEQ requires qualified personnel (provided by the site owner or operator) to conduct inspections of all areas disturbed by construction activity and all storage areas that are exposed to precipitation. The inspectors must look for evidence of, or the potential for, pollutants to enter the stormwater system. Locations where vehicles enter or exit the site, discharge locations, and locations where erosion and sediment control measures are installed shall also be inspected. In addition, the City of Rogers, DEQ or EPA may conduct inspections at any time.

Issuance of a Notice of Violation (NOV) by the City, State or EPA sets the stage for enforcement action and fines. This is a regulatory program with many potential consequences and has to be taken seriously by site owners or operators. Conducting construction activities without coverage under the Construction General Permit when one is needed has the potential of criminal action enforcement being taken against the violating party, which not only can carry much higher fines, but has a potential for jail sentences.

The following are objectives for erosion and sediment control during construction:

1. Conduct all land disturbing activities in a manner that effectively reduces accelerated soil erosion and reduces sediment movement and deposition off site.
2. Schedule construction activities to minimize the total amount of soil exposed at any given time to reduce the period of accelerated soil erosion.
3. Establish temporary or permanent cover on areas that have been disturbed as soon as possible after grading is completed.
4. Design and construct all temporary or permanent facilities to limit the flow of water to non-erosive velocities around, through or from disturbed areas.
5. Remove sediment from surface runoff water before it leaves the site.
6. Stabilize the areas of land disturbance with permanent vegetative cover and stormwater quality control measures.

The owner is responsible for providing the Stormwater Pollution Prevention Plan (SWPPP). It is recommended that the owner secure the services of a qualified professional knowledgeable in construction management practices to develop the SWPPP. The SWPPP must meet the requirements listed in the DEQ Construction General Permit No. ARR150000, "Authorization to Discharge under the National Pollutant Discharge Elimination System and the Arkansas Water and Air Pollution Control Act" available at www.adeq.state.ar.us.

Two copies of the SWPPP shall be submitted for review and approval to the City of Rogers for sites with disturbed areas of five (5) acres or more. The final SWPPP must be consistent with the Drainage Report accepted by the City of Rogers. However, approval of the SWPPP does not imply acceptance or approval of Drainage Plans, Street Plans, Design of Retaining Walls, or any other aspect of the site development.

The City of Rogers will review the SWPPP submitted for the site and will return either an approval of the SWPPP or a request for revisions. Construction activity, including any soil disturbance or removal of vegetation, shall not commence on the site until the City of Rogers and DEQ has issued an approval of the SWPPP.

For sites with disturbed areas greater than or equal to one (1) acre and less than five (5) acres, the City of Rogers will review the proposed BMPs based on the submitted erosion control plans. The SWPPP doesn’t need to be submitted to the City of Rogers unless requested.

Preparation and implementation of SWPPPs for construction activity shall comply with the following:

1. Preparation:
   1. The SWPPP shall be prepared under the direction of a qualified person.
   2. The SWPPP shall provide the name, address and phone number of the project owner for purposes of correspondence and enforcement.
   3. The SWPPP shall identify existing natural resources such as streams, forest cover, and other established vegetative cover.
   4. The SWPPP shall specify and provide detail for all BMPs necessary to meet the requirements of this Manual, including any applicable BMPs that have been adopted and imposed by the City.
   5. The SWPPP shall specify when each BMP will be installed, and for how long it will be maintained within the construction sequence. Multiple plans may be required for major phases of construction such as rough grading, building construction and final grading.
   6. The SWPPP shall delineate all anticipated disturbed areas and specify the vegetative cover that must be established in those areas to achieve final stabilization.
2. Implementation:
   1. BMPs shall be installed and maintained by qualified persons. The owner/developer or their representative shall be able to provide upon the stormwater coordinator's request a copy of the SWPPP on site and shall be prepared to respond to unforeseen maintenance of specific BMPs.
   2. The owner/developer or their representative shall inspect all BMPs at least once per month and within 24 hours after a rainfall of one quarter of an inch or more as measured at the site or generally reported in the City area.
   3. Based on inspections performed by the owner/developer or by authorized City personnel, modifications to the SWPPP will be necessary if at any time the specified BMPs do not meet the objectives of this Manual. In this case, the owner/developer or authorized representative shall meet with authorized City personnel to determine the appropriate modifications. All modifications shall be completed within seven days of the referenced inspection, except in circumstances necessitating more timely attention, and shall be recorded on the owner's copy of the SWPPP.

Any person proposing to engage in clearing, filling, cutting, quarrying, construction, or similar activities on any piece of disturbed land of one-half (1/2) acre or larger shall apply for a Land Disturbance Permit with the City of Rogers.

For sites with disturbed areas of five (5) acres or more, the SWPPP must be approved by the City of Rogers prior to issuance of a Land Disturbance Permit.

For sites with disturbed areas greater than or equal to one-half (1/2) acre and less than five (5) acres, the erosion control plans must be approved by the City of Rogers prior to issuance of a Land Disturbance Permit.

During the construction phase, the following sequence is recommended for the implementation of the project and the SWPPP:

1. The owner and/or the contractor shall designate a manager for the implementation of the SWPPP. This person shall be responsible for implementing all permit conditions and shall communicate with inspectors from the City of Rogers and other agencies.
2. Install all BMPs shown on the SWPPP that need to be installed in advance of proceeding with construction, such as construction entrances and exits, perimeter silt fences, etc.
3. Identify construction equipment and materials storage and maintenance areas. Install BMPs to prevent pollutant migration from these areas.
4. Install any additional BMPs that are called for in the SWPPP before overlot grading begins.
5. Strip off and stockpile topsoil for reuse.
6. Open areas not planned for immediate use shall be seeded or sodded. Soil which is exposed for more than fourteen (14) days with no construction activity shall be seeded, mulched, or re-vegetated.
7. Exposed soil within stormwater facilities, including but not limited to detention/retention ponds, swales and sedimentation basins, must be stabilized and sodded immediately, but no later than fourteen (14) days, following construction of the facility.
8. Insure that BMPs are installed and fully operational in advance of each construction phase as called for in the SWPPP.
9. After construction and revegetation is complete, permanent post-construction BMPs that were used as construction sediment controls shall be cleaned and restored.

Once revegetation has been deemed acceptable by the City of Rogers, the owner shall request release of any surety, letters of credit or other financial guarantees that the City of Rogers may have required the permit holder to provide at the time the permit was issued. A closure of the Construction General Permit from DEQ shall also be pursued at this time.

The City of Rogers shall require a bond at the time of approval by the Planning Commission for revegetation of the site if construction halts. The value of the bond shall be set based on an estimate provided by the engineer of record.

Upon completion of permitted construction activity on any site, the property owner and subsequent property owners will be responsible for continued compliance with the requirements of this article, in the course of maintenance, reconstruction or any other construction activity on the site.

Erosion controls limit the amount and rate of erosion occurring on disturbed areas. Sediment controls attempt to capture the soil that has been eroded before it leaves the construction site. Despite the use of both erosion control and sediment control measures (referred to as Best Management Practices (BMPs)), it is recognized that some amount of sediment will remain in runoff leaving a construction site.

The purpose of BMPs is to potentially minimize the sediment to the extent feasible. Construction activities management shall address six major elements:

1. The erosion control measures that will be used to limit erosion of soil from disturbed areas at a construction site;
2. The sediment and runoff control measures to limit transport of sediment off-site to downstream properties and receiving waters;
3. The waterway protection measures to protect waterways located on or downstream of the construction site from erosion and sediment damages;
4. The construction practices management to limit pollutant movement off site resulting from construction equipment maintenance and storage and from materials storage and handling.
5. The stabilization practices to return the site to either a vegetative state or employ non-erosive surfaces where disturbances have occurred. Stabilization may include both temporary and permanent stabilization methods.
6. The onsite infiltration measures used to infiltrate stormwater runoff onsite where appropriate.

Stormwater planning should occur early in the site development process. The planning process can be divided into five separate steps:

1. Gather information on topography, soils, drainage, vegetation and other predominant site features.
2. Analyze the information in order to anticipate erosion, sedimentation, and runoff problems.
3. Devise a plan which schedules construction activities and minimizes the amount of erosion created by development.
4. Develop a SWPPP which specifies effective erosion, sediment and runoff control measures as well as waste management and construction phasing.
5. Follow the SWPPP and revise it when necessary.
6. Remove temporary BMPs once the site has reached final stabilization and file a Notice of Termination (NOT) with DEQ.

Erosion control practices include the BMPs listed in Table CS-2.

Sediment Control and Runoff Control practices include the BMPs listed in Table CS-3.

Many of the temporary controls used for sediment control can be modified into permanent structural controls. In addition, permanent stormwater quality controls can often be constructed at the initial stages of the project and modified to control sediment during construction phases. When that occurs, they will need to be modified and restored to the post-construction BMP configuration at the end of construction. Restoration of the post-construction BMPs may involve removing sediment that may have accumulated during construction.

When working immediately adjacent to a waterway, the use of erosion and sediment control practices described earlier in this Manual is crucial. Activities such as minimizing disturbed areas adjacent to the waterways, timing construction during low flows, using surface roughening techniques, mulching disturbed areas as quickly as possible, using silt fence, and using temporary slope diversions to direct runoff to sediment basins before runoff enters the waterway. The inspection and maintenance of the erosion and sedimentation controls needs to be more aggressive.

When working within a waterway, steps must be taken to stabilize the work area during construction to control erosion. The channel banks and channel bed must be restabilized by the use of seeding, mulching, and/or erosion control matting, as quickly as possible. If it is not practical to do final seeding due to site conditions (e.g., frozen ground, prolonged wet weather, etc.), mulch shall be applied to the surface, and then seed and final mulch when conditions permit.

A permit is required for placement of fill in a waterway under Section 404 of the Clean Water Act. The U.S. Army Corps of Engineers has issued nationwide permit Number 14 for Linear Transportation Projects (roads, highways, railways, trails, airport runways, etc.) along with the placement of temporary fill associated with the construction. Appropriate measures must be taken to maintain normal downstream flows and floodplain capacity. The U.S. Army Corps of Engineers has issued nationwide permit Number 12 for Utility Line Activities for construction of utility lines within Waters of the United States provided there is no change in pre-construction contours. The local office of the Corps of Engineers should be contacted concerning the requirements for obtaining a 404 permit.

In addition, a permit from the U.S. Fish and Wildlife Service may be needed if endangered species are of concern in the work area. For a list of endangered or threatened species, contact the Arkansas Natural Heritage Commission at (501) 324-9619 or www.naturalheritage.com or the U.S. Fish and Wildlife Services (USFWS) at (501) 324-5643 or www.fws.gov. Typically the USFWS issues are addressed by a 404 permit if one is required. The City of Rogers should also be consulted and can provide assistance.

Applicants of a Corps of Engineers 404 permit shall also contact DEQ for a Short Term Activity Authorization (STAA) needs determination for activities that have the potential to violate water quality criteria.

Besides permitting with the U.S. Army Corps of Engineers and USFWS, it may be necessary to submit the proper map revision application [(C)LOMA, (C)LOMR-F, (C)LOMR)] to FEMA depending on the type and level of work taking place within a waterway. Should any of the work occurring in and around a waterway create a situation that permanently alters the future hydraulic characteristics of the waterway (e.g. by placement of fill in a waterway or realignment of the waterway) it will be necessary to coordinate such work with FEMA and the City’s Floodplain Administrator to ensure all necessary maps and hydraulic information are revised/updated for the impacted area of the waterway.

Where an actively-flowing watercourse must be crossed regularly by construction vehicles, a temporary stream crossing shall be provided. Three primary methods are available: (1) a culvert crossing, (2) a stream ford, and (3) a bridge crossing. Refer to Figures CS-4 through CS-5 for examples of temporary stream crossings. Also refer to the Temporary Stream Crossing BMP fact sheet EC-12.

Construction vehicles shall be kept out of a waterway to the maximum extent practicable.

When working within a waterway, temporary facilities shall be installed to divert clean flowing water around the construction activities taking place within a waterway.

Whenever possible, construction in a waterway shall be sequenced to begin at the most downstream point and work progressively upstream installing required channel and grade control facilities.

Complete work in small segments, exposing as little of the channel at a time as possible.

Where feasible, it is best to perform all in-channel work during historically low stream flow periods. This is the period when the chances of flash floods and flows higher than the 2-year flood peak flows are least likely.

Some construction activities within a waterway are short lived, namely a few hours or days in duration, and are minor in nature. These are typically associated with maintenance of utilities and stream crossings and minor repairs to outfalls and eroded banks. In these cases, construction of temporary diversion facilities can often cause more soil disturbance and sediment movement than the maintenance activity itself. However, this determination will have to be made in conjunction with the Corps of Engineers, DEQ, the City and any other appropriate jurisdictions.

Limiting construction activities within a waterway will significantly reduce erosion and sediment movement downstream. Construction berms can be used on portions of large channels to carry water around construction activities. The berms shall be tall enough to contain at least the 2-year flood peak without being overtopped.

Temporary diversion channels that divert the entire waterway are appropriate for work in smaller waterways and for the construction of detention basins and dams located on waterways. Refer to Figure CS-6 for an example of a temporary channel diversion.

Whenever the temporary diversion is around the construction site of a detention basin or a dam, the detention basin behind the dam should be considered for use as a temporary sediment basin. During construction such basins will need to be maintained as any other sediment basin. Once the construction site is stabilized, and before the temporary diversion is removed, all the accumulated sediment will need to be removed. The basin and its outlet facility will need to be configured to meet the requirements of the final design plans and specifications.

Description

Premature and/or excessive grading can increase the erosion potential and is therefore prohibited.

Construction Sequencing coordinates land disturbing activities with construction requirements to minimize the amount of soil exposed to erosion at any time.

Applicability

Projects on larger sites and on projects that land disturbing activities can be phased are best suited for Construction Sequencing.

Design Criteria

The potential for erosion is reduced when construction is performed in stages and the entire construction site is not disturbed all at the same time.

Areas of the site to be preserved should be clearly marked on the plans and delineated on the site. The timing of clearing and access to different areas of the site should be indicated in the contract documents.

Only land needed for building activities and vehicular traffic should be cleared.

Another way to phase construction is to minimize the disturbed areas during times of the year that traditionally receive large precipitation events.

Limitations

Sometimes, smaller projects do not lend themselves to sequencing of land disturbing activities.

Maintenance Requirements

Maintenance of protective BMP as needed.

Description

Often materials are used at a construction site that present a potential for contamination of stormwater runoff. Hazardous Waste Management is the proper staging, storage, handling, and disposal of construction material listed as hazardous by EPA and/or DEQ to prevent pollutants from being released from the site to receiving waters.

Applications

All construction materials that are listed as hazardous by EPA and/or DEQ.

Criteria

Guidelines published by EPA and OSHA for the types of materials to be used on the construction site should be incorporated into the SWPPP.

The types of materials that are generally considered hazardous are:

• Fuels (diesel, gas, etc.)
• Oils and greases (lubricating, cutting, etc.)
• Petroleum based materials (asphalt, emulsions, solvents)
• Paints (including wood preservatives, stains, and lead based)
• Solvents (paint thinners, cleaners, etc.)
• Pesticides, herbicides, insecticides

Proper management of hazardous materials entails:

• Replace hazardous materials with non-hazardous materials
• Minimize the use of hazardous materials
• Reuse and recycle hazardous materials
• Proper use of hazardous materials
• Proper storage and handling of hazardous materials
• Proper disposal of hazardous materials

Employees must be trained in the use, storage, and disposal of hazardous wastes. Hazardous materials should be stored so only authorized personnel can use the material.

Areas at the construction site that are used for storage of toxic materials and petroleum products should be designed with an enclosure, under a roof if possible, with a container, or with a dike located around the perimeter of the storage area to prevent discharge of these materials in runoff from the construction site. These barriers will also function to contain spilled materials.

Measures to prevent spills or leaks of fuel, gear oil, lubricants, antifreeze, and other fluids from construction vehicles and heavy equipment should be considered to protect groundwater and runoff quality. All equipment maintenance should be performed in a designated area and measures, such as drip pans, used to contain petroleum products. Spills of construction-related materials, such as paints, solvents, or other fluids and chemicals, shall be cleaned up immediately and disposed of properly.

The following methods shall be followed for spill prevention and clean-up:

• The manufacturers recommended methods for spill clean-up shall be clearly posted and personnel shall be trained in the location of clean-up supplies and clean-up procedures.
• Clean-up supplies shall be kept in a secure area.
• Personnel shall wear proper protective clothing when cleaning up the spill.
• Spills shall be cleaned up immediately and the waste properly disposed of.
• Licensed hazardous waste haulers must be used to transport hazardous wastes to approved treatment and disposal sites.
• Additional measures for spill prevention, response, and material storage practices may be required.

Description

Solid wastes that are improperly disposed of can be blown or washed from construction sites causing others to pick up the wastes from their property. Solid Waste Management refers to the proper handling and disposal of all construction wastes.

Applications

All construction sites.

Criteria

Areas shall be designated for the storage and disposal of construction material waste (both solid and liquid) to prevent discharge or movement of these materials off of the construction site.

These sites shall be located away from all storm drainage facilities and waterways. Consider covering the waste storage areas and fencing them, if necessary, to contain windblown materials. Consider constructing a perimeter dike to exclude or to contain runoff. These measures may not be necessary if all waste is placed immediately in covered waste containers at the site and is otherwise controlled in an effective manner. Trash receptacles shall be placed in convenient locations throughout the job site.

All waste shall be disposed only at approved landfill sites.

Maintenance Requirements

Trash and waste construction materials shall be picked up and disposed of daily.

Description

Concrete waste from washout of ready mix trucks, concrete pumps, and other concrete equipment increases sediment and changes the pH of stormwater runoff.

Concrete Waste Management is the practice of providing a basin for disposing of concrete residue and to wash out concrete truck mixers.

Applications

All construction sites with concrete work.

Design Criteria

The concrete washout area shall have sufficient storage volume to accept the wash water and allow the suspended particles to settle out.

The concrete washout area shall provide a minimum of six (6) cubic feet of containment volume for every ten (10) cubic yards of concrete to be poured.

Limitations

Improperly sized washout area can overflow and washout will not be contained.

Maintenance Requirements

The washout pit shall be cleaned weekly, when two-thirds (2/3) full, or as necessary to maintain capacity for wasted concrete. The waste material shall be disposed of properly.

Description

Ideally, vehicle maintenance occurs in garages and wash facilities, not on active construction sites. However, if these activities must occur onsite, operators shall follow appropriate BMPs to prevent untreated nutrient-enriched wastewater or hazardous wastes from being discharged to surface or ground waters.

Applications

Vehicle maintenance and BMPs prevent construction site spills of wash water, fuel, or coolant from contaminating surface or ground water. They apply to all construction sites.

A covered, paved or gravel-lined area shall be dedicated to vehicle maintenance. A spill prevention and cleanup plan should be developed. Prevent hazardous chemical leaks by properly maintaining vehicles and equipment. Properly cover and provide secondary containment for fuel drums and toxic materials. Properly handle and dispose of vehicle wastes.

Implementation

Construction vehicles shall be inspected daily, and any leaks repaired immediately. All used oil, antifreeze, solvents and other automotive-related chemicals shall be disposed of according to manufacturer instructions. These wastes require special handling and disposal. Used oil, antifreeze, and some solvents can be recycled at designated facilities, but other chemicals must be disposed of at a hazardous waste disposal site. Local government agencies can help identify such facilities.

Limitations

There are construction costs for the enclosed maintenance area, along with labor costs for hazardous waste storage, handling, and disposal.

Maintenance

Vehicle maintenance operations produce substantial amounts of hazardous and other wastes that require regular disposal. Clean up spills and dispose of cleanup materials immediately. Inspect equipment and storage containers regularly to identify leaks or signs of deterioration.

(Source: EPA)

Description

Construction dewatering practices involve the removal of sediment from trench or groundwater prior to it being discharged from the construction site. It is also appropriate for the removal of stormwater from depressed areas at a construction site.

Implementation

If trench or ground waters contain sediment, it must pass through a sediment settling pond or other equally effective sediment control device, prior to being discharged from the construction site. Sediment may be removed by settling in place or by dewatering into a sump pit, filter bag, or comparable practice.

Groundwater dewatering which does not contain sediment or other pollutants is not required to be treated prior to discharge. However, care must be taken when discharging groundwater to ensure that it does not become pollutant-laden by traversing over disturbed soils or other pollutant sources.

Dewatering discharges must not cause erosion at the discharge point.

Limitations

Dewatering operations will require, and must comply with, applicable local permits. Dewatering that discharges water in a manner that may enter any waters of the State require a Construction General Permit from the Arkansas Department of Energy & Environment Division of Environmental Quality (DEQ). This permit will need to be obtained by the owner and all conditions stipulated in that permit strictly adhered to. It is the responsibility of the owner and their SWPPP manager to insure that this occurs.

Description

Erosion is caused by rainfall impact detaching soil particles and runoff carrying the particles downslope. Chemical stabilization is the practice of spraying chemicals (tackifiers, soil binders) on the soil to hold the soil particles in place and protect against erosion.

Applicability

Areas that have been cleared of vegetation or do not have a protective cover on the soil. If temporary seeding cannot be used or would not be effective due to the time of year, steepness of slope, or other reasons; chemical stabilizers can be applied to protect against erosion. Chemical stabilization can be used in conjunction with seeding and mulching.

Design Criteria

The type of chemical used (asphalt emulsion, polyacrylamides (PAM), vinyl, or rubber), the application rate, and application method should meet the manufacturer’s recommendations.

Limitations

Improper application methods or rates can result in over application which can diminish infiltration and cause additional runoff.

Maintenance Requirements

Chemically stabilized areas shall be inspected regularly and after one-half (1/2) inch or greater rainfalls and stabilizer reapplied as required.

Description

A compost blanket is a layer of loosely applied compost or composted material that is placed on the soil in disturbed areas to control erosion and retain sediment resulting from sheet-flow runoff. It can be used in place of traditional sediment and erosion control tools such as mulch, netting, or chemical stabilization. When properly applied, the erosion control compost forms a blanket that completely covers the ground surface. This blanket prevents stormwater erosion by (1) presenting a more permeable surface to the oncoming sheet flow, thus facilitating infiltration; (2) filling in small rills and voids to limit channelized flow; and (3) promoting establishment of vegetation on the surface.

Compost blankets can be placed on any soil surface: rocky, frozen, flat, or steep. The method of application and the depth of the compost applied will vary depending upon slope and site conditions. The compost blanket can be vegetated by incorporating seeds into the compost before it is placed on the disturbed area (recommended method) or the seed can be broadcast onto the surface after installation.

Applications

Compost blankets are most effective when applied on slopes between 4:1 (horizontal:vertical) and 2:1 (horizontal:vertical), such as stream banks, road embankments, and construction sites, where stormwater runoff occurs as sheet flow.

Compost blankets can be used on steeper slopes, such as 1:1 (horizontal:vertical), if netting or confinement systems are used in conjunction with the compost blanket to further stabilize the compost and the slope or if the compost particle size and compost depth are specially designed for the application.

Limitations

Compost blankets are not applicable for locations with concentrated flow.

Compost blankets are not generally used on slopes greater than 2:1

(Source: US Environmental Protection Agency)

Description

Geotextiles are porous fabrics also known as filter fabrics, road rugs, synthetic fabrics, construction fabrics, or simply fabrics. Geotextiles are manufactured by weaving or bonding fibers that are often made of synthetic materials such as polypropylene, polyester, polyethylene, nylon, polyvinyl chloride, glass, and various mixtures of these materials. As a synthetic construction material, geotextiles are used for a variety of purposes such as separators, reinforcement, filtration and drainage, and erosion control (USEPA, 1992).

Some geotextiles are made of biodegradable materials such as mulch matting and netting. Mulch mattings are jute or other wood fibers that have been formed into sheets and are more stable than normal mulch. Netting is typically made from jute, wood fiber, plastic, paper, or cotton and can be used to hold the mulching and matting to the ground. Netting can also be used alone to stabilize soils while the plants are growing; however, it does not retain moisture or temperature well. Mulch binders (either asphalt or synthetic) are sometimes used instead of netting to hold loose mulches together. Geotextiles can aid in plant growth by holding seeds, fertilizers, and topsoil in place. Fabrics come in a wide variety to match the specific needs of the site and are relatively inexpensive for certain applications.

Applications

Geotextiles can be used in various ways for erosion control on construction sites. Use them as matting to stabilize the flow of channels or swales or to protect seedlings on recently planted slopes until they become established. Use matting on tidal or stream banks, where moving water is likely to wash out new plantings. Geotextiles can be used to protect exposed soils immediately and temporarily, such as when active piles of soil are left overnight. They can also be used as a separator between riprap and soil, which prevents the soil from being eroded from beneath the riprap and maintains the riprap's base. Geotextiles can also be used on stockpiles.

Design Considerations

There are many types of geotextiles available; therefore, the selected fabric shall match its purpose. To ensure the effective use of geotextiles, keep firm, continuous contact between the materials and the soil. If there is no contact, the material will not hold the soil, and erosion will occur underneath the material.

Limitations

Geotextiles (primarily synthetic types) have the potential disadvantage of disintegrating when exposed to light. Consider this before installing them. Some geotextiles might increase runoff or blow away if not firmly anchored. Depending on the type of material used, geotextiles might need to be disposed of in a landfill, making them less desirable than vegetative stabilization. If the geotextile fabric is not properly selected, designed, or installed, its effectiveness may be reduced drastically.

Maintenance

Inspect geotextiles regularly to determine if cracks, tears, or breaches have formed in the fabric; if so, repair or replace the fabric immediately. It is necessary to maintain contact between the ground and the geotextile at all times. Remove trapped sediment after each storm event.

(Source: EPA)

Description

Terraces are earthen embankments or ridge and channel systems that reduce erosion by slowing, collecting and redistributing surface runoff to stable outlets that increase the distance of overland runoff flow. Terraces hold moisture and help trap sediments, minimizing sediment-laden runoff.

Sediment can be controlled on slopes that are particularly steep by the use of terracing. During grading, relatively flat sections or terraces, are created and separated at intervals by steep slope segments. The steep slope segments are prone to erosion, however, and must be stabilized by mulching or other techniques. Retaining walls, gabions, cribbing, deadman anchors, rock-filled slope mattresses and other types of soil retention systems are available for use. These should be specified in the plan and installed according to manufacturer's instructions.

Applications

Terraces perform most effectively in barren areas with an existing or expected water erosion problem. Gradient terraces are effective only if suitable runoff outlets are available. Do not build terraces on slopes comprised of rocky or sandy soil because these soil types may not adequately redirect flows.

Implementation

Terraces should be properly spaced and constructed with an adequate grade, and they should have adequate and appropriate outlets toward areas not susceptible to erosion or other damage. Whenever possible, use vegetative cover in the outlet.

Terraced (stair-stepping) slopes shall have the vertical cuts no more than two (2) feet deep and the horizontal steps shall be wider than the depth of the vertical cut. The horizontal step shall slope backward to the vertical cut upslope on the hill.

Limitations

Terraces are inappropriate for use on sandy or shallow soils, or on steep slopes. If too much water permeates a terrace system's soils, sloughing could occur; potentially increasing cut and fill costs.

Maintenance

Terraces shall be inspected after major storms and at least once annually to ensure that they are structurally sound and have not eroded.

(Source: US Environmental Protection Agency)

Description

Erosion is caused by rainfall impact detaching soil particles and runoff carrying the particles downslope. Mulch can be applied to the area to hold the soil particles in place and protect against erosion.

Mulching is the practice of applying a layer of organic material (hay, straw, wood fiber, paper fiber, etc.) to protect the soil from impact of precipitation.

Applicability

Areas that have been cleared of vegetation or do not have a protective cover on the soil. Mulches are typically used to protect areas that have been seeded. Mulching can be used in conjunction with chemical stabilization.

Design Criteria

Mulch should be applied consisting of clean, weed-free and seed-free, long-stemmed grass hay (preferred) or cereal grain straw. Hay is preferred as it is less susceptible to removal by wind. At least 50 percent of the grass hay mulch, by weight, shall be ten (10) inches or more in length.

Straw mulch shall be evenly applied at a rate of two (2) tons of dry straw per acre. The mulch shall be attached to the soil immediately after application as an anchor and not merely placed on the surface. This can be accomplished mechanically by crimping or with the aid of tackifiers or nets. Anchoring with a crimping implement is preferred, and is the recommended method for all areas equal to or flatter than 3:1. Mechanical crimpers must be capable of tucking the long mulch fibers into the soil four (4) inches deep without cutting them.

Mulch is typically applied using a mulch blower, but it can be applied by hand in small or hard to reach areas.

Soil which is exposed for more than fourteen (14) days with no construction activity shall be seeded, mulched, or revegetated.

On small areas sheltered from the wind and from heavy runoff, spraying a tackifier on the mulch is satisfactory for holding it in place. Hydraulic mulching consisting of wood cellulose fibers must be mixed with water and a tackifying agent and applied at a rate of no less than 2,000 pounds per acre with a hydraulic mulcher.

Mats, blankets, and nets are required to help stabilize steep slopes (3:1 and steeper) and waterways. Depending on the product, these may be used alone or in conjunction with grass or straw mulch. Normally, use of these products will be restricted to relatively small areas. Mats made of straw and jute, straw-coconut, coconut fiber, or excelsior can be used instead of mulch. Whichever material is used, blankets need to be bio-degradable.

Some synthetic tackifiers or binders may be used to anchor mulch in order to limit erosion and, if approved by review agency, provide soil stabilization. Caution should be used to prevent the introduction of any potentially harmful and non-biodegradable materials into the environment. Manufacturer's recommendations should be followed at all times.

Rock (gravel, slag, crushed stone, river rock) can also be used as mulch. It provides protection of exposed soils to wind and water erosion and allows infiltration of precipitation. Rock of aggregate base-coarse size can be spread on disturbed areas for temporary or permanent stabilization. Rock must be removed from those areas to be planned for vegetation establishment.

Limitations

Wind and concentrated water flows can blow or wash mulch from the application area. Mulch should not be applied in areas with concentrated flows.

For steep slopes and special situations where greater control is needed, blankets anchored with stakes should be required instead of mulch.

Road cuts, road fills, and parking lot areas shall be covered as early as possible with the appropriate aggregate base course where this is specified as part of the pavement in lieu of mulching.

Maintenance Requirements

Mulched areas shall be inspected regularly and after one-half (1/2) inch or greater rainfalls and mulch reapplied as required.

Description

Water exiting a channel, swale, pipe, or culvert (any water carrying conduit) typically is in a concentrated stream with a relatively high velocity. This high energy stream of water erodes unprotected soil.

Energy Dissipation is a structural BMP placed at the exit of a water carrying conduit to slow the velocity and decrease the turbulence of the water. Permanent energy dissipation controls can be used as temporary devices during the construction phase of the project, and shall be designed according to methods described in Chapter 6 – Open Channel Flow Design. A riprap apron is considered the most cost effective type of temporary energy dissipation device; meaning that the energy dissipation device is only needed during construction and will be removed once construction is complete. However, should a permanent energy dissipation device be required at the outlet end of a conduit it may be more economical to install a permanent energy dissipation device early in construction as a structural BMP, making sure to maintain and service the device so it can be used permanently once construction is over. Other types of energy dissipation devices include: Plunge Pools, ScourStop© Mats, ShoreMax© Mats, Velocity Dissipaters, etc. The only type of temporary energy dissipation device that will be discussed in this Manual will be the riprap apron. The use of riprap as a permanent energy dissipation device is discouraged and its use requires city approval.

Applicability

All channels or pipes carrying runoff at velocities that will erode the soil in the discharge area.

Design Criteria

See Section 6.0 – Outlet Protection of Chapter 7 –Culvert and Bridge Hydraulic Design for additional design information on sizing riprap apron outlet protection.

Determine the required median size (d₅₀) of riprap using graph in the “Riprap Apron Sizing” Figure(s) below for the condition at hand. Enter the graph on the X-axis with the discharge in cubic feet per second, move vertically to intersect either the appropriate depth of flow (d) line or the velocity of flow (v) line, and then read horizontally to the Y-axis on the right side to determine the required median diameter of riprap (d₅₀).

Determine the minimum required apron length using the graph in the “Riprap Apron Sizing” Figure(s). Enter the graph on the X-axis with the discharge in cubic feet per second, move vertically to the second set of lines to intersect the appropriate depth of flow (d), and then read horizontally to the left to determine the minimum required length of apron (L_a) in feet.

Limitations

Riprap aprons are best suited for applications where the Froude Number at the conduit exit is less than 2.5.

Maintenance Requirements

The apron should be inspected after large storms to ensure that the riprap is in place. Riprap should be replaced when it is dislodged or missing.

Description

Erosion is caused by rainfall impact detaching soil particles and runoff carrying the particles downslope. Vegetation (seeded or sodded) can hold the soil particles in place and protect against erosion.

Applicability

Any area of a construction site that the natural vegetation has been removed. Seeding or sodding can be used as a temporary or a permanent erosion control measure.

Topsoil and Seedbed Preparation

Areas to be revegetated shall have soils capable of supporting vegetation. Overlot grading will oftentimes bring to the surface subsoils that have low nutrient value, little organic matter content, few soil microorganisms, rooting restrictions, and conditions less conducive to infiltration of precipitation. As a result, rototilling and adding topsoil, compost, and other soil amendments can be essential to achieve successful revegetation.

Topsoil should be salvaged during grading operations and used for spreading on areas to be revegetated later. Topsoil shall be viewed as an important resource to be utilized for vegetation establishment, primarily due to its water-holding capacity. Native topsoil located on a construction site also has good soil structure, organic matter content, biological activity, and nutrient supply that support vegetation.

At a minimum, the upper six (6) inches of topsoil can be stripped and stockpiled, and respread to a thicker depth on surfaces not planned for buildings or impervious areas. Stockpiled soils shall be seeded with a temporary or permanent grass cover. Mulching is recommended to ensure vegetation establishment. If stockpiles are located within one hundred (100) feet of a waterway, additional sediment controls, such as diversion dikes or embedded silt fences, should be provided.

If the soils have become compacted, they shall be loosened to a depth of at least six (6) inches.

Soil roughening will assist in placement of a stable topsoil layer on steeper slopes, and allow percolation and root penetration to greater depth. Soil roughening techniques shall be used for slopes greater than 3:1 (33%).

Where topsoil is not available or utilized, subsoils can be treated to provide a plant-growth medium. Organic matter, such as well digested compost, can be added to improve nutrient levels necessary for plant growth. Other treatments, such as liming, can be used to adjust soil pH conditions when needed. If the pH of the soil is less than 6, lime shall be added to the top six (6) inches of soil. Soil testing needs to be done to determine appropriate amendments required. Fertilizer (10-10-10) shall also be incorporated into the top six (6) inches of soil at a rate of 100 lb/acre.

A suitable seedbed will enhance the success of revegetation efforts. The surface should be rough and the seedbed should be firm, but neither too loose nor compacted. The seed bed should be loose, without large clods, and uniform before seeding. The upper layer of soil should be in a condition suitable for seeding at the proper depth and conducive to plant growth.

Temporary Revegetation

The appropriate temporary vegetation for a site is dependent upon the time of year. Prior to application of seed, grading of the site shall be complete including all erosion and sediment control practices.

Soil which is exposed for more than fourteen (14) days with no construction activity shall be seeded, mulched, or re-vegetated. All temporary seeding shall be protected with mulch.

Typical broadcast rates for temporary vegetation are listed in Table 8.1 below.

Description

Wind erosion or dust control consists of applying water or other dust palliatives as necessary to prevent or alleviate dust nuisance generated by construction activities. Covering small stockpiles or areas is an alternative to applying water or other dust palliatives.

Applications

Wind erosion control BMPs are suitable for construction vehicle traffic on unpaved roads, for drilling and blasting activities, for sediment tracking onto paved roads, for soil and debris storage piles, for batch drops from front-end loaders, for areas with unstabilized soil, and for final grading and site stabilization.

Limitations

Watering only prevents dust for a short period of time and should be applied daily (or more often) to be effective.

Over watering may cause erosion.

The effectiveness of wind erosion control depends on soil, temperature, humidity, and wind velocity.

Implementation

Dust control BMPs generally stabilize exposed surfaces and minimize activities that suspend or track dust particles. For heavily traveled and disturbed areas, wet suppression (watering), chemical dust suppression, gravel or asphalt surfacing, temporary gravel construction entrances, equipment washout areas, and haul truck covers can be employed as dust control applications. Permanent or temporary vegetation and mulching can be employed for areas of occasional or no construction traffic. Preventative measures would include minimizing surface areas to be disturbed, limiting onsite vehicle traffic to fifteen (15) mph, and controlling the number and activity of vehicles on a site at any given time.

Maintenance

Most dust control measures require frequent, often daily, or multiple times per day attention.

(Source: California Stormwater BMP Handbook, January 2003)

Description

Hydroseeding typically consists of applying a mixture of wood fiber, seed, fertilizer, and stabilizing emulsion with hydromulch equipment, to temporarily protect exposed soils from erosion by water and wind and provide an environment conducive to plant growth. Hydromulching is applying a slurry of water, wood fiber mulch, and often a tackifier, to prevent soil erosion. These terms are often used interchangeably. For our purposes we will refer to hydroseeding only in this section, but all information shared below can and should be applied to hydromulching if its application is warranted in a design.

Applications

Hydroseeding is suitable for soil disturbed areas requiring temporary protection until permanent stabilization is established. Hydroseeding is also suitable for disturbed areas that will be re-disturbed following an extended period of inactivity.

Implementation

In order to select the appropriate hydroseeding mixture, an evaluation of site conditions shall be performed with respect to soil conditions, site topography, season and climate, vegetation types, maintenance requirements, sensitive adjacent areas, water availability, and plans for permanent vegetation.

Prior to application, roughen the area to be seeded with the furrows trending along the contours.

Hydroseeding can be accomplished using a multiple step or one step process. The multiple step process ensures maximum direct contact of the seeds to soil. When the one step process is used to apply the mixture, the seed rate shall be increased to compensate for all seeds not having direct contact with the soil.

Follow up applications shall be made as needed to cover weak spots and to maintain adequate soil protection.

Avoid over spray onto roads, sidewalks, drainage channels, and existing vegetation.

Limitations

Hydroseeding may be used alone only when there is sufficient time in the season to ensure adequate vegetation establishment and coverage to provide adequate erosion control. Otherwise, hydroseeding must be used in conjunction with mulching.

Steep slopes are difficult to protect with temporary seeding.

Temporary vegetation may have to be removed before permanent vegetation is applied.

Temporary vegetation is not appropriate for short term inactivity.

Maintenance

Hydroseeding BMPs, along with irrigation systems, shall be inspected prior to forecast rain, daily during extended rain events, after rain events, weekly during the rainy season, and at two-week intervals during the non-rainy season.

Where seeds fail to germinate, or they germinate and die, the area must be re-seeded, fertilized, and mulched within the planting season, using not less than half the original application rates.

(Source: California Stormwater BMP Handbook, January 2003)

Description

Water flowing down a bare slope will erode soil and transport soil to the bottom of the slope. Surface roughening provides temporary stabilization of disturbed areas from wind and water erosion.

Soil roughening is the practice of increasing the roughness of exposed soil by making grooves, tracks, or terraces (stair-steps) which run perpendicular to the flow path (parallel to slope) slowing flow and trapping sediment.

Applications

Soil roughening can be used on a wide variety of slopes and in conjunction with seeding and mulching.

Soil roughening is particularly useful where temporary revegetation cannot be immediately established due to seasonal planting limitations.

Design Criteria

Surface roughening shall be performed after final grading. Fill slopes can be constructed with a roughened surface. Cut slopes that have been smooth graded can be roughened as a subsequent operation. Roughening ridges and depressions should follow along the contours of the slope.

Tracking with lugged tracked equipment is appropriate on sandy material so as to not excessively compact the soil.

Grooving can be accomplished using a plow with the furrows three (3) inches deep and less than fifteen (15) inches apart.

Terraced (stair-stepping) slopes shall have the vertical cuts no more than two (2) feet deep and the horizontal steps shall be wider than the depth of the vertical cut. The horizontal step shall slope backward to the vertical cut upslope on the hill.

The slope shall be seeded immediately after roughening and mulch or chemical stabilization should be utilized where appropriate.

Limitations

Soil roughening should not be used on rocky soils or soils that are high in clay content. Tracking may cause excessive compaction which can lead to greater erosion.

Care should be taken not to drive vehicles or equipment over areas that have been roughened. Tire tracks will smooth the roughened surface and encourage runoff to collect into rills and gullies. As surface roughening is only a temporary control, additional treatments may be necessary to maintain the soil surface in a roughened condition.

Maintenance Requirements

Roughened slopes shall be inspected after ½ inch and greater storms and problem areas noted. After a rain event, slopes may need reconstruction, re-roughening, re-seeding, and re-mulching.

Description

Gullying and excessive erosion will take place on slopes subjected to concentrated flows of runoff.

Slope Drains are conduits (open or closed) used to direct water down a slope while protecting the slope from erosion.

Applicability

Slopes with the potential for intended or unintended concentrated flows.

Design Criteria

Slope drains (rundowns, pipe slope drains, etc.) should be placed where runoff from uphill drainage areas will concentrate. Slope drains shall be sized to handle a 10-year storm from an area no greater than five (5) acres. Minimum size for a pipe slope drain is 18-inch diameter. Appropriate energy protection should be placed at the outlet of the pipe. Slope rundowns (stone or riprap lined channels) should be constructed with the middle sufficiently lower than the sides to ensure flow stays in the rundown. Slope drains operate best when used in conjunction with interceptor swales and dikes on the top of the slope. The discharge from all slope drains must be directed to a stabilized outlet, temporary or permanent channel, or sediment basin.

Limitations

For larger storms, the slope drain may not operate properly and can cause excessive gullying and slope erosion as well as damage to the construction site. Slope drains that are improperly designed or constructed such that the flow does not stay in the drain will cause excessive erosion.

Maintenance Requirements

Slope drains shall be inspected weekly and kept clear of trash, debris, and vegetation.

Description

A temporary stream crossing is a temporary culvert, ford, or bridge placed across a waterway to provide access for construction purposes. Temporary stream crossings are not intended to maintain traffic for the public. The temporary access will eliminate erosion and downstream sedimentation caused by vehicles.

Applications

Temporary stream crossings shall be installed at all designated crossings of perennial and intermittent streams on the construction site, as well as for dry channels that may be significantly eroded by construction traffic.

Temporary stream crossings shall be installed at sites when alternate access routes impose significant constraints, when crossing perennial streams or waterways causes significant erosion, and when appropriate permits have been obtained for the stream crossing (such as Corps of Engineers 404 permit).

Implementation

Temporary stream crossings are used to provide a safe, erosion-free access across a stream for construction equipment. Minimum standards and specifications for the design, construction, maintenance, and removal of the structure shall be established by a professional engineer registered in the State of Arkansas. Design and installation requires knowledge of stream flows and soil strength. Both hydraulic and construction loading requirements should be considered.

The following types of temporary stream crossings should be considered:

• Culverts – A temporary culvert is effective in controlling erosion, but will cause erosion during installation and removal. A temporary culvert can be easily constructed and allows for heavy equipment loads.
• Fords – Fords are appropriate during the dry season and on low-flow perennial streams. A temporary ford provides little sediment and erosion control and is ineffective in controlling erosion in the stream channel. A temporary ford is the least expensive stream crossing and allows for maximum load limits. It also offers very low maintenance.
• Bridges – Bridges are appropriate for streams and high flow velocities, steep gradients, and where temporary restrictions in the channel are not allowed.

The temporary stream crossing should be located where erosion potential is low. They should be constructed during dry periods to minimize stream disturbance and reduce costs.

Temporary stream crossings should be constructed at or near the natural elevation of the streambed to prevent potential flooding upstream of the crossing.

A culvert crossing should be designed to pass at least the 2-year design flow accounting for the headwater and tailwater controls to meet its design capacity.

When a ford needs to and can be used, namely a culvert is not practical or the best solution, it shall be lined with at least a twelve (12) inch thick layer of 6” riprap (D₅₀) or 9” riprap (D₅₀) with void spaces filled with 1-1/2 inch diameter rock.

Limitations

Installation and removal of the temporary stream crossings usually disturb the waterway, therefore additional BMPs will be required to minimize soil disturbance.

Appropriate permits will need to be obtained for the fill associated with temporary stream crossings (such as a Corps of Engineers 404 permit).

Installation may require dewatering or temporary diversion of the stream.

Fords shall only be used in dry weather.

Temporary stream crossings are not intended to maintain traffic for the public, only for construction purposes.

Maintenance

Inspect and verify that activity-based BMPs are in place prior to the commencement of associated activities. While activities associated with the BMP are under way, inspect weekly during the rainy season and at two week intervals in the non-rainy season to verify continued BMP implementation.

Check for blockage in the channel, sediment buildup or trapped debris in culverts, and for blockage behind fords or under bridges. Remove sediment that collects behind fords, in culverts, and under bridges periodically.

Check for erosion of abutments, channel scour, riprap displacement, or piping in the soil.

Check for structural weakening of the temporary crossings, such as cracks, and undermining of foundations and abutments.

Remove temporary crossings promptly when they are no longer needed.

Description

A level spreader receives concentrated flow from channels, outlet structures, or other conveyance structures and converts them to sheet flow. Although a level spreader by itself is not considered a pollutant reduction device, it improves the efficiency of other facilities, such as vegetated swales, filter strips, or infiltration devices, which are dependent on sheet flow to operate properly. The slight depression allows water to collect and then disperse uniformly over the surrounding vegetated area to reduce erosion and concentrated stormwater runoff.

Applications

Level spreaders are used in wide, level areas where concentrated runoff occurs. The level spreader converts the concentrated runoff to sheet flow and releases it onto an area stabilized by vegetation. Flows to the spreader should be relatively free of sediment or the spreader will be quickly overwhelmed by sediment and lose its effectiveness.

Implementations

The spreader should be constructed absolutely level. Height of the spreader is based on depth of design flow, allowing for sediment and debris deposition. The length of the spreader is based on the design flow for the site.

The slope leading to the level spreader shall be less than one (1%) percent for at least twenty (20) feet immediately upstream in order to keep velocities less than two (2) feet per second at the spreader during the 10-year storm event. Slope of the outlet from the spreader shall be six (6%) percent or less.

Limitations

If the spreader is not absolutely level, flows will concentrate at the low point and may cause more problems than if no level spreader were used.

The drainage area shall be limited to five (5) acres or forty (40) cubic feet per second (cfs).

Maintenance

Regular inspection and maintenance is essential to ensure sheet flow discharge and to avoid channeling across the crest of the depression. The level spreader shall be inspected regularly and after large rainfall events. Inspection shall note and repair any erosion and low spots in spreader. Sediment shall be removed from behind spreader.

Description

Mud and sediment carried off-site on the tires of equipment and vehicles will be deposited on the neighboring streets. This sediment will end up in the local streams if not swept up.

Construction Entrances are systems that clean vehicles of mud, sediment, and aggregate prior to leaving the site.

Applicability

Any entrance/exit of a construction site.

Design Criteria

A six (6) inch layer of B-stone (ranging in size from 1-1/2” minimum to 6” maximum, where the stone shall be uniformly graded and the amount passing the 1-1/2” sieve shall be not more than 10% by weight) can be used to stabilize construction site entrances. The stabilized construction entrance shall be a minimum length of twenty percent (20%) of the lot depth or fifty (50) feet, whichever is greater, up to a maximum of one hundred (100) feet and of adequate thickness to minimize tracking onto the city street. The stabilized construction entrance shall be at least 50 feet long. The entrance shall be as long as the longest vehicle that will enter the site. If larger volumes of traffic are expected, a two-lane entrance is appropriate.

Construction access shall be limited to locations as approved by the City of Rogers.

A stabilized construction entrance and a dunk or mechanical wheel wash are required on all sites.

Other methods of removing mud from vehicles may be acceptable such as rumble strips (cattle guard, logs, etc.).

A dunk wheel wash is a water filled, stabilized (1 inch or greater gravel or stone) pit. The water depth shall be at least two feet deep and the pit shall be at least 20 foot long. The pit shall be two vehicle lengths from the construction site exit and the entrance and exit to the pit shall be stabilized. These shall be provided on all sites. If there is not enough room to install a dunk wheel wash, a hand-operated pressure wash may be used instead with the approval of the city.

Limitations

In order to avoid puncturing tires, stabilized entrances shall not be constructed with sharp edge stones.

Maintenance Requirements

Stabilized entrances require periodic cleaning or addition of stone as the voids in the stones fill with mud and sediment.

Wheel wash facilities and rumble strips will need to be cleaned as the pits fill in order to provide more room to store new mud and sediment.

The street in front of the entrance shall be cleaned as required to remove sediment that has been tracked off site.

Whenever sediment is transported onto a public road, regardless of the size of the site, the road shall be cleaned immediately. Sediment shall be removed from roads by shoveling and sweeping and be transported to a controlled sediment disposal area. Washing of the street with a water hose or flushing the water downstream shall not be allowed.

Description

Water flowing in sheet or shallow flow will carry sediment down a slope and off-site.

An embedded silt fence is a barrier made of geotextile fabric placed along a contour to capture water, slow the flowrate, trap sediment, and allow water to filter through the fabric.

Applications

Small drainage areas with sheet flow or shallow flow.

Design Criteria

The embedded silt fence shall be placed on a contour and designed to hold runoff from the 10 year storm from an area of 100 sq. ft for each foot of fence. The maximum depth of retained water on the upstream side of the fence shall be two (2) feet. The maximum slope length above the fence shall be no more than one hundred (100) feet. The maximum slope above the fence is 3:1.

The fabric shall be buried in a trench that is at least eight inch deep and eight inches wide. The fabric shall be place on the upstream side of the posts.

Post shall be made of metal (T-post) or wood (2”x2”) and placed no more than six feet apart.

All embedded silt fence shall be wire-backed except when used as inlet protection.

Limitations

Silt fence must be embedded or it will not function properly and should not be installed in rocky soil where it cannot be properly embedded.

Silt fence is not designed to hold back concentrated flow and therefore shall not be placed across channels, gullies, or streams.

Silt fence shall not be run down slopes as it will concentrate flow causing gully erosion and causing downstream BMPs to fail.

Maintenance Requirements

The embedded silt fence shall be inspected weekly and after one-half (1/2) inch or greater rainfalls for proper installation, defective fencing, erosion on the ends, and excessive sediment buildup behind the fence (half the fence height). Any sediment accumulated behind them must be removed and disposed of properly. Any defective measures shall be repaired or replaced within 24 hours.

Description

Runoff from a construction site often carries sediment into the stormwater sewer system, which discharges into local streams. Besides the problems caused by sediment, other pollutants (e.g. oil, grease, and nutrients) are often attached to the sediment.

Inlet Protection is the practice of placing gravel, sand bags, silt fence or other proprietary systems around or in an inlet to allow runoff to pond and sediment to settle out prior to entering the stormwater sewer system.

Applications

Any storm drain inlet that could receive runoff from the construction site.

Description

Chemical treatment includes the application of chemicals to stormwater to aid in the reduction of turbidity caused by fine suspended solids.

Applications

Chemical treatment can reliably provide exceptional reductions of turbidity and associated pollutants and should be considered where turbid discharges to sensitive waters cannot be avoided using other BMPs. Typically, chemical use is limited to waters with numeric turbidity standards.

Implementation

Turbidity is difficult to control once fine particles are suspended in stormwater runoff from a construction site. Sedimentation ponds are effective at removing larger particulate matter by gravity settling, but are ineffective at removing smaller particulates such as clay and fine silt. Chemical treatment may be used to reduce the turbidity of stormwater runoff. Very high turbidities can be reduced to levels comparable to what is found in streams during dry weather.

Chemically treated stormwater discharged from construction sites must be non-toxic to aquatic organisms.

Maintenance

Chemical treatment systems must be operated and maintained by individuals with expertise in their use. Chemical treatment systems should be monitored continuously while in use.

(Source: California Stormwater BMP Handbook, January 2003)

Description

Water carrying sediment off-site can cause damage to neighboring property and local streams. Sediment Traps provide an area for sediment to settle out of the runoff prior to discharge from the site.

Applications

Sediment traps are well suited for sites that will be required to have a permanent stormwater control basin; but, should be used for any concentrated flow (culvert, pipe, swale, etc.) that could have sediment in the runoff leaving the site.

Design Criteria

The removal efficiency of sediment traps is a function of the total surface area of the pond, the shape of the pond, the influent flow rate, and the type of soil in the runoff. The maximum drainage area for a sediment trap shall be three (3 acres), for larger areas a sediment basin shall be used.

The minimum bottom area and spillway width for sediment traps are given in the Table 8.4 below. The berm or levee shall curve upstream to hold the water; the berm shall have 3:1 side slopes (maximum) and have a maximum depth of three (3) feet. The outlet spillway shall be made of six (6) inches of stone (6 inch diameter minimum) and be placed on a geotextile fabric, or approved equal, in lieu of rip rap.

Description

A sediment basin is a temporary basin formed by excavation or by constructing an embankment so that sediment-laden runoff is temporarily detained under quiescent conditions, allowing sediment to settle out before runoff is discharged.

Applications

Sediment basins should be considered for use when the drainage area is three (3) acres or more, for smaller areas a sediment trap shall be used.

Sediment basins should be considered where post construction detention basins are required.

Implementation

A sediment basin is a controlled stormwater release structure formed by excavation or by construction of an embankment of compacted soil across a drainage way, or other suitable location. It is intended to trap sediment before it leaves the construction site. The basin is a temporary measure and is to be maintained until the site area is permanently protected against erosion or a permanent detention basin is constructed.

Sediment basins shall be located at the stormwater outlet from the site, but not in a natural or undisturbed stream.

Limit the contributing area to the sediment basin to only the runoff from the disturbed soil areas. Use temporary concentrated flow conveyance controls to divert runoff from undisturbed areas away from the sediment basin.

The volume of the sediment basin shall be three-thousand (3,000) cubic feet per acre for property with average slope greater than five percent (5%), or fifteen-hundred (1,500) cubic feet per acre for property with an average slope less than five percent (5%). A properly sized sediment basin is required for each separate drainage area within the property being developed.

The outlet from a sediment basin shall be designed to empty its volume over an extended period of time. This is needed to permit the smaller sediment particles to settle to the bottom of the basin.

Maintenance

Sediment basins shall be inspected weekly and after one-half (1/2) inch or greater rainfalls for proper installation, erosion, and excessive sediment buildup and defective measures repaired or replaced within 24 hours.

Check inlet and outlet structures for any damage, obstructions, or erosion. Repair damage and remove obstructions as needed. The sediment basin must be maintained until final stabilization of the site.

Description

A compost filter sock is a type of contained compost filter berm. It is a mesh tube filled with composted material that is placed perpendicular to sheet flow runoff to control erosion and retain sediment in disturbed areas. The compost filter sock is oval to round in cross section, and it provides a three-dimensional filter that retains sediment and other pollutants while allowing the cleaned water to flow through. The filter sock can be used in place of a traditional sediment and erosion control tools, such as a silt fence.

Applications

Compost filter socks can be used on disturbed sites where stormwater runoff occurs as sheet flow.

Implementation

Compost filter socks are generally placed along the perimeter of a site, or at intervals along a slope, to capture and treat stormwater that runs off as sheet flow. They can be laid adjacent to each other, perpendicular to stormwater flow, to reduce flow velocity and soil erosion. They can be used on pavement as inlet protection.

No trenching is required; therefore, soil is not disturbed upon installation. Once the filter sock is filled and put in place, it shall be anchored to the slope. The preferred anchoring method is to drive stakes through the center of the sock at regular intervals; alternatively, stakes can be placed on the downstream side of the sock. The ends of the filter sock shall be directed upslope, to prevent stormwater from running around the end of the sock. The filter sock may be vegetated by incorporating seed into the compost prior to placement in the filter sock. Since compost filter socks do not have to be trenched into the ground, they can be installed on frozen ground or even pavement.

Limitations

The drainage areas for compost filter sock use shall not exceed 0.25 acre per 100 feet of device length and flow shall not exceed one (1) cubic foot per second. To ensure optimum performance for compost filter socks, heavy vegetation should be cut down or removed and extremely uneven surfaces should be leveled to ensure that the compost filter sock uniformly contacts the ground surface. Filter socks can be installed perpendicular to flow in areas where a large volume of stormwater runoff is likely, but should not be installed perpendicular to flow in perennial waterways and large streams.

Maintenance

Compost filter socks shall be inspected regularly, as well as after each rainfall event, to ensure that they are intact and the area behind the sock is not filled with sediment.

If there is excessive ponding behind the filter sock or accumulated sediments reach the top of the sock, an additional sock shall be added on top or in front of the existing filter sock.

If the filter sock was overtopped during a storm event, the operator should consider installing an additional filter sock on top of the original, placing an additional filter sock further up the slope, or using an additional BMP, such as a compost blanket.

(Source: US Environmental Protection Agency)

Description

Fiber rolls help reduce sediment loads to receiving waters by filtering runoff and capturing sediments.

Fiber rolls (also called fiber logs or straw wattles) are tube-shaped erosion control devices filled with straw, flax, rice or coconut fiber material. Each roll is wrapped with a UV-degradable polypropylene netting for longevity or with 100 percent biodegradable materials like burlap, jute, or coir. Fiber rolls also help to slow, filter, and spread overland flows. This helps to prevent erosion and minimizes rill and gully development. Fiber rolls also help reduce sediment loads to receiving waters by filtering runoff and capturing sediments.

Description

A gravel bag berm is a series of gravel-filled bags placed on a level contour to intercept sheet flows. Gravel bags pond sheet flow runoff, allowing sediment to settle out. They also release runoff slowly to prevent erosion.

Applications

Gravel bag berms are suitable for sediment control when placed down slope of exposed soil areas, as sediment traps at pipe outlets, along the perimeter of a site, around temporary stockpiles, parallel to a roadway to keep sediment off paved areas, and along streams and channels.

Gravel bag berms are suitable for erosion control when placed at the top of slopes to divert runoff away from disturbed slopes, when placed along the face and at grade breaks of exposed and erodible slopes to shorten length and spread runoff as sheet flow, and as check dams across mildly sloped construction roads.

Implementations

Gravel bag berms are to be placed on level contours. For slopes between 20:1 and 2:1 (horizontal:vertical), gravel bags should be placed at a maximum interval of fifty (50) feet. For slopes 2:1 (horizontal:vertical) or steeper, gravel bags should be placed at a maximum interval of twenty-five (25) feet. Turn the ends of the gravel bag barriers up slope to prevent runoff from going around the berm. Allow sufficient space up slope from the gravel bag berm to allow ponding, and to provide room for sediment storage. Use a pyramid approach when stacking bags.

Limitations

Gravel bag berms may not be appropriate for drainage areas greater than five (5) acres.

Runoff will pond upstream of the berm, possibly causing flooding if sufficient space does not exist.

Installation can be labor intensive.

Maintenance

Gravel bag berms shall be inspected prior to forecasted rain, daily during extended rain events, after rain events, and at two (2) week intervals during the non-rainy season.

Gravel bags exposed to sunlight will need to be replaced every two (2) or three (3) months due to the degrading of the bags.

Sediment shall be removed when the sediment accumulation reaches one-third (1/3) of the barrier height.

Remove gravel bag berms when no longer needed.

(Source: California Stormwater BMP Handbook, January 2003)

Description

Vegetative buffers are areas of natural or established vegetation to protect the water quality of neighboring areas. Buffer zones slow stormwater runoff, provide an area where runoff can permeate the soil, contribute to ground water recharge, and filter sediment. Slowing runoff also helps to prevent soil erosion and stream bank collapse.

Applications

Vegetated buffers can be used in any area able to support vegetation. They are most effective and beneficial on floodplains, near wetlands, along stream banks, and on unstable slopes.

Implementations

Most vegetation will be removed from a construction site during clearing and grading operations. A perimeter buffer strip shall be temporarily maintained around disturbed areas for erosion control purposes and shall be kept undisturbed except for reasonable access for maintenance. The width of this strip shall be six percent (6%) of the lot width and depth. The minimum width shall be twenty-five (25) feet and the maximum shall be forty (40) feet. In no event shall these temporary buffer strips be less than the width of the permanent buffers required for the development.

Vegetative buffers shall be used along streams, creeks, rivers, lakes, and other water bodies. The stricter criteria should be used between the DEQ Construction General Permit buffer requirements and the following: A minimum strip twenty-five (25) feet wide, undisturbed except for reasonable access, shall be provided along each side of streams having a peak ten-year storm flow rate of greater than one hundred fifty (150) cubic feet per second. The twenty-five (25) foot strip shall be measured from the top of bank. An exception to this requirement is allowed where the only work being done on the site is public street construction.

Limitations

Adequate land and soil must be available for a vegetative buffer.

Maintenance

Once established, vegetated buffers do not require maintenance beyond the routine procedures and periodic inspections. Inspect them after heavy rainfall of 0.5-inch or greater and at least once every fourteen calendar days. Focus on encroachment, gully erosion, the density of the vegetation, evidence of concentrated flows through the areas, and any damage from foot or vehicular traffic. If more than six (6) inches of sediment has accumulated, remove it and restore vegetative buffer.

(Source: US Environmental Protection Agency)

Description

Sediment filters are sediment-trapping devices typically used to remove pollutants (mainly particulates) from stormwater runoff. Sediment filters have four components: (1) inflow regulation, (2) pretreatment, (3) filter bed, and (4) outflow mechanism. Sediment chambers are one component of a sediment filter system.

Inflow regulation is diverting stormwater runoff into the sediment-trapping device. After runoff enters the filter system, it enters a pretreatment sedimentation chamber. This chamber is used as a preliminary settling area for large debris and sediments. It is usually no more than a wet detention basin. As water reaches a predetermined level, it flows over a weir into a bed of some filter medium. The medium is typically sand, but it can consist of sand, soil, gravel, peat, compost, or a combination. The filter bed removes small sediments and other pollutants from the stormwater as it percolates through the filter medium. Finally, treated flow exits the sediment filter system via an outflow mechanism. It returns to the stormwater conveyance system.

Sediment filter systems can be confined or unconfined, on-line or off-line, and aboveground or belowground. Confined sediment filters are constructed with the filter medium contained in a structure, often a concrete vault. Unconfined sediment filters are made without a confining structure. For example, sand might be placed on the banks of a permanent wet pond detention system to create an unconfined filter. On-line systems retain stormwater in its original stream channel or storm drain system. Off-line systems divert stormwater.

Applications

Sediment filters might be a good alternative for small construction sites where a wet pond is being considered as a sediment-trapping device. They are widely applicable, and they can be used in urban areas with large amounts of highly impervious area. Confined sand filters are man-made systems, so they can be applied to most development sites and have few constraining factors. However, for all sediment filter systems, the drainage area to be serviced shall be no more than ten (10) acres.

The available space is important to the design of sediment filters. Another important consideration is the amount of available head. Head is the vertical distance available between the inflow of the system and the outflow point. Because most filtering systems depend on gravity to move water through the system, if enough head is not available, the system will not be effective.

Limitations

For sediment filter systems, the drainage area to be serviced shall be no more than ten (10) acres.

Sediment filters are usually limited to removing pollutants from stormwater runoff. To provide flood protection, they have to be used with other stormwater management practices.

Sediment filters are likely to lose effectiveness in cold regions because of freezing conditions.

(Source: US Environmental Protection Agency)

Description

Excessive velocity of water in swales or channels causes erosion and transports the sediment downstream to local streams.

Check Dams (ditch check) slow water in channels and provide an area for sediment to settle out of the water before it flows over the dam.

Applications

Any unlined channel or any channel that the vegetative protection has not developed. Steeper slopes are more subject to erosion than flatter slopes.

Design Criteria

Place ditch checks such that the top of the downstream check is at the same elevation as the bottom of the next upstream check.

Checks must be constructed such that the top elevation of the center of the check is at least six (6) inches below the bottom elevation of both ends of the check. The dam must be excavated into the channel no less than six (6) inches as shown in the below figures.

Limitations

If improperly constructed, water will flow around or through the check dam and erode the banks of the channel. Large flows (less frequent storms) can washout the check dams, erode the banks at the end of the check dams, or cause excessive scour at the outfall of the check dam.

Maintenance Requirements

Sediment that collects behind a check dam shall be removed when the sediment reaches fifty percent (50%) of the depth to the spillway crest. Check Dams shall be inspected weekly and after one-half (1/2) inch or greater rainfalls for proper installation, erosion, and excessive sediment buildup and defects shall be repaired or replaced within 24 hours.

Check dams constructed in permanent swales shall be removed when perennial grasses have become established, or immediately prior to installation of a non-erodible lining. All of the rock and accumulated sediment shall be removed. The area shall be dressed to match surrounding grades, then seeded and mulched, or otherwise stabilized.

Description

A triangular silt dike is a triangular-shaped foam block covered with geotextile fabric. When laid in a channel and placed perpendicular to the flow of water, it provides an area for sediment to settle out of the water. A triangular silt dike is a reusable alternative to rock check dams. It conforms to curves and rough terrain.

Applications

Any channel where the vegetative protection has not developed. Steeper slopes are more subject to erosion than flatter slopes.

Triangular silt dikes can also be used as diversion dikes and as inlet protection.

Design Criteria

Place ditch checks such that the top of the downstream check is at the same elevation as the bottom of the next upstream check.

A protective apron shall be installed on both sides of the dike to prevent erosion and failure and are secured using U-shaped wire staples.

A trench shall be excavated that is approximately three to six (3 to 6) inches deep on the upslope side of the dike. The trench shall then be backfilled and the soil compacted over the textile.

Limitations

If improperly constructed, water will flow around or the triangular silt dike and erode the banks of the channel. Large flows (less frequent storms) can washout the triangular silt dike, erode the banks at the end of the check dams, or cause excessive scour at the outfall of the check dam.

Maintenance Requirements

Triangular silt dikes shall be inspected weekly and after one-half (1/2) inch or greater rainfalls for proper installation, erosion, and excessive sediment buildup. Any damage shall be repaired immediately. Sediment must be removed when it reaches six (6) inches high on the dike. If the geotextile has deteriorated due to ultraviolet breakdown, it shall be replaced.

Triangular silt dikes constructed in permanent swales shall be removed when perennial grasses have become established, or immediately prior to installation of a non-erodible lining. All of the accumulated sediment shall be removed. The area shall be dressed to match surrounding grades, then seeded and mulched, or otherwise stabilized.

Description

A grass-lined or sod-lined channel conveys stormwater runoff through a stable conduit. Vegetation lining the channel reduces the velocity of concentrated runoff and provides water quality benefits through filtration and infiltration. Because grassed channels are not usually designed to control peak runoff loads by themselves, they are often used with other BMPs, such as subsurface drains and riprap stabilization.

Where moderately steep slopes require drainage, grassed channels can include excavated depressions or check dams to enhance runoff storage, decrease flow rates, and improve pollutant removal. Peak discharges can be reduced by temporarily holding them in the channel. Pollutants can be removed from stormwater by filtration through vegetation, by deposition, or in some cases by infiltration of soluble nutrients into the soil. The degree of pollutant removal in a channel depends on how long the water stays in the channel and the amount of contact with vegetation and the soil surface. Local conditions affect the removal efficiency.

Applications

The first choice of lining should be grass or sod because this reduces runoff velocity and provides water quality benefits through filtration and infiltration. If the velocity in the channel would erode the grass or sod, riprap, concrete, or gabions can be used. Geotextile materials can be used in conjunction with either grass or riprap linings to provide additional protection at the soil-lining interface.

Use grassed channels in areas where erosion-resistant conveyances are needed, including areas with highly erodible soils and moderately steep slopes (though less than five (5%) percent). Install them only where space is available for a relatively large cross section.

Grassed channels have a limited ability to control runoff from large storms, so do not use them in areas where flow rates exceed five (5) feet per second.

Implementations

Site grass-lined channels in accordance with the natural drainage system. The channel should not receive direct sedimentation from disturbed areas and should be sited only on the perimeter of a construction site to convey relatively clean stormwater runoff. To reduce sediment loads, separate channels from disturbed areas by using a vegetated buffer or another BMP.

Consider using geotextiles to stabilize vegetation until it is fully established. Consider covering the bare soil with sod, mulches with netting, or geotextiles to provide reinforced stormwater conveyance immediately.

Use triangular channels with low velocities and small quantities of runoff; use parabolic grass channels for larger flows and where space is available; use trapezoidal channels with large, low-velocity flows (low slope).

Install outlet stabilization structures if the runoff volume or velocity might exceed the capacity of the receiving area.

Limitations

If grassed channels are not properly installed, they can change the natural flow of surface water and adversely affect downstream waters. And if the design capacity is exceeded by a large storm event, the vegetation might not be adequate to prevent erosion and the channel might be destroyed. Clogging with sediment and debris reduces the effectiveness of grass-lined channels for stormwater conveyance.

Grassed channels have a limited ability to control runoff from large storms, so do not use them in areas where flow rates exceed 5 feet per second.

Maintenance

The maintenance requirements for grass channels are relatively minimal. While vegetation is being established, inspect the channels after every rainfall. After vegetation is established, mow it, remove litter, and perform spot vegetation repair. The most important objective in grassed channel maintenance is to maintain a dense and vigorous growth of turf.

Periodically clean the vegetation and soil buildup in curb cuts so that water flow into the channel is unobstructed.

During the growing season, cut the channel grass no shorter than the level of the design flow.

(Source: US Environmental Protection Agency)

Description

Water running onto the site will increase erosion and be a nuisance to construction activities. Additionally, runoff from the construction site can have excessive amounts of sediment that can end up in local streams.

Interceptor and Diversion Swales and Dikes are diversion systems used to divert runoff around a site or to direct runoff from a site to a pond in order to settle out sediment prior to discharge from the site.

Applicability

Any area that is subject to runoff from uphill drainage areas.

Design Criteria

There are two types of temporary slope diversion dikes:

1. A diversion dike located at the top of a slope to divert upland runoff away from the disturbed area. The runoff from undisturbed upland areas may be directed by such dikes to a permanent channel or temporary diversion channel.
2. A diversion dike located at the base or mid-slope of a disturbed area to divert sediment-laden water to a sediment basin. The discharge intercepted by these diversion dikes may be directed to a temporary slope drain and/or sediment basin.

Temporary diversion dikes shall be provided whenever:

S 2L > 2.5 for undisturbed tributary areas:

S 2L > 1.0 for disturbed tributary areas:

S 2L > 0.25 for paved tributary areas:

where: S = slope of the upstream tributary area (in feet/foot); and,
L = length of the upstream slope (in feet).

and

Undisturbed Tributary Area = area tributary to the temporary diversion dike that is, and will remain, in a natural condition undisturbed by development activities.

Disturbed Tributary Area = area tributary to the temporary diversion dike that has been disturbed by development activities, including removal of native vegetation and/or compaction of native soils.

Impervious Tributary Area = area tributary to the temporary diversion dike that is largely comprised of impervious surfaces, such as buildings and pavement.

The swale (channel) and dike shall be situated to capture runoff uphill of the work area with a vegetative buffer uphill of the swale to remove sediment before it enters the swale. The stabilized swale and ditch shall be in-place prior to all other earth work on the project. The channel shall be designed to handle the 10-year storm, with the bottom and sides protected for the anticipated water velocity. Typically, the ditch will be two (2) foot wide at the bottom and six (6) foot wide at the top. Maximum water velocity in the swale shall not exceed four (4) feet per second. Side slopes shall be no steeper than 3:1 (horizontal:vertical). Energy dissipation shall be provided at the exit from the swale as needed.

Description

Rough-cut street controls are dirt berms, sandbag dikes, or gravel filled geotextiles socks used to prevent rill, channel and gully erosion on unpaved streets.

Rough cut street controls are runoff barriers that are constructed at intervals down an unpaved road. These barriers are installed perpendicular to the longitudinal slope from the outer edge of the roadside swale to the crown of the road. The barriers are positioned alternately from the right and left side of the road to allow construction traffic to pass in the lane not barred. Refer to the rough-cut street control detail below.

Applicability

Rough-cut street controls shall be considered for roadways that are not paved for thirty (30) days of final grading and have not received an application of road base.

Maintenance

Rough-cut street controls shall be inspected immediately following the initial installation, once a week while the site is under active construction, and immediately following a rain event.

Accumulated sediment shall be removed when the sediment depth is a quarter (1/4) the height of the berm.

Rough-cut street control shall be repaired immediately following any sign of wear or alteration of the original shape and dimensions.

Rough-cut street control shall be kept in place and maintained until subgrade preparation begins for paving.

Figure CS-17 – Rough-Cut Street Control (Source: Orange County, CA)

Description

A water bar is a ridge of compacted soil, loose rock, or gravel constructed diagonally across disturbed rights-of-way and similar sloping areas. The height and side slopes of the water bar are designed to divert water and allow vehicles to cross. Water bars are used to shorten the flow length within a long sloping right-of-way, thereby reducing the erosion potential by diverting storm runoff to a stabilized outlet or sediment trapping device.

Applications

Water bars can be used in areas where earthen diversions are applicable and where there will be little or no construction traffic within the right-of-way. Gravel structures are more applicable to roads and rights-of way which accommodate vehicular traffic.

Implementations

Construction of utility lines and roads often requires the clearing of long strips of right-of-way over sloping terrain. The volume and velocity of stormwater runoff tend to increase in these cleared strips and the potential for erosion is much greater since the vegetative cover is diminished or removed. To compensate for the loss of vegetation, it is usually a good practice to break up the flow length within the cleared strip so that runoff does not have an opportunity to concentrate and cause erosion. At proper spacing intervals, water bars can significantly reduce the amount of erosion which will occur until the area is permanently stabilized.

Limitations

Water bars shall not be used for drainage areas less than one (1) acre.

The water bar spacing must be close enough to dissipate water flow energy.

Water bars can be used where there will be little or no construction traffic within the right-of-way. Gravel structures are more applicable to roads and rights-of way which accommodate vehicular traffic.

Maintenance

Water bars shall be inspected after every rainfall and repairs made if necessary. Approximately once every week, whether a storm has occurred or not, the measure shall be inspected and repairs made if needed. Earth fill that is subject to damage by vehicular traffic shall be reshaped at the end of each working day.

(Source: NRCS Planning and Design Manual)

The purpose of this chapter is to provide guidance for selecting, designing, and maintaining stormwater Best Management Practices (BMPs) to minimize potential adverse impacts on stormwater quality caused by urbanization in the City of Rogers. The City, along with many communities around the United States, encourages the widespread use of stormwater BMPs on all development sites.

To comply with the Federal Clean Water Act, the Arkansas Department of Energy & Environment Division of Environmental Quality (DEQ) issued Arkansas State Operating Permit ARR040041 to the City of Rogers to authorize discharges from the City’s Municipal Separate Storm Sewer System (MS4) to waters of the State. In accordance with the MS4 permit, the City is required to develop and implement a comprehensive Stormwater Management Program that includes controls to identify illicit discharges and reduce the discharge of pollutants from the MS4 to the Maximum Extent Practicable. The design tools provided in this chapter are intended to improve the quality of stormwater runoff from development sites in the City.

The historic, traditional approach for managing stormwater was to convey the water away from developed areas as quickly as possible. Today, sound stormwater management programs require new developments to be designed in a manner to reduce runoff volumes, runoff velocities and reduce pollutant loads. This can be achieved with properly designed, implemented, and maintained stormwater BMPs.

The City requirements for stormwater quality protection described in this chapter apply to:

• All development and additions to existing sites that increase the site’s impervious area by one tenth (0.1) of an acre or more, including projects that are less than one tenth (0.1) of an acre that are part of a larger common plan of development or sale;
• Sites that are being redeveloped; or
• Other developments, increasing the impervious area by less than one tenth (0.1) of an acre, that have been specifically identified by the City as having a significant potential to adversely impact the quality of stormwater runoff.

To achieve basic objectives related to stormwater quality such as protecting drinking water supplies, protecting human health and the environment, and complying with the federal National Pollutant Discharge Elimination System (NPDES) permit requirements, the City will require new developments to implement several fundamental principles with respect to stormwater management, including:

• Minimizing the amount of runoff from developed areas;
• Minimizing the amount of Directly Connected Impervious Area (DCIA);
• Maximizing the contact of runoff with grass and vegetated soil;
• Maximizing holding and settling times in detention basins;
• Designing BMPs for small, frequent storms;
• Utilizing BMPs in series where feasible;
• Incorporating both flood control and stormwater quality objectives in designs;
• Providing special treatment for runoff from fueling areas and other areas having a high concentration of pollutants; and
• Stabilizing drainageways downstream from developments.

The quantifiable objective of these principles is to capture and manage the Water Quality Capture Volume (WQCV) of each development site. To achieve this objective, specific types of BMPs that can be used are described in this chapter. These include:

• Extended Dry Detention Basin
• Extended Wet Detention Basin
• Constructed Wetland Basin
• Permeable Pavers
• Porous Landscape Detention
• Vegetated Filter Strip/Grass Buffer
• Grass Swale

For each of the BMPs listed above, a description is provided of design considerations, the design procedure and criteria, maintenance considerations, and a design example. Other BMPs are also discussed briefly, such as design considerations for material storage and handling areas, spill containment and control measures, and alternative structural BMPs (e.g., proprietary stormwater treatment units). Low Impact Development (LID), a design approach that incorporates many of the design elements described in this chapter, is also presented.

Other important considerations identified include restrictions imposed by the United States Fish and Wildlife Service (USFWS) on development in the Karst Formation/Cave Springs Recharge Area and regulatory requirements of the United States Army Corps of Engineers (USACE) associated with the development of constructed wetlands.

To comply with the City requirements for protection of stormwater quality, new developments or redevelopments must satisfy one of the following two requirements outlined below:

1. The onsite or regional stormwater BMPs must store and treat the calculated Water Quality Capture Volume (WQCV) for the site.
   or
2. A fee must be paid in lieu of implementing the onsite facilities necessary to store and treat the WQCV. The fee-in-lieu option is an alternative only when the development or redevelopment site disturbs between one tenth (0.1) and one half (0.5) of an acre, or the site has not been specifically identified by the City as having a significant potential to adversely impact the quality of stormwater runoff. Fees collected from this option are used for other stormwater quality protection projects throughout the City. Although the fee-in-lieu option eliminates the need to store and treat the WQCV on-site, it does not eliminate the need to provide water quality BMPs on the site upstream of the discharge point to the receiving stream or storm sewer.

For development sites that have been specifically identified by the City as having a significant potential to adversely impact stormwater quality (e.g., development on steep slopes or highly erosive soils), then the only acceptable option, regardless of the size of the site, is to capture and manage the WQCV calculated for the site. The fee-in-lieu option is not applicable to these sites.

Urban stormwater runoff can contain a variety of pollutants that can adversely impact waterbodies. The Nationwide Urban Runoff Program (USEPA 1983) and other studies widely document the types and concentrations of pollutants associated with various land use types. Urban runoff may contain contaminants such as metals, lubricants, solvents, pesticides, herbicides, fertilizers, pet waste, litter and suspended sediments.

The quality of stormwater runoff from the City is of particular importance given its location in the watersheds of Beaver Lake to the east and the Illinois River to the west. This chapter discusses specific engineering measures that can be implemented to improve stormwater quality.

Traditional engineering approaches for stormwater management historically focused on moving water away from people, structures, and transportation systems as quickly and efficiently as feasible. This was accomplished by creating conveyance networks of impervious storm sewers, roof drains, and lined channels, which concentrated runoff discharges to receiving waters.

While the historical focus on stormwater was not on water quality, the potential adverse effects of urban runoff on the physical, chemical, and biological characteristics of receiving waters have been widely documented (e.g., WEF/ASCE 1992, 1998; Debo and Reese 2002; Horner, et al. 1994; Schueler and Holland 2000). Potential water quality implications of the traditional approach to drainage design include the following:

• Introduction of new pollutant sources and types (e.g., sediment from streets and parking lots).
• Increased runoff temperature.
• Habitat damage and ecosystem disruption associated with increased runoff from impervious surfaces, resulting in streambed and bank erosion and associated sediment and pollutant transport.
• Channel widening and instability.
• Destruction of both aquatic and terrestrial physical habitats.
• Increased contaminant transport, leading to increased water quality degradation that often may result in regulatory consequences such as stream segments being listed as impaired on the State 303(d) list and requirements for Total Maximum Daily Load (TMDL) allocations for dischargers to the stream.
• Production of potentially toxic concentrations of contaminants in receiving waters and long-term accumulation of contaminants.

To comply with the Federal Clean Water Act, the Arkansas Department of Energy & Environment Division of Environmental Quality (DEQ) issued Arkansas State Operating Permit ARR040041 to the City of Rogers to authorize discharges from the City’s Municipal Separate Storm Sewer System (MS4) to waters of the State. In accordance with the MS4 permit, the City is required to develop and implement a comprehensive Stormwater Management Program that includes controls to identify illicit discharges and reduce the discharge of pollutants from the MS4 to the Maximum Extent Practicable. The design tools provided in this chapter are intended to improve the quality of stormwater runoff from development sites in the City.

To comply with the NPDES requirements and to minimize the potential adverse impacts of urbanization on water quality, the City, along with many communities around the United States, encourages the widespread use of stormwater BMPs on all development sites. The purpose of this chapter is to provide guidance for selecting, designing, and maintaining BMPs. This section is primarily targeted at protecting water quality in conjunction with development and redevelopment of residential and commercial areas. However, BMPs for light industrial areas and other types of land uses are also addressed.

Structural BMPs are constructed facilities designed to passively treat urban stormwater runoff, including practices such as detention basins (both dry basins and wet ponds), wetlands, permeable pavement, and designed vegetated zones, among others. Structural BMPs can be designed to treat small volumes of stormwater on development sites or to serve larger regional drainage areas.

Non-structural BMPs are practices and procedures that minimize or prevent pollution and control it at its source. Examples of non-structural BMPs include proper handling and storage of materials, minimizing directly connected impervious areas to reduce the transport of pollutants in runoff, and implementing public education programs to protect stormwater quality.

The design guidelines in this chapter represent current BMP technology and are anticipated to evolve as BMP technology is evaluated and refined, new BMPs are developed, or as new standards are promulgated by the State. This chapter significantly draws from the Denver Urban Drainage and Flood Control District (UDFCD) Urban Storm Drainage Criteria Manual (UDFCM), Volume 3,Best Management Practices, first published in 1992 and regularly updated since then. Volume 3 updates and other information are available from the UDFCD website (www.udfcd.org).

Design requirements are presented for both structural and non-structural water quality BMPs. General BMP descriptions, design considerations and criteria, maintenance considerations, design forms and completed examples are provided for each structural BMP. The discussion in this section is limited to permanent, post-development BMPs. For information on construction-phase erosion and sediment control BMPs, the Chapter 8 –Construction Site Stormwater Management shall be referenced.

To achieve the stormwater quality design objectives for a new development, designs shall incorporate one or more of the following principles:

1. Minimize the amount of runoff. The total quantity of pollutants transported to receiving waters can be minimized most effectively by minimizing the amount of runoff. Both the quantity of runoff and the amount of pollutant wash-off can be reduced by minimizing the Directly Connected Impervious Area (DCIA) at a site. Impervious areas are considered connected when runoff travels directly from roofs, driveways, pavement, and other impervious areas to street gutters, closed storm drains, and concrete or other impervious lined channels. Impervious areas are considered disconnected when runoff travels as sheet flow over grass areas or through properly designed BMPs, prior to discharge from the site.
   Minimizing DCIA is a land development design philosophy that seeks to reduce paved areas and direct stormwater runoff to landscaped areas, grass buffer strips, and grass-lined swales to slow down the rate of runoff, reduce runoff volumes, attenuate peak flows, and facilitate the infiltration and filtering of stormwater. This approach increases the time of concentration for runoff, in contrast to the historic stormwater engineering approach that resulted in drainage systems with a relatively rapid, large peak runoff rate and increased runoff volumes, even for relatively small storms.
   A design approach that minimizes DCIA can be integrated into the landscape and drainage planning for any development. Drainage from rooftop collection systems, sidewalks, and driveways can be directed to landscaped areas, infiltration areas such as porous landscape detention and permeable pavement, grassed buffer strips, or to grass swales. Instead of using traditional solid curbing, curbing can be eliminated in some areas or slotted curbing can be used along with stabilized grass shoulders and swales or can direct flows towards BMPs within street bulb-outs. Residential driveways can use permeable pavement or their runoff can be redirected to the lawn rather than the street. Large parking lots can minimize DCIA by using permeable pavement to capture runoff and encourage local infiltration or storage. Green roofs may also be used as a tool to minimize DCIA.
2. Maximize contact with grass and vegetated soil. The opportunity for pollutants to settle can be maximized by providing maximum contact with grass and vegetated soil. Directing runoff over vegetative filter strips and grass swales enhances settling of pollutants as the velocity of flow is reduced. Street bulb-outs must have plantings proposed within them in the design to be utilized as BMPs to capture and treat runoff from the impervious areas within the right-of-way.
3. Maximize holding and settling time. The most effective runoff quality controls reduce both the runoff peak and volume. By reducing the rate of outflow and increasing the time of detention storage, settling of pollutants and infiltration of runoff are maximized.
4. Design for small, frequent storms. Drainage stormwater systems for flood control are typically designed for large, infrequent storm events. In contrast, water quality controls shall be designed for small, frequent storm events. In Rogers, approximately 90 percent of all rainfall events are 1 inch or less. Studies indicate that many pollutants are frequently washed off in the “first flush,” typically considered the first ½ inch of runoff from directly connected impervious areas.
5. Utilize BMPs in series where feasible. Performance monitoring of BMPs throughout the country has shown that the combined effect of several BMPs in series can be more effective in reducing the level of pollutants than just providing a single BMP at the point of discharge. To the extent practical, impervious areas shall be disconnected with runoff directed first to vegetative filter strips, then to grass swales or channels, and then to extended detention basins, etc.
6. Incorporate both flood control and stormwater quality objectives in designs, where practical. Incorporating both flood control and water quality enhancement into a single stormwater management facility is encouraged whenever practical. Combining several objectives, such as water quality enhancement and flood control, maximizes the cost-effectiveness of stormwater management facilities.
7. Provide special care for runoff from fueling areas and other areas having a high concentration of pollutants. Runoff from areas that pose a specific high hazard to the quality of runoff must be directed to a properly designed BMP that provides both filtration and settling prior to discharge to receiving waters.

Studies indicate that small-sized, frequently occurring storm events account for the majority of events that result in stormwater runoff from urban drainage basins. Consequently, these frequent storms also account for a significant portion of the annual pollutant loads. Capture and treatment of stormwater from these small and frequently occurring storms is the recommended design approach for water quality enhancement, as opposed to designs for flood control facilities that focus on larger, less frequent storm events. Incorporation of both sets of criteria (i.e., small, frequent storms for water quality purposes and larger storms for flood control) into a single stormwater management facility is encouraged, where practical. However, unless exempt from the requirement, no single BMP shall address more than 50 percent of the site’s calculated WQCV.

For sites where the water quality requirements apply, water quality BMPs shall be designed to capture and treat the WQCV of the site. The required WQCV (measured in cubic feet [ft³]) is a function of the total area tributary to the storage facility and the impervious percentage of the tributary area. The WQCV curves in Figure WQ-1 are calculated for the City of Rogers and are based on approximately the 85th percentile runoff event (i.e., the top 85 percent of storm events in Rogers that generate runoff) (WEF and ASCE, 1998). The required WQCV will need to be dispersed throughout the site. No single BMP can address more than 50 percent of the required WQCV unless a regional water quality BMP is present or if the site is within an industrial zone. The three curves represent the required WQCV for drain times of 12, 24, and 48 hours. Different drain times are required depending on the type of BMP and their relative effectiveness in removing suspended sediments and other contaminants. Storage and treatment of the WQCV can be achieved through the use of five BMPs described in Section 4.0. These BMPs and their respective drain times are:

• Extended dry detention basin 48 hour drain time
• Constructed wetland basin 24 hour drain time
• Permeable pavers 24 hour drain time
• Extended wet detention basin 12 hour drain time
• Porous landscape detention 12 hour drain time

*Refer to Chapter 5 – Detention Design for drain times associated with volumetric detention.

With an understanding of the type of BMP to be employed and the associated drain time, and the impervious percentage of the area tributary to the BMP, Figure WQ-1 graphically shows the WQCV (in cubic feet) per square foot of area tributary to the storage facility. The required WQCV shall be computed using the WQCV Worksheet in the BMP spreadsheet.

The required quantity of the WQCV can be reduced through the use of BMPs that minimize the DCIA at a site. Such BMPs promote infiltration and reduce the runoff volume from a site. These BMPs also serve to filter runoff that does leave the site. Three BMPs that can be used to reduce the necessary WQCV include:

• Permeable pavers
• Vegetated filter strip/grass swale
• Grass swale

As seen above, the use of permeable pavers can potentially store and treat the WQCV as well as reduce the necessary WQCV. The pavers’ function would depend on the design of the system. Sites that are increasing the overall impervious area, not including any building’s footprint, beyond 2,000 square feet must make at least 10 percent of that impervious area permeable pavement so as to minimize the DCIA. Areas where heavy vehicles or equipment are expected would be exempt of implementing permeable pavement for this purpose. The reduction in the amount of necessary WQCV provided by use of permeable pavement and these BMPs is described in Appendix A.

Figure WQ-1
Water Quality Capture Volume for Rogers, Arkansas

The City may allow the property owner to pay a fee in-lieu-of implementing the water quality control measures described in this section. The fee paid in-lieu-of water quality protection measures is an acceptable alternative only if the development site disturbs less than one half (0.5) an acre and the site has not been specifically identified by the City as having a significant potential to adversely impact the quality of stormwater runoff. Sites that have an existing regional water quality control facility with adequate capacity, as determined by the City, are exempt of having to pay a fee-in-lieu of water quality protection. Proceeds from fees collected from this option will be used by the City to fund regional stormwater facilities or other measures that will benefit the quality of stormwater in the community. The methodology for calculating the in-lieu-of fees is described in Appendix B.

In addition to the design considerations above, the following factors shall be considered when selecting BMPs for a site:

• Pollutants Controlled - The BMPs shall effectively control pollutants known to be associated with the tributary land use.
• Reliability/Sustainability - Measures shall be effective over an extended time and be able to be properly maintained over time.
• Public Acceptability - BMP selection shall consider the expected response from the public, particularly neighboring residential properties, if any.
• Agency Acceptability - BMP selection shall consider the expected response of agencies that will oversee the BMPs and their relationship to regulatory requirements.
• Public Safety - Control measures shall be evaluated in terms of public safety and the risks or liabilities that occur during implementation. Public safety is always one of the most important design considerations, not only for “traditional” drainage structures, but also for BMPs.
• Performance Verification – All hydrodynamic separators employed on a site with the intention of being utilized as a BMP for water quality mitigation will need to be listed on the most up to date list of New Jersey Department of Environmental Protection (NJDEP) Certified Stormwater Manufactured Treatment Devices (MTDs). MTDs must to be regularly maintained by the agency overseeing BMPs on a site.
• Mosquito Control - The potential for mosquito breeding and the spread of mosquito-borne illnesses in stormwater BMPs must be addressed. In general, the biggest concern is the creation of areas with shallow stagnant water and low dissolved oxygen that creates prime mosquito habitat. Other habitat characteristics that may enhance breeding include dense stands of vegetation that may protect larvae from natural predators and soils with high organic content. While stormwater BMPs such as detention ponds and constructed wetlands often include these features, careful design and proper management and maintenance of systems can effectively control mosquito breeding.

The key to minimizing breeding is to avoid creating, or allowing the formation of, areas of shallow standing water. Studies indicate that pools of deep water (≥ 5 feet) and pools with residence times less than 72 hours are less likely to breed mosquitoes. Stormwater BMPs with permanent pools are generally less of a concern than dry detention basins because of their greater depth. Therefore, dry detention basins must have outlets designed to drain within 48 hours.

Once the BMPs are implemented, it is necessary to ensure that structural BMPs are properly operated and maintained and that the relevant non-structural BMPs are also being implemented. This may involve requiring subdivision covenants, inspecting BMPs, designating individuals responsible for BMPs, and pollution prevention education. Modifications to BMPs over time may also be necessary if land uses or other factors change or if BMPs prove to be ineffective or a nuisance. For permeable paver systems, see the Annual Inspection and Maintenance Checklist.

Covering of storage and handling facilities and proper handling of potential industrial or commercial pollutants, such as salt piles, oil products, pesticides, fertilizers, etc., is a requirement under the City’s Municipal Separate Storm Sewer System (MS4) discharge permit. A copy of the City’s MS4 discharge permit can be provided upon request. In addition, these practices reduce the likelihood of stormwater contamination and help prevent loss of material from wind or rainfall erosion. Development plans for these facilities must specify how potential pollutants will be covered and handled to prevent discharge of the pollutant into the City’s MS4. Covering is appropriate for areas where solids (e.g., gravel, salt, compost, building materials, etc.) or liquids (e.g., oil, gas, tar, etc.) are stored, prepared, or transferred. Coverings shall be permanent in nature and handling procedures must be carried through plans and policies in place at the operating facility.

Spill containment within industrial and some commercial sites includes berms, walls, and gates that control spilled material. Berms consist of temporary or permanent curbs or dikes that surround a potential spill site, preventing spilled material from entering surface waters or storm sewer systems. The berm or wall may be made of concrete, earthen material, metal, synthetic liners, or any material that will safely contain the spill. The containment area must have an impermeable floor (asphalt or concrete) or liner so that contamination of groundwater does not occur.

Two methods of berming can be used: 1) containment berming that contains an entire spill, or 2) curbing that routes spill material to a collection basin. Both methods shall be sized to safely contain a spill from the largest storage tank, rail car, tank truck, or other containment device located inside the possible spill area. A collection basin shall be provided to hold stormwater and spills until removal is possible.

Except as otherwise provided herein, the Department of Community Development shall administer, implement, and enforce the provisions of this section.

The following prohibitions must be adhered to:

1. No person shall release or cause to be released into the storm drainage system any discharge that is not composed entirely of uncontaminated stormwater, except as allowed herein. Common stormwater contaminants include trash, yard waste, lawn chemicals, pet waste, wastewater, oil, petroleum products, cleaning products, paint products, hazardous waste, and sediment;
2. Any discharge shall be prohibited by this section if the discharge in question has been determined by the coordinator to be a source of pollutants to the storm drainage system;
3. The construction, use, maintenance or continued existence of illicit connections to the storm drain system are prohibited. This prohibition expressly includes, without limitation, illicit connections made in the past, regardless of whether the connection was permissible under law or practices applicable or prevailing at the time of connection;
4. No person shall connect a line conveying sanitary sewage, domestic sewage, or industrial waste to the storm drainage system, or allow such a connection to continue;
5. No person shall maliciously destroy or interfere with BMPs implemented pursuant to this article.

The following nonstormwater discharges are deemed acceptable and not a violation of this chapter:

1. A discharge authorized by an NPDES permit other than the NPDES permit for discharges from the MS4;
2. Uncontaminated waterline flushing and other infrequent discharges from potable water sources;
3. Infrequent uncontaminated discharge from landscape irrigation or lawn watering;
4. Uncontaminated discharge from foundation, footing or crawl space drains, sump pumps, and air conditioning condensation drains;
5. Uncontaminated groundwater, including rising groundwater, groundwater infiltration into storm drains, pumped groundwater and springs;
6. Diverted stream flows and natural riparian habitat or wetland flows; and
7. A discharge or flow of fire protection water that does not contain oil or hazardous substances or materials.

1. Private drainage system maintenance - The owner of any private drainage system shall maintain the system to prevent or reduce the discharge of pollutants. This maintenance shall include, but is not limited to, sediment removal, bank erosion repairs, maintenance of vegetative cover, and removal of debris from pipes and structures.
2. Minimization of irrigation runoff - A discharge of irrigation water that is of sufficient quantity to cause a concentrated flow in the storm drainage system is prohibited. Irrigation systems shall be managed to reduce the discharge of water from a site.
3. Cleaning of paved surfaces required - The owner of any paved parking lot, street or drive shall clean the pavement as required to prevent the buildup and discharge of pollutants. In addition to any other criminal penalties that may be prescribed by state law, the visible buildup of mechanical fluid, waste materials, sediment or debris is a violation of this ordinance and is subject to the penalty provisions as outlined in Rogers City Code Section 1-5. Paved surfaces shall be cleaned by dry sweeping, wet vacuum sweeping, collection, and treatment of wash water or other methods in compliance with this Manual. This section does not apply to pollutants discharged from construction activities.
4. Maintenance of equipment - Any leak or spill related to equipment maintenance in an outdoor, uncovered area shall be contained to prevent the potential release of pollutants. Vehicles, machinery, and equipment must be maintained to reduce leaking fluids.
5. Materials storage - In addition to other requirements of this Code, materials shall be stored to prevent the potential release of pollutants. The uncovered, outdoor storage of unsealed containers of hazardous substances is prohibited.
6. Pet waste - Pet waste shall be disposed of as solid waste or sanitary sewage in a timely manner in order to prevent discharge to the storm drainage system.
7. Pesticides, herbicides and fertilizers - Pesticides, herbicides and fertilizers shall be applied in accordance with manufacturer recommendations and applicable laws. Excessive application shall be avoided.
8. Prohibition on use of pesticides and fungicides banned from manufacture - Use of any pesticide, herbicide or fungicide, the manufacture of which has been either voluntarily discontinued or prohibited by the Environmental Protection Agency, or any federal, state, or city regulation is prohibited.
9. Open drainage channel maintenance - Every person owning or occupying property through which an open drainage channel passes shall keep and maintain that part of the drainage channel within the property free of trash, debris, excessive vegetation, and other obstacles that would pollute, contaminate, or retard the flow of water through the drainage channel. In addition, the owner or occupant shall maintain existing privately owned structures adjacent to a drainage channel, so that such structures will not become a hazard to the use, function or physical integrity of the drainage channel.

Any person responsible for a known or suspected release of materials which are resulting in or may result in illegal discharges to the storm drainage system shall take all necessary steps to ensure the discovery, containment, abatement and cleanup of such release. In the event of such a release of a hazardous material, said person shall comply with all state, federal, and local laws requiring reporting, cleanup, containment, and any other appropriate remedial action in response to the release. In the event of such a release of nonhazardous materials, said person shall notify the Department of Community Development no later than 5:00 p.m. of the next business day.

The City may adopt and impose requirements identifying the best management practices for any activity, operation, or facility, which may cause a discharge of pollutants to the storm drainage system. Where specific BMPs are required, every person undertaking such activity or operation, or owning or operating such facility shall implement and maintain these BMPs at their own expense.

The following personnel employed by the city/county shall have the power to issue notices of violations and implement other enforcement actions under this division as provided by the City:

1. All authorized personnel under the supervision of the Department of Community Development;
2. All inspectors and code enforcement officers under the supervision of the director of code enforcement; and
3. All county health officers that are authorized representatives of the director of the county health department.

Whenever the stormwater coordinator has cause to believe that there exists, or potentially exists, in or upon any premises any condition which constitutes a violation of this article, the stormwater coordinator shall have the right to enter the premises at any reasonable time to determine if the discharger is complying with all requirements of this article. In the event that the owner or occupant refuses entry after a request to enter has been made, the City is hereby empowered to seek assistance from a court of competent jurisdiction in obtaining such entry. The Department of Community Development shall have the right to set up on the property of any discharger to the storm drainage system such devices that are necessary to conduct sampling of discharges.

This policy establishes a formal enforcement procedure to be followed by the City stormwater coordinator when enforcement action is necessary on sites that do not comply with the City's codes. Enforcement cases can be generated in any of three ways: through the construction review process; through complaints from individuals, groups, etc.; and through referrals from city/state agencies. Procedures to be followed for each of these methods are outlined in this section.

1. Construction review - Every effort is made to use the construction review process to correct deficiencies in site compliance whenever possible. Should that process fail to achieve expected results or if the site reviewer feels that a violation is serious enough to warrant enforcement action, the following procedures shall be followed:
   1. Issuance of notice of violation - If site deficiencies are noted, the owner/developer or authorized agent shall be given a notice of violation. The notice of violation shall be specific as to the noted violation, corrective measures to be taken, and time frame allowed to complete the work.
   2. Compliance review - At the end of the time period specified above, a follow-up site inspection shall take place to determine whether compliance has been achieved. Depending on that determination, the following actions may occur:
      1. If all previous site violations have been corrected, the site reviewer shall issue an inspection report stating that fact and the site shall be returned to a normal construction review status.
      2. If previously noted violations have not been satisfactorily corrected, the further actions may be initiated.
2. Submissions from the general public - Members of the general public may submit information pertaining to this article to the Department of Community Development. The stormwater coordinator will consider such submissions as they pertain to the implementation and enforcement of this article and will provide written or verbal response to the person submitting the information.
3. Referrals from other agencies - Referrals from other agencies will be handled in the following manner:
   1. Cases will be referred directly to the stormwater coordinator. At this point the stormwater coordinator will determine if enforcement actions are warranted and if proper documentation has been obtained. If the stormwater coordinator determines that action is required, the enforcement process will be set into motion.
   2. Cases received by the stormwater coordinator will be handled on a first come, first served basis. All enforcement actions will be initiated by a site inspection to verify site conditions that caused the case to be referred. If conditions have been corrected or do not exist as stated in the referral, the case will be returned to file for documentation and reporting purposes. If conditions exist as stated in the referral, enforcement actions will proceed.
   3. Once site conditions have been verified and the site is determined to be in a state of noncompliance two avenues of enforcement can be pursued, one for the infrequent offender and one for the frequent offender.
      1. If an individual or company is being reviewed by the stormwater coordinator for the first time or it has been at least three years since the last violation (36 months has elapsed since last review), a notice to comply will be issued to the owner/developer informing them they are not in compliance with the City's codes, the steps needed to be taken to get into compliance, and that they have an established time frame to complete the work. At the end of the period the stormwater coordinator will reinspect to check for compliance. If all work has been satisfactorily completed the case will be returned to file for documentation and reporting purposes. If the work has not been satisfactorily completed within the established time frame a citation (ticket) will be issued to the owner/developer and follow-up will be done until the site is brought into compliance.
      2. If an individual or company has been reviewed by the stormwater coordinator at any time in the preceding 36 months they will be considered repeat offenders. Repeat offenders will be issued a citation (ticket) by the stormwater coordinator upon verification of noncompliance with the City's codes and after consulting with the Department of Community Development. Follow up will continue until the site has been brought into compliance.
4. Enforcement options for failure to comply are as follows:
   1. The City stormwater coordinator in conjunction with the Department of Community Development may issue a stop work order to any persons violating any provision of this article by ordering that all site work stop except that necessary to comply with any administrative order.
   2. The City stormwater coordinator may request that the Department of Community Development or building inspections office refrain from issuing any further building or grading permits until outstanding violations have been remedied.
   3. The city stormwater coordinator may initiate penalties as stipulated herein. Complete information concerning enforcement and penalties is described below.
5. Action without prior notice - Any person who violates a prohibition or fails to meet a requirement of this article will be subject, without prior notice, to one or more of the enforcement actions, when attempts to contact the person have failed and the enforcement actions are necessary to stop an actual or threatened discharge which presents or may present imminent danger to the environment, or to the health or welfare of persons, or to the storm drainage system.
6. Enforcement actions are as follows:
   1. Recovery of costs - Within 30 days after abatement by City representatives, the Department of Community Development shall notify the property owner of the costs of abatement, including administrative costs, and the deadline for payment. The property owner may protest the assessment before the City Council. The written protest must be received by the Mayor's office within 15 days of the date of the notification. A hearing on the matter will be scheduled before the City Council. The decision of the City Council shall be final. If the amount due is not paid within the protest period or within ten days of the decision of the City Council, the charges shall become a special assessment against the property and shall constitute a lien on the property for the amount of the assessment. A copy of the resolution shall be turned over to the county clerk so that the clerk may enter the amounts of the assessment against the parcel as it appears on the current assessment roll, and the treasurer shall include the amount of the assessment on the bill for taxes levied against the parcel of land.
   2. Termination of utility services - If the Department of Community Development determines that the disconnection of utility services will remedy or mitigate a violation of this article that poses a public health hazard or environmental hazard, they shall notify Rogers Water Utilities and request the disconnection of city water, sanitary sewer, and/or sanitation services.
   3. Performance bonds - Where necessary for the reasonable implementation of this article, the Department of Community Development may, by written notice, order any owner of a construction site or subdivision development to file a satisfactory bond, payable to the City, in a sum not to exceed a value determined by the Department of Community Development to be necessary to achieve consistent compliance with this article. The City may deny approval of any building permit, subdivision plat, site development plan, or any other city permit or approval necessary to commence or continue construction or to assume occupancy, until such a performance bond has been filed. The owner may protest the amount of the performance bond before the City Council. The written protest must be received by the Mayor's office within 15 days of the date of the notification. A hearing on the matter will be scheduled before the City Council. The decision of the City Council shall be final.
   4. Criminal prosecution - Any person who violates or continues to violate a prohibition or requirement of this article shall be subject to criminal prosecution to the fullest extent of the law, and shall be subject to criminal penalties.
7. Non-compliance with any provision of this article shall be deemed a violation. Any person violating this code shall, upon an adjudication of guilt or a plea of no contest, be fined according to the schedule of fines. Each separate day on which a violation is committed or continues shall constitute a separate offense.
8. Notwithstanding any other remedies or procedures available to the City, if any person discharges into the storm drainage system in a manner that is contrary to the provisions of this code, the senior staff attorney may commence an action for appropriate legal and equitable relief including damages and costs in any court of competent jurisdiction. The senior staff attorney may seek a preliminary or permanent injunction or both which restrains or compels the activities on the part of the discharger.

Primary objectives of the CSKRegulationsand this chapter include:

1. Provide additional protection for the Cave Springs Recharge Area through the use of stream buffers, runoff reduction practices, filtration, source controls, construction practices and control measures, wastewater policies and practices, requirements for buried facilities that are potential contaminant sources, and spill prevention and control practices to protect the quality of water entering the groundwater system.
2. Reduce the rate at which surface water is entering the shallow groundwater system that sustains the cavefish through disconnection of impervious area, detention and filtration.
3. Provide a reasonable and practical framework for water quality protection that recognizes risk and vulnerabilities and how site-specific circumstances and mitigating factors including stormwater control measures/BMPs must be taken into account, building upon the technical foundation and recommendations made by OUL and The Nature Conservancy.
4. Develop practical criteria and methods to enhance treatment of detained runoff in the Direct Recharge Area that will allow for development of the area in a responsible manner that protects water quality through implementation of BMPs.

Exhibit 10-2 of the CSK Regulations illustrates the geographic extent of the Direct Recharge Area. Municipal boundaries of the Cities of Cave Springs, Rogers, Lowell, and Springdale are shown on Exhibit 10-1. In addition, Exhibit 10-2 illustrates the following key features of the Recharge Area:

• Losing Streams –These reaches were identified through hydrogeologic analysis and tracer studies by OUL (OUL 2015). Note that not all reaches of streams are identified as losing streams – upper reaches of some streams further east and north of Cave Springs are not identified as losing reaches.
• Cave Springs Groundwater Trough – The trough extends east from Cave Springs and is an area of rapid groundwater recharge and generally poor to fair soil treatment capability. The trough encompasses an area of approximately 1.8 square miles and is an area of heightened vulnerability for the karst system.
• Soil Treatment Capability – These ratings are based on soil gradation and infiltration characteristics. In general, soils with more rapid infiltration have lower treatment capability and higher potential for direct recharge to the karst system. Soils are classified as follows:
  • Good Treatment Capability Soils - The major soil series in this group are Captina and Peridge. Captina and Peridge Series soils are in this group because they have a high percent of material that will pass through a Number 4 sieve (particle size less than 4.76 millimeters). These are loamy materials, excluding gravels and coarse materials that provide less filtration, and provide good natural cleansing. When constructing stormwater detention basins the upper horizons of these soils can be stockpiled and amended to create a media filtration layer to blanket the bottom of detention ponds to enhance contaminant removal.
  • Fair Treatment Capability Soils - Major soil series that are in this group are the Nixa and Tonti soil series. Nixa soils are in this group because they have a relatively low percentage of material that will pass through a Number 4 sieve and because they have a fragipan typically located 17 to 30 inches below the surface. If undisturbed, soils above the fragipan become saturated during wet-weather conditions and appreciable lateral flow often occurs along preferential flow routes toward discrete recharge zones where the fragipan is breached or where other soil series lacking a fragipan exist. If the fragipan is breached in constructing stormwater detention ponds the low percentage of material that will pass through a Number 4 sieve and the moderate permeability of the soils above and below the fragipan provide ineffective natural cleansing for passing waters. Tonti Series soils are in this group because they have more fine textured material in their upper horizons than the Poor Treatment Capability Soils. However, below a depth of about a foot and a half the Tonti Series soils have characteristics similar to Poor Treatment Capability Soils.
  • Poor Treatment Capability Soils - The major soil series in this group is the Noark soil series. These soils are classified as poor for treatment capability because they have high percolation rates and a relatively low percentage of material that will pass through a Number 4 sieve. The texture is too coarse to provide much of the natural cleansing present in soils.
• I-49 Corridor – The I-49 corridor lies in the southwestern portion of the Indirect Recharge Area. ARDOT and the USFWS have developed a plan for BMPs along this corridor that includes lined ponds and other practices. Because BMPs have already been developed for this area, this area is not addressed in the chapter.

Implementation of BMPs in accordance with the Drainage Criteria Manual is required throughout the Direct and Indirect Recharge Area. Conservation Regulations and criteria in this chapter, which provide for buffers and enhanced treatment methods, are required only in the Direct Recharge Area.

Exhibit 10-2 of the CSK Regulations provides mapping of risk-based water quality protection zones for the Cave Springs Recharge Area. Zones are color-coded by the zone designations 1-4:

Zone 1 (Extremely High Vulnerability) represents the highest risk to the water quality and hydrology of the groundwater system. These are lands where the hydrobiological setting and existing and/or foreseeable land uses pose extremely high risks of groundwater impacts that could potentially adversely affect Ozark Cavefish and the associated biological community. Zone 1 includes areas that are mapped for:

• Poor soil treatment capability - highly permeable or karst soils at shallow depths that directly and rapidly recharge the groundwater system.
• Proximity to losing stream corridors and other sensitive features - areas within a zone of high vulnerability from losing streams (measured from center of stream), or other known direct/high- permeability recharge areas included in Zone 1.
• Areas within the Cave Springs Groundwater Trough are included in Zone 1.

Zone 2 (High Vulnerability) is an area that has a high risk to water quality of the groundwater system. These are lands where the hydrobiological setting and existing and/or foreseeable land uses pose high risks of groundwater impacts with potential to adversely affect Ozark Cavefish and the associated biological community. This area has some overlying soils that can provide filtration of stormwater; however, excavations for development projects, including utilities, building foundations, detention and water quality ponds have the potential to penetrate to highly permeable or karst layers.

Zone 3 (Moderate Vulnerability) is an area with moderate risk to the groundwater system water quality based on native soils that have good potential for filtration, greater thickness of overlying soil layers, and greater distance from sensitive features. These are lands where the hydrobiological setting and existing and/or foreseeable land uses pose moderate risks of groundwater impacts likely to adversely affect the Ozark Cavefish and the associated biological community. Within the Direct Recharge Area these are typically upland areas underlain by soils capable of removing many contaminants. They are remote from sinkholes or losing streams and are areas where land use does not include localized groundwater contamination hazards, such as suburban development utilizing on-site disposal of sewage or concentrated or confined animal operations (including poultry).

Zone 4 (Low Vulnerability, Indirect Recharge Area) is an area of relatively low risk to the groundwater system. These are areas that contribute relatively minor amounts of water to the groundwater system recharging Cave Springs cave and are unlikely to have significant deleterious impacts on water quality and cave fauna. Chapter 9 of the Drainage Criteria Manual is applicable to Zone 4. The I-49 corridor is a part of Zone 4. The existence of I-49 poses greater groundwater quality vulnerability than the Indirect Recharge Area lands located further to the east. Because ARDOT and USFWS have developed approved plans for BMPs for the corridor, the area is included in Zone 4.

For an amendment to the vulnerability zone designation, a property owner may apply to change the vulnerability zone designation from vulnerability zone 2 or zone 3 to vulnerability zone 3 or zone 4 pursuant to the procedures and review criteria set forth herein.

The application for an amendment to the vulnerability zone designation shall include the following minimum information:

1. A site map or diagram depicting the following features:
   1. Delineation of inner buffer and outer buffer as determined by these CSK regulations.
   2. Slope study map that indicates the areas of less than three percent grade and areas of three percent or greater grade in the inner buffer and outer buffer areas.
   3. Areas of erosive soils.
   4. Areas with poor vegetative cover and areas of existing erosion.
   5. Unstable stream reaches.
   6. Storage areas for hazardous materials, fertilizers, or pesticides.
   7. Wetlands and waterbodies.
   8. Sanitary wastewater collection, storage, treatment, pumping facilities.
2. A map or diagram separately depicting the boundaries of the Cave Springs groundwater trough, the boundaries of zone 1, zone 2, and zone 3 vulnerability areas, depicting the boundary of losing streams, and depicting the boundary of losing streams as defined on the Cave Springs direct recharge area vulnerability zone map as it affects the proposed development site.
3. A detailed analysis that accurately depicts the soil and hydrogeological conditions on the subject property.
4. The Director of the Department of Community Development or his or her designee or review authority may request additional information, studies, or peer review as deemed appropriate and relevant to providing sufficient information to evaluate the application for compliance with the applicable review criteria.
5. The burden of proof shall be on the applicant to demonstrate that the existing vulnerability zone designation is not appropriate and that a new vulnerability zone designation is clearly warranted.

The Planning Commission shall review and make a recommendation and the City Council shall review and take final action to approve or disapprove an application to change the vulnerability zone designation on the subject property.

The review authority shall use the following review criteria as the basis for a decision on an application to change the vulnerability zone designation:

1. The application clearly demonstrates and provides convincing evidence that the soil and hydrogeologic conditions on the entire subject property warrant inclusion in the requested vulnerability zone district; and
2. The change of vulnerability zone district designation will not create a non-uniform or haphazard vulnerability zone district map, or result in vulnerability zone designations that split a property that will complicate administration and implementation of these CSK regulations on a property by property basis

Inner restrictive buffers are generally “no development” zones. Planning for development should seek to preserve these areas and minimize impacts from utilities, roads and other features that must cross or encroach on the buffer area. Inner buffer widths vary by zone from 100 feet in the Extremely High Vulnerability zone to 50 feet in the High Vulnerability zone to 25 feet in the Moderate Vulnerability zone.

Allowable uses/activities within inner buffers include:

• Utility crossings – Buffer areas should not be viewed as preferred utility corridors but, if necessary, can be used for a development. In some cases utility crossings with enhanced BMPs may be required. When a crossing or encroachment cannot be avoided, it is allowable subject to the Drainage Criteria Manual erosion and sediment control requirements and additional BMPs listed herein. An approved Land Disturbance Permit from the City of Rogers is required prior to construction activities.
• Open space – Open space or park uses with heavy concentrations of animals (dog parks, horse paths, etc.) are not allowed in the inner buffer, but many other open space uses are compatible.
• Trails, biking/hiking paths – Pedestrian paths also provide benefit of maintenance access along stream corridors.
• Herbicide use in native landscaped areas should be as limited as possible within the buffer zone to small spot treatments. No utility corridor spraying is allowed. Herbicides must not be used when there is ponded or flowing water on the surface, all labeled instructions must be followed.
• Road and bridge crossings constructed in accordance with applicable water quality regulations – the number of crossings should be minimized to the extent practical through land planning.
• Wetland mitigation and stream stabilization/restoration projects.
• Projects to enhance or restore functions of the buffer or stream. Buffer restoration is one of the factors that can be used to reduce the width of the outer buffer.
• Stormwater BMPs that cannot feasibly be located in the outer buffer or that must be located in the inner buffer to achieve desired functions.
• Maintenance activities associated with these allowable uses.
• Uses and activities that are determined by the Director of the Department of Community Development or his or her designee to be similar to the uses and activities described above.

Prohibited and restricted use/activities within inner buffers include:

• Grading, stripping, or other soil-disturbing practices should be minimized to the extent practical.
• Filling or dumping or storage of material not related to a permitted use or activity.
• Draining the buffer area by ditching, underdrains, or other systems, or any grading or excavation work which has the effect of draining that buffer area which is not related to a permitted use or activity.
• Use, storage, or application of pesticides, herbicides (except as permitted), fertilizers, or hazardous/toxic materials.
• Fueling facilities and bulk storage of fuel or petroleum products above or below ground.
• Storage, repair, or operation of motorized vehicles other than for maintenance or emergency use.
• Structures or impervious surfaces, with the case-by-case exception of paved trails (preferred for maintenance) and associated facilities such as picnic tables/sitting areas subject to the requirements of a Land Disturbance Permit.
• Land application of biosolids.
• Other activities or land uses that are determined by the Director of the Department of Community Development or his or her designee to pose an unacceptable risk to water quality of the receiving waters and cave system.

The outer variable-width buffer (outer buffer) is in addition to the inner buffer. The outer buffer can be reduced, as described below, to a minimum width for each zone. Inner and/or outer buffers are not required for Zone 4. The outer buffer, with a maximum width of 300 feet in Zone 1 (inclusive of restrictive inner buffer), is intended to encompass sensitive site features including high-permeability recharge areas, karst features, losing streams, and areas that have high potential to contribute to contaminant loading. Areas within the outer buffer that pose high risks to water quality including erodible soils, areas of erosion/poor vegetative cover, steep slopes, existing areas of hazardous material, or waste storage shall be mapped. To the extent that these or other known risk factors to water quality exist outside of the outer buffer on a site, these areas must be mapped by the applicant as a part of the Land Disturbance Permit Application, and BMPs must be provided to mitigate potential water quality impacts.

Allowable activities and land uses in the outer buffer are the same as those allowed in the inner buffer and also include BMPs associated with a project that provide similar functions to the buffer or enhance functions of the buffer. Additional land uses, including those that consist of mostly pervious areas, can be incorporated within the buffer, depending on site-specific conditions, BMPs, and other factors that justify reductions in the maximum extent of the outer buffer, as determined by an Arkansas-registered Professional Engineer.

The width of the outer buffer may be reduced depending on site-specific conditions, BMPs, and other factors including the following:

• Soil treatment capability
• Land use characteristics
• Losing stream corridors
• Average ground slope
• Buffer zone filtration characteristics
• Stormwater detention and stormwater treatment practices
• Wastewater disposal quantities and quality
• Proximity to Cave Springs Groundwater Trough

BMPs that are designed that provide similar functions to the buffer upon which they encroach in terms of water quality may be used to justify reductions in the outer buffer width. Site-specific conditions, which justify a variance, may be used to support a reduction in the outer buffer width. Restoration activities within degraded buffers to improve function are encouraged and can be used to reduce the overall outer buffer width while stabilizing degraded areas adjacent to streams.

Inner buffers are to be maintained in a “natural” condition that promotes dense and diverse vegetation. Periodic maintenance may involve removing fallen trees, repairing areas of erosion, weed control and vegetation management. A “manicured” turf appearance is generally undesirable in the inner buffer and landscaping and maintenance practices such as less frequent mowing can help to discourage foot traffic and other urban/suburban activities in this area.

The outer buffer may be used for a broader set of activities than the inner buffer, and maintenance should be performed to keep a dense vegetated cover. Typical activities include mowing, weed control, clearing of debris/fallen trees, spot revegetation, and aeration.

Inner and outer buffer areas should be maintained in accordance with BMP SC-10 Vegetative Buffers in Chapter 9. Given the extent of buffers used in the Direct Recharge Area it will generally only be necessary to perform sediment removal in portions of buffers adjacent to disturbed and/or developed areas. Areas should be revegetated to match adjacent buffer areas following sediment removal.

Non-structural source control BMPs are required in all zones and are important for keeping sources of pollution from getting into the karst system. Education on source control BMPs is key to ensuring that BMPs are implemented and that the public is aware of the water quality concerns with practices such as petroleum and chemical storage, etc. Preservation of buffers is one of the most effective non-structural BMPs and is a major component of the water quality protection strategy for the Cave Springs Recharge Area. Chapters 8 and 9 of the Drainage Criteria Manual provide guidance on many non-structural practices that are applicable to the Direct Recharge Area. The following lists additional requirements related to selected practices:

1. BMP CM-1 Construction Sequencing and Phasing – The Land Disturbance Permit Application and Report should describe how disturbances within buffer areas are minimized or avoided. Disturbances in buffer areas should be promptly revegetated and mulched to provide cover. Phasing is limited to 20 acres of disturbance in Zone 1 and 40 acres of disturbance in Zone 2.
2. BMP CM-2 Hazardous Waste Management and Chemical Storage – All requirements of CM-2 apply both to construction and post-construction activities involving hazardous wastes and chemical storage. In addition, storage of hazardous materials and chemicals that are potential contaminants is not allowed within the inner or outer buffer areas. These requirements apply to construction activities as well as the long-term use of the site.
3. BMP CM-3 Solid Waste Management – Solid and liquid wastes should not be stored within buffer areas and areas/containers for liquid waste disposal should be watertight and covered. For commercial, industrial and office sites, waste storage areas and other activities that generate waste should be located on a portion of the site outside of the buffer area.
4. BMP CM-4 Concrete Washouts – Concrete washouts are not allowed in buffer areas. Concrete washouts should be avoided in Zone 1 to the extent practical, and those installed in Zones 1 and 2 should be lined with a 30-mil plastic liner. A 10-mil liner may be used in Zone 3.
5. BMP CM-5 Construction Staging and Access – Construction staging is not allowed in buffer areas and access to buffer areas where work is occurring should occur at limited, controlled access points. Vehicle maintenance and fueling must occur outside of buffer areas.

In general, land uses requiring Spill Prevention, Containment, and Control (SPCC) Plans (SPCC Regulations in Title 40 Code of Federal Regulation Part 112) for bulk storage of fuel or other sites for storage of waste (junkyards, landfills, transfer stations, etc.) are not allowed in the Direct Recharge Area. To the extent that a project is proposed requiring creation or modification of a SPCC Plan within the Recharge Area, the City will review the SPCC Plan and may add requirements as appropriate for protection of the karst groundwater system.

Construction BMPs in Chapter 8 of the Drainage Criteria Manual apply throughout the Cave Springs Recharge Area. In the Direct Recharge Area, special care should be taken with construction BMPs because of the adverse affects of sediment and turbidity in discharges to karst areas. The BMPs identified in Section 4.1 Working in or Crossing a Waterway should be followed for any construction activities within the inner buffer, including work on utilities, roads and bridges.

The following requirements apply in addition to the criteria in Chapter 8:

1. Disturbances in inner and outer buffer areas should be minimized to the extent practical.
2. Topsoil and soil that is rated as having “good” treatment capability must be segregated and stockpiled for use in revegetation and/or as a primary component of the filtration media mix for enhanced treatment in detention facilities.
3. Redundant perimeter controls are required between areas of disturbance and designated undisturbed buffer areas (double row of silt fence, double row of wattle, berm and wattles, etc.) in Zones 1 and 2.
4. Construction should be phased to limit the maximum area of disturbance at a given time to 20 acres in Zone 1 and 40 acres in Zone 2.
5. If a karst feature (spring, sinkhole or other direct recharge feature associated with karst hydrology) is encountered during construction, construction in the vicinity should be halted and the area protected with redundant perimeter controls. An Arkansas-registered Professional Geologist or Engineer should evaluate to recommend design modifications to limit impacts and direct recharge of runoff.
6. Construction dewatering discharges must receive treatment prior to discharge to buffer areas. Treatment methods including filter bags, sedimentation basins and/or sheet flow/infiltration are required upgradient of buffer areas. Criteria in Chapter 8 BMP CM6 Construction Dewatering Dischargesapply.

The minimum requirements applicable to all zones are those in Chapter 9 of the Drainage Criteria Manual. These BMPs include runoff reduction measures, providing the water quality capture volume (WQCV), implementing source controls, and stabilizing drainageways. Additional requirements apply in Zones 1, 2, and 3.

Future state and/or federal highway projects within the Cave Springs Direct Recharge Area must provide BMP treatment systems comparable to or exceeding the system that has been approved for the I-49 corridor. These systems may include lined ponds, filtration practices and other BMPs.

The need for a gravity sewer collection system is paramount in the City of Cave Springs especially, but also in the City of Lowell as it relates to vulnerability of the Direct Recharge Area groundwater quality. Based on data from the Northwest Arkansas Conservation Authority (NACA) studies and rough sewer interceptor plans, preliminary cost to bring a 24” interceptor from NACA to the Osage Creek crossing of Highway 264 just west of Cave Springs involves 25,800 linear feet at $350+ per linear foot or $9,000,000±. Several million dollars more will be required to tie in existing subdivisions currently using pumped effluent systems. STEPP systems should be avoided. Meanwhile, recommended policy for sewer service in new subdivisions will be to require gravity sewer systems to each lot with a “drainage basin” type lift station installed to collect sewage that when gravity sewer is available would provide one point for connection to said new gravity sewer.

With exception of service lines, all public and privately maintained pressure and gravity sewer lines located within 300 feet of a losing stream, anywhere in Zone 1 or the Cave Springs Groundwater Trough, must meet all criteria and design standards of the Cities of Rogers or Springdale (which are based on the Ten State Standards) and Cave Springs pumped effluent systems until a gravity sewer system is available. As shown in Exhibit 10-5, these sewer lines must be constructed using geomembranes installed in the bottom of the sewer trench under the pipe bedding up the sides of the trench to 12” above the top of the pipe. Trench dams shall be installed at 400’+/- intervals and at mainline branches. These trench dams shall extend 6” into undisturbed soil on the bottom and sides of the trench and must extend to within 12” of finished grade. Trench dams may be composed of bentonite clay, flowable concrete fill, manufactured anti-seep collars, or other widely accepted materials and methods of practice.

An 8” PVC monitoring well shall be installed on the upstream side of each trench dam from the surface down to the sewer pipe flow line designed to intercept any flow along or immediately beneath the sewer pipe. Each monitoring well shall be sampled annually for dye that will be introduced into the upstream sewer main to check for sewer leakage. As an alternative to dye tracing, the utility owner may implement a monitoring program to test annual samples for the presence of wastewater by commonly accepted laboratory practice or implement an annual inspection program capable of detecting defects and exfiltration points in sewer mains.

All pressure and gravity sewers in Zones 1, 2, 3 & 4 must meet all criteria and design standards of Rogers or Springdale Water Utilities.

All wastewater lift stations shall be equipped with two or more pumps such that the wastewater lift station is capable of pumping the total rated capacity with one pump out of service.

A control system with remote transmission of data or equivalent shall be implemented at wastewater lift stations. Graphical display screens shall show, and the control system shall be capable of transmitting, the instantaneous station discharge, wet well water level, pump run times, pump starts, pump status (such as running, off, or alarm conditions), power status, and standby generator status. Wastewater lift station alarm systems shall transmit and identify alarm conditions to a municipal facility that is staffed 24 hours a day. If such a facility is not available, the alarm shall be transmitted to municipal offices during normal working hours and to the home of the responsible person(s) in charge of the lift station during off- duty hours. The control system and communication method shall be coordinated with the local municipal agency for integration with their SCADA system and operational procedures.

Fuel or gas standby generators with appropriate fuel storage containers, or an approved alternative secondary power supply, shall be provided as a backup power source for wastewater lift stations. In the event of a power failure, the redundant power system shall automatically operate the wastewater lift station, shall have sufficient capacity to start up and maintain the total rated running capacity of the station. A portable pump connection to the force main with rapid connection capabilities and appropriate valving shall be provided outside the dry well and wet well, similar to Exhibit 10-6.

Industrial and commercial land uses are not allowable in Zones 1 or 2 of the Direct Recharge Area per zoning without a variance, and discharges requiring permitting under DEQ industrial stormwater and/or wastewater discharge permits are prohibited in these zones. Under special circumstances, these types of land uses may be allowed when there is not a feasible location outside of Zone 1 or Zone 2; however, this would require a special Land Disturbance Permit review process to identify BMPs that would adequately protect water quality given the nature of the discharge and the proximity to direct recharge areas. In cases where a variance is allowed the applicant must demonstrate that, with BMPs, the proposed land use will pose no higher risk to water quality than permitted land uses.

For commercial and industrial land uses or other land uses requiring a permit from DEQ and/or a federal agency in Zone 3, the requirements of Chapter 9 and this chapter apply. Any potential stormwater, wastewater, SPCC or other plans and associated permits must be submitted as a part of the Land Disturbance Permit Application.

Bulk storage of fuel and other hazardous materials is discouraged in the Direct Recharge Area because of the threat that spill and leaks pose to groundwater quality. Bulk storage of fuel and similar substances must comply with SPCC regulations and hazardous materials and waste must be managed in accordance with the Resource Conservation and Recovery Act (RCRA).

The following additional restrictions apply:

1. Underground storage tanks are prohibited in Zones 1 and 2.
2. Aboveground storage of bulk fuel or other hazardous materials is also prohibited in Zones 1 and 2.
3. Underground storage tanks are allowed in Zone 3; however, any such underground storage tanks must have secondary containment.
4. Aboveground storage of bulk fuel or other hazardous material is allowed in Zone 3 with appropriate secondary containment, spill prevention, control and countermeasure BMPs and appropriate federal, state and local regulations.

The plan sheets for improvements shall be submitted on 22”x34” sheets with all sheets in a plan set being the same size. Plan drawings shall be of an appropriate scale to be legible; the suggested scale is typically 1”=100’. Legibility will be determined by the City’s engineer or planning staff. Profile drawings shall be provided for all storm sewers and drainage ditches at a suggested scale of 1”=20’ horizontal and 1”=5’ (minimum) vertical.

Plan sheets shall conform to generally accepted engineering practices; special conditions may require additional information.

The title sheet shall include:

• Project name, nature of the project, city and state.
• Index of sheets.
• A location or vicinity map showing the project in relation to existing streets, railroads and physical features. The location map shall have a north arrow and appropriate scale.
• A project control benchmark identified and referenced to the City of Rogers GPS control monuments.
• The name and address of the owner of the project and the engineer preparing the plans.
• Engineer’s seal, signature and date.

In general, layout sheets shall contain to the following:

• North arrow and scale.
• Legend of symbols.
• Name of project.
• Boundary line or project area.
• Location and description of existing major drainage facilities within or adjacent to the project area.
• Location of proposed drainage facilities, including cross sections or reference to detail sheets.
• Location and description of utilities within or adjacent to the project area.
• Provide match lines if more than one sheet is necessary.
• The date, registration seal and signature of the Engineer of Record.
• Elevations shown in the plans shall be based on City of Rogers GPS control monuments.
• The top of each page shall be either north or west. The stationing of street plans and profiles shall be from left to right and downstream to upstream for channels.
• Show topography a minimum of 20’ beyond the project area; 50’ for channel improvements.
• Show existing and proposed property and easement lines with dimensions.
• Minimum finish floor elevations shall be shown a minimum of 3-feet above the 100-year water surface elevation on each lot when located in a designated floodplain, areas adjacent to a designated floodplain and in areas where flooding is known to occur. All occupied buildings, whether in or out of a designated floodplain shall have the finished floor elevation a minimum of 12-inches above the land immediately surrounding the building and all buildings in a subdivision are required to be have the finish floor 12” above the curb per the Subdivision Ordinance.
• Include current City of Rogers Standard Details as needed.

There are two major goals with respect to floodplain management:

Floodplain Management Goal 1 - Reduce the vulnerability of the residents in the City of Rogers to the danger and damage of floods.

Floodplain Management Goal 2 - Preserve and enhance the natural characteristics of the City’s floodplains.

These two goals are achievable through appropriate management shared by the agencies involved. A multi-pronged approach to achieve the floodplain management goals described above is summarized below:

• Adopt effective floodplain regulations.
• Appropriately modify local land use practices, programs, and regulations in flood-prone areas.
• Provide a balanced program of measures to reduce losses from flooding.
• Foster the preservation and/or creation of greenbelts, with associated wildlife and other ecological benefits, in urban areas.

Floodplain management practices must be implemented to be of value. Although hydrologic data are critical to the development of a floodplain management program, the program is largely dependent on a series of policy, planning, and design decisions.

Flood insurance should be an integral part of a strategy to manage flood losses. The City is a participant in the National Flood Insurance Program (NFIP), which is administered by the Federal Emergency Management Agency (FEMA). As a participant, the City must maintain and enforce regulations meeting minimum requirements of the NFIP and restricting development in designated flood hazard areas shown on FEMA Flood Insurance Rate Maps (FIRMs). Federal requirements mandate that flood insurance be purchased for mortgaged properties within a FEMA flood hazard area. Because the City is an NFIP participant in good standing, all property owners in the City are able to obtain flood insurance for their property with premiums based on the flood hazard zones shown on the FIRM. For additional information related to flood hazard zones, refer to the City of Rogers Code of Ordinances Chapter 22 FLOODS(http://library.municode.com/).

While floodplain management includes some utilization of the flood fringe (i.e., areas outside of the formal floodway), the planner and engineer should proceed cautiously when planning facilities on lands below the expected elevation of the 100-year flood. Flood peaks from urbanized watersheds are high and short-lived, and filling the flood fringe tends to increase downstream peaks. Per Chapter 22 – Floodplain Administration – any fill material introduced within the boundaries of the 100-year flood and below the expected elevation of the 100-year flood will be required to be mitigated with an equal amount of cut somewhere else onsite within the floodplain boundary.

The following type of maps can be referenced to identify flood-prone areas in the City of Rogers for use in drainage planning. (FEMA) Flood Insurance Rate Maps (FIRM) are an important tool to assist with good floodplain management. The National Flood Insurance Act of 1968 established the National Flood Insurance Program (NFIP), which included a national floodplain mapping effort. Certain areas in the City of Rogers have been designated as floodplains and are regulated as required by the NFIP. While these maps were created to indicate risk factors for determining appropriate flood insurance rate premiums, they are also useful for designating flood prone areas. Anyone considering developing property in the City of Rogers should obtain a copy of the FEMA FIRM panels and understand the effects any floodplain may have on a proposed development. Refer to Map Panel ID No. 05007CIND0A for an Index Map of the FIRM panels in the City of Rogers area (FEMA 2009).

The runoff coefficient, C, in the Rational Formula is also dependent on the character of the surface soil. The type and condition of the soil determines its ability to absorb precipitation. The rate at which a soil absorbs rainfall typically decreases if the rainfall continues for an extended period of time. The soil absorption or infiltration rate during a rainfall event is also influenced by the degree of soil saturation before a rain (antecedent moisture condition), the rainfall intensity, the proximity of ground water, the degree of soil compaction, the porosity of the subsoil, vegetation, ground slopes, and surface topography (or relief). Detailed soil information is described in Section 3.3.1 – Hydrologic Soil Group.

The runoff coefficient, C, is the variable of the Rational Method which is most difficult to precisely determine. Proper selection requires judgment and experience on the part of the design engineer, and its use in the formula implies a fixed ratio for any given drainage area over the course of a rainfall event, which in reality is not the case. A reasonable runoff coefficient must be chosen in order to determine accurate volumes for runoff.

To standardize City design computations, Table RO-2 provides standard runoff coefficient values based on current zoning and land use designations. However, if the designer chooses, Table RO-3 provides runoff coefficient values for specific types of land/surface areas that can be used to evaluate a composite analysis that may provide a more accurate runoff coefficient value for an area.

Additionally, the values in Table RO-2 and Table RO-3 are typical for design storms with recurrence intervals of 1 to 10 years. For less frequent recurrence intervals (i.e., larger storm events), the runoff coefficient, C, must be adjusted upward using the correction factors shown in Table RO-4 due to saturated soil conditions that typically increase the runoff during larger storm events. Table RO-4 contains correction factors for the 1-, 5-, 10-, 25-, 50-, and 100-year events. To determine the appropriate runoff coefficient for these events, the runoff coefficient from either Table RO-2 or Table RO-3 shall be multiplied by the appropriate factor in Table RO-4.

A convenient tool that complements the City’s Manual is the RDM-Rational Method spreadsheet. The Weighted C tab within this spreadsheet calculates the area-weighted runoff coefficient given the collective areas and corresponding runoff coefficients for subareas within the watershed being analyzed. Refer to Section 4.0 for additional information on using this spreadsheet. All composite analyses shall be completed using the Weighted C spreadsheet and included in the drainage report.

Overland flow occurs over plane surfaces. With overland flow, the effective roughness coefficient (Manning’s n value) includes the effect of raindrop impact; drag over the plane surface; obstacles such as litter, crop ridges, and rocks; and erosion and transportation of sediment. Table RO-6 gives Manning’s n values for sheet flow for various surface conditions. These n values are for overland flow depths of approximately 0.1 foot.

The overland flow time, t_o, may be calculated using Equation RO-3:

(Equation RO-3)

in which:

t_o = overland flow time (minutes)

n = Manning’s roughness coefficient (Table RO-6)

L = length of overland flow in feet (300-ft maximum in non-urban areas; 100-ft maximum in urban areas)

P₂= 2-year, 24-hour rainfall (inches) calculated from Table RO-5 (or obtained from Table RO-9)

S = average basin slope (feet-per-feet) expressed as a decimal

Equation RO-3 is a simplified form of the Manning’s kinematic solution, taken from TR-55(1986), and is based on the following assumptions:

1. shallow steady uniform flow
2. constant intensity of rainfall excess (that part of a rain event available for runoff)
3. rainfall duration of 24 hours, and
4. minor effect of infiltration on travel time

Rainfall depth can be calculated from Table RO-5 (and/or can be obtained directly from Table RO-9). Engineering judgment should be used when determining the maximum overland flow distance. For example, in non-urban, gently sloping areas, with ground cover in good condition a maximum overland flow distance of 300-feet can be used. But in urban areas, where more impervious areas exist and ground cover condition is poor a maximum length of 100-feet shall be used. The engineer needs to be aware under what conditions and in what areas overland flow transitions to shallow concentrated or channelized flow when determining the overland flow distance. If the overland flow time is calculated to be in excess of 20 minutes, the designer should check to be sure that the time is reasonable considering the projected ultimate development of the area.