A.   Purpose: The purpose of this document is to direct land development in such a way as to minimize the negative effects or impacts that increased storm water runoff can have on ground water, surface water, and public and private property. It is the specific intention of the planning administrator to require natural, on site control of storm water and to prohibit the release of untreated storm water, sediments, and other materials from development sites.

   B.   Best Management Practices: This natural control can be afforded by a series of techniques collectively called best management practices (BMPs). These BMPs and their applications are described and referred to throughout ordinance 364, on file in the office of the city clerk. In some situations BMPs may not allow certain specific development goals. For these situations more structural storm water controls may be used and some are discussed herein. In either case, however, the collection and treatment efficiencies of the BMPs shall stand as the measure against which the alternative measures must perform. The control of storm water runoff and its effects as a result of BMPs or equivalent technologies is considered to be the essence of storm water management.

   C.   Institutional Controls Plan: Most of the terrain in the city of Kellogg and its area of city impact are either mountainous or in the floodplain. This area has a long history of mining and smelting activities which have left a variety of wastes, including high concentrations of lead, arsenic, antimony, copper, cadmium, mercury, and zinc. A twenty one (21) square mile area of the Silver Valley, which includes the city of Kellogg, was declared a superfund site by the EPA and has undergone extensive remediation of hillsides, areas of mining

   The size and complexity of the site have resulted in the extensive use of capping and contaminants stabilization techniques. The ICP was developed to ensure that these contaminants remain controlled and away from direct human contact to the greatest extent possible. "Institutional controls" are defined as nonengineering mechanisms used to prevent or limit access to contaminated soils, thereby eliminating health risks. The ICP at the Bunker Hill superfund site (hereinafter referred to as the site) includes a barrier management program (barriers are the physical means of limiting human contact with contaminated soils; barriers can be as simple as a layer of clean soil, asphalt paving, or a concrete sidewalk, but may also include access controls such as fencing) to control large excavations and building renovations, an education program to promote barrier awareness, a testing and monitoring program, and a disposal program for contaminated materials which have been removed. PHD is the manager of the ICP. The program maintains a property tracking system to assist lending institutions as they continue to complete property transactions valley wide, in addition to the education and enforcement of the ICP.

   D.   Areas Of Concern: The areas of greatest concern within the site with regard to contaminants are the low lying areas, especially the floodplain areas. The sediments which have been washed from the hillsides over the last one hundred (100) years have collected in these areas and are associated with high concentrations of contaminants to depths up to ten feet (10') in areas, creating a considerable remediation challenge. Although the hillsides have contaminated soils, they are usually of a superficial nature and may be remediated by using methods as simple as deep tilling, which mixes the surface soils with the clean underlying material and lowers the contaminant concentrations to acceptable levels. Soil pH is also a concern in many areas. Soils with pH as low as four (4) or five (5) are common within the site with some as low as two (2). There is the existing hazard of mine dumps and tailings located on the hillsides and these should be avoided when possible. For the most part, the need for barriers is limited to the floodplains, mine waste areas, and populated areas within three (3) miles of the smelter site.

   E.   Intent Of Regulations: It is the intent of these regulations to limit migration of these contaminants, as well as direct human contact with them, to the extent possible by controlling storm water.

   F.   Acknowledgments: The storm water control measures discussed in this manual are supplemented versions of the "Best Management Practices For Storm Water Management And Erosion And Sedimentation Control" and the "Storm Water Management Plan Criteria And Engineering Standards", prepared for the interagency storm water committee of Kootenai County by Kennedy Engineers of Spokane, WA. They have been edited to reflect the environmental conditions and concerns present in the site. Although they contain provisions for storm water control in areas of heavy metal contamination and low pH, they are still useful for, and it is recommended that they be practiced in, areas within Shoshone County which includes the city of Kellogg which do not suffer the environmental problems of the site.

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)

   The following performance standards shall be applicable to the design, construction, and maintenance of all storm water management systems developed pursuant to the storm water management ordinance:

   A.   It is the specific intention of the planning administrator that there shall be as little increase in the peak rate (flow) of surface runoff from a site as possible when compared with the rate which is determined to have existed from the natural, undeveloped state of that site. Designs and contingencies shall accommodate runoff events up to and including that peak flow from a 25-year storm event (see article G of this chapter).

   B.   All sediment resulting from site development activities shall be detained on site during runoff events to the extent possible. Detained sediment shall be either stabilized on site or removed in an approved manner. Contaminated sediments shall be handled and disposed of in a manner and place approved by the PHD and the Idaho department of health and welfare (IDHW-DEQ).

   C.   No storm water shall be collected or concentrated except in on site channels which are projected from erosion (through the establishment of ground cover [vegetation or other], installation of energy dissipating structures and/or design of sufficiently low velocity producing slopes) (see article D of this chapter). The potential effects of on site collected and concentrated storm water, especially that from greater than design storm events, to off site areas shall be considered during project design.

   D.   All storm water from the completed development shall be directed to grassed infiltration areas (GIAs), where possible and appropriate, for treatment by natural biological, chemical and physical processes which occur during infiltration (percolation) into the ground. Grassed infiltration areas (or their acceptable alternatives) shall be sized to store and treat the first one-half inch (1/2") of runoff draining from the "impervious surface area from a storm (precipitation or snowmelt) event (see article F of this chapter). Further, the GIA shall be designed such that excess runoff volumes overflow to a drywell or other approved on site system without damage to that storm water system or adjacent land improvements. If the use of a GIA is impractical due to slope or soil type, a written explanation of why must accompany the storm water plan when it is submitted to the planning administrator. Any storm water from the site which is not directed to a GIA must be detained on site long enough to assure removal of all sediments in a sediment trap. No runoff released from the sight shall have passed over contaminated soils which have been exposed by activities on the sight without first having been detained in a sediment trap for a sufficient time to have removed any sediments.

   E.   A storm water management plan shall be submitted for review and approval prior to any site clearing or construction documenting the design considerations of all storm water controls which are needed to meet the requirements of the storm water management ordinance. In addition, a post construction (as built) drawing(s) shall be submitted showing details of completed control features, including the installation or repair of any barriers, to PHD for inclusion in the tracking system (see article C of this chapter).

   F.   An operation and maintenance plan shall be submitted for review and approval, prior to any site clearing or construction, documenting the methods and responsibility for the continued functioning and integrity of constructed storm water controls and barriers (see article E of this chapter).

   G.   It is the intention of the planning administrator that all development activities within his jurisdiction shall be conducted so as to achieve these performance standards. It is acknowledged, however, that the cost of preparation of a detailed storm water management plan for a single-family residential dwelling may pose a financial hardship to the owner. As a result, simplified storm water control planning procedures may be developed for smaller projects; however, the intent and spirit of these regulations shall still be met.

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)

Storm water management definitions shall be as follow:

BIOFILTER: A designed, vegetated treatment facility where the more or less simultaneous processes of filtration, infiltration, absorption, and biological uptake of pollutants in storm water takes place when runoff flows over and through. Vegetation growing in these facilities acts as both a physical filter which causes gravity settling of particulates by regulating velocity of flow, and also as a biological sink when direct uptake of dissolved pollutants occurs.

CHANNEL STABILIZATION: Erosion prevention and stabilization of velocity distribution in a channel using vegetation, jetties, drops, revetments, and/or other measures.

DETENTION: The temporary storage of storm runoff in a BMP, which is used to control the peak discharge rates, and which provides gravity settling of pollutants.

FLOODPLAIN: For a given flood event, that area of land adjoining a continuous watercourse which has been covered temporarily by water.

IMPERVIOUS SURFACE: A hard surface area which either prevents or retards the entry of water into the soil mantle as under natural conditions prior to development, and/or a hard surface area which causes water to run off the surface in greater quantities or at an increased rate of flow from the flow present under natural conditions prior to development. Common impervious surfaces include, but are not limited to, roof tops, walkways, patios, driveways, parking lots, or storage areas, concrete or asphalt paving, gravel roads, packed earthen materials, and oiled, macadam or other surfaces which similarly impeded the natural infiltration of storm water. Open, uncovered retention/detention facilities shall not be considered as impervious surfaces.

PERMANENT STORAGE: The portion of a pond or infiltration BMP which is below the elevation of the lowest outlet of the structure.

STORM FREQUENCY: The time interval between major storms of predetermined intensity and volumes of runoff for which storm sewers and other structures are designed and constructed to handle hydraulically without surcharging and backflooding, e.g., a 2-year, 25-year, or 100-year storm.

WATERSHED: A geographic region within which water drains into a particular river, stream, or body of water.

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)

   A storm water management plan shall be submitted for review and approval by the planning administrator for all proposed development activities within their jurisdiction (except as specifically exempted). This plan shall describe the proposed development with emphasis on the storm water control methods which are planned to meet the provisions of the performance standards stated above and any IC barriers already existing or required. Each submitted plan shall satisfy the following objectives:

   A.   Answer the question: How are the features and systems going to work, and thus provide a basis from which the planning administrator can review and approve storm water drainage systems.

   B.   Identify the individual who can interpret the design for which he or she is responsible (plan documents shall be stamped by a qualified, licensed professional).

   C.   Convey the design of the storm water control system(s) to the builder (contractor) and building inspectors.

   D.   Provide a record for present and future development owners to maintain and perpetuate the drainage systems and for the planning administrator to monitor their functioning and effectiveness.

   E.   Provide a record of any barrier alterations or additions for PHD's tracking system, present and future development owners, and utilities.

   The frequency and difficulty of future maintenance should be minimized by thorough consideration of all possible failures in the system during design and what would be required to correct the problem. Design adjustments to ease maintenance should be a major consideration.

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)

Submitted storm water management plans shall have the following parts:

   A project summary with design calculations;

   Vicinity drainage plan;

   A site plan;

   An erosion and sedimentation control plan;

   A barrier option plan (BOP), including access control;

   A contaminated soil disposal plan;

   An operation and maintenance (O&M) plan;

   A dust control plan.

Specific contents of each of these parts are given below. A summary checklist of required plan contents is presented in article H of this chapter. All or most of these elements may be required as part of the ICP permitting process. Separate plans may not be required by the planning administrator.

   A.   Project Summary And Design Calculations: This part of the storm water management plan shall present, in text form, an overview of the proposed project. Pertinent details include site acreage, a summary of the planned development (number of units, type of construction for instance), proposed construction schedule (with anticipated start and completion dates) and any design problems or constraining environmental conditions that are anticipated or have been encountered. Any pertinent information which supports the design calculations should also be included in this part. For example, each development proposal should include a list of appropriate soil characteristics (soil erodability factors, engineering properties, lead levels in ppm, soil pH, etc.) for all soil types found (or expected) on the site from the USDA soil conservation service "Interim Soil Survey Of Silver Valley Area Idaho" and any site specific soil data which is available or necessary. Site specific soil test results for metal contamination may be required by PHD for development or improvement plans submitted for locations within the boundaries of the site. Test results for many locations within the site are on file with the PHD and may be used for submittal purposes if no substantial changes have occurred on the location since the test was performed. A new set of tests must be conducted in compliance with the sampling analysis protocol (SAP) on file with PHD.

   The plan submitted shall present (in an organized manner with references to the plan drawings where appropriate) all pertinent calculations performed for the determination of the required size of the storm water controls system elements. Included in this category is an analysis of off site flow upstream of the site, a determination of pre and post development runoff from the site (both total volume and peak flow) with the associated volumes of expected sediment generation; grassed infiltration area and detention/retention facility capacities and infiltration rates; swale, ditch, culvert and pipe system capacities and velocities; and any other appropriate design detail.

   B.   Vicinity Drainage Plan: A vicinity map, with a scale of 1:24,000 (2,000 feet to the inch) or larger, shall be prepared showing the relationship of the development site to its surroundings. Storm water drainage patterns (excepted or observed) shall be shown for all lands, surface waters, and environmentally sensitive areas within one mile of the site. This map should also include delineations showing areas of heavy metal contamination within the drainage pattern of the area to be developed.

   C.   Site Plan: A site plan shall be a scale drawing(s) of not more than one hundred feet (100') to the inch and shall show property (site) boundaries and existing natural and manmade features on and within five hundred feet (500') of the site boundary including roads, structures including collection channels and pipes, erosion checks, water sources and drainage channels, utilities, easements, topography (at 2 foot intervals, referenced to datum) soil types, and vegetative cover types.

      In addition, the plan shall show areas to be cleared of vegetation; where topsoil is to be removed and stockpiled; where excavation, filling and grading is planned (with final contours); where structures, paving, utilities, barriers, lawns, and landscaping are planned; anticipated post construction runoff patterns, and storm water control features. The storm water control features shall further show (either on the plan or attached) profile/cross sections, bottom elevations, temporary and permanent IC barrier locations and cross sections, slopes and lengths of swales and open ditches; sizes, type, invert areas, detention (retention facilities and other storm water controls with references to appropriate capacity calculations); and bottom elevations and dimensions of any off site drainage channels into which overflow on site storm water controls will pass. The plan shall also show storm water easements as described in article E of this chapter.

   D.   Erosion And Sedimentation Control Plan: An erosion and sedimentation control plan shall be submitted and be approved prior to the initiation of any site clearing, excavation, grading, or other development activity. The theory and application of erosion and sedimentation control, as well as required elements are discussed in article D of this chapter.

   E.   Barrier Option Plan (BOP): A barrier option plan detailing the condition of the soils on the site, including test results from a qualified lab or agency showing the types and concentrations of heavy metals present and the pH may be required by PHD. The types and locations of existing IC barriers, areas most likely to become contaminated if soil migration occurs and the uses of those areas, temporary IC barriers to be used during the construction phase including a maintenance plan, and permanent IC barrier designs may also be required for review. Details and cross sections will be required for all soil cap barriers and shall conform to the minimum standards shown in the barrier performance standards. Details of any temporary or permanent access control for vehicular and pedestrian traffic shall be included in the BOP. This will include a plot plan showing access and egress locations, staging sites, decontamination locations, fencing, and public detours. More information concerning the requirements of this plan are specified in article D of this chapter.

   F.   Contaminated Soil Disposal Plan: A plan and schedule for removing, handling, transporting, and disposing of any contaminated soils is required for sites which have lead levels in excess of the action levels listed in the environmental rule and the institutional controls program; barrier design criteria and permitting requirements (ICP;BDC&PR). Soils with lead levels exceeding one thousand (1,000) ppm must either be off-hauled to the PHD approved soil repository or used as fill material under a barrier. An equipment decontamination plan to ensure that no contaminants are tracked off of the job site by equipment or vehicles and a source for any imported borrow material (including soil tests) may also be required. More information concerning the requirements of this plan are specified in article D of this chapter.

   G.   Operation And Maintenance Plan: An operation and maintenance plan shall be required for all developments describing the planned efforts which will be followed to ensure future functioning of constructed storm water controls and institutional controls as they relate to storm water management. This also will be reviewed and approved separate from the storm water management plan. Maintenance efforts should include annual inspections, replacement of damaged vegetation in grassed infiltration areas, replacement of eroded soils in channels, repair of any damaged IC barriers, schedule of sediment removal from control structures with associated disposal protocol, and remediation of any contaminated soils which may have resulted from a damaged IC barrier. Other efforts and specific requirements of operation and maintenance plans are given in article E of this chapter.

   H.   Dust Control Plan: If the proposed project involves the use of heavy equipment for the purpose of earthwork, a dust control plan may be required by PHD. The plan must detail a realistic work schedule for all activities which will create airborne dust and the means by which the contractor or on site agent plans to control it. More information concerning the requirements of this plan are specified in article D of this chapter.

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)

   A.   Two (2) sets of prints of the plan drawings and two (2) sets each of the project summary and design calculations, erosion and sedimentation control plan, IC barrier protection and soil handling plan and operation and maintenance plan, each signed and stamped by the design professional, shall be submitted to the planning administrator for review and approval. Acceptance of the submitted materials shall be based on the determination that the proposed development is in compliance with the storm water ordinance. One marked up set of prints and plans plus written comments/questions will be returned to the applicant.

   B.   Engineered submittals will be reviewed by a qualified professional engineer. If any of the parts of the plan are returned for corrections or additional information, changes must be marked or highlighted in color on revised copies and returned. The colors red and yellow are reserved the planning administrator's use. The date that changes are made must be clearly indicated.

   C.   To help expedite the review of a development plan, the sponsor and/or his designer may request a meeting with the planning administrator following receipt of initial review comments. However, prior to such a meeting, the sponsor and his designer shall review all initial comments and be prepared to discuss alternate solutions.

   D.   A copy of the approved plan must be on site whenever construction is in progress. During any aspect of site development, if field conditions prove to be substantially different from conditions assumed by the design professional, such that storm water controls may not function as planned, the approved plan will be deemed invalid. Revised portions of the plan shall be submitted for review and approval, prior to resuming construction work. Any deviation from the approved plan shall be approved in writing by the planning administrator prior to further work.

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)

   Prior to acceptance of plat improvements, release of bond, issuance of a certificate of occupancy, or final sign off by the planning administrator, a postconstruction drawing(s) shall be submitted by the applicant's design professional to the planning administrator. All substantive differences from the original, approved, design drawing(s) shall be approved and a reproducible copy of the accepted drawings shall be labeled "POSTCONSTRUCTION DRAWINGS". The following statement shall be lettered on these drawing(s) and stamped and signed by the design professional certifying the postconstruction drawing(s):

   A postconstruction drawing(s) shall be submitted in timely manner following the completion of construction. No certificate of occupancy shall be issued without the submission and approval of a postconstruction drawing(s).

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)

   A.   A storm water drainage control system shall be provided before, during, and after all pertinent land disturbance or construction. The goal of this system is to minimize, to the maximum extent possible, the effect that sediments and storm water runoff (including snow melt) can have on ground water and surface water quality or on public or private downstream property, especially with respect to heavy metal contaminant migration.

   B.   An erosion and sedimentation control plan shall be required to be submitted to, and approved by, the planning administrator prior to the initiation of any site clearing, excavation, construction, or other development activity. This plan will address both temporary (primarily construction related) and permanent erosion controls. Specific erosion/sedimentation controls shall be consistent with those described in the Panhandle Area Councils' "Handbook Of Best Management Practices For Storm Water Management And Erosion Sedimentation Control Revised For The Environmental Conditions Of The Bunker Hill Superfund Site". An approved erosion and sedimentation control plan shall represent the minimum requirements for anticipated site conditions. Additional erosion and/or sedimentation controls may be required in the event of unexpected storm occurrences, repair, or maintenance of existing systems, temporary or permanent barrier protection, or replacement of nonfunctioning systems.

   C.   For the purpose of this section, "erosion" is considered to mean the detachment and movement of soil or rock as a result of the action of water, wind, ice, gravity, or human activities. "Sedimentation" shall mean the deposition or accumulation of sediment (soil, dust, debris, or other materials) on ground surfaces and in watercourses.

   D.   Note the suggestions and recommendations made herein are not intended to limit, in any way, the use of the best available technology on achieving erosion and sedimentation control. However, for other runoff control techniques which are proposed for use in the erosion and sedimentation control plan, it may be required to show that equivalent control or treatment can be achieved utilizing alternatives not described.

(Ord. 364, 4-5-1989; amd. Ord. Ord. 637, 4-9-2025)

   A.   Prior to any clearing or grading for any land disturbance or development which falls under the authority of the city of Kellogg storm water management ordinance, devices or structures which shall intercept and collect all runoff from (or flowing across) the proposed disturbed areas shall be installed or constructed. This interception and collection shall make use of best available runoff management practices, and shall preclude the direct, uncontrolled discharge of sediment laden runoff to downstream properties, and provide for the collection of sediment particles on site prior to runoff discharge off site.

   B.   It is recognized that sediment removal efficiencies of temporary runoff collection devices will vary depending on soil type and other factors. Very fine grained suspended sediment carried in runoff may not be stopped by the required interception and collection. In addition to causing turbid conditions in water bodies, this fine sediment can carry a significant level of heavy metal contaminants, nutrients, and other pollutant which can harm water quality, pollute neighboring soils, and even pose significant health risks. While higher sediment removal efficiencies can be achieved by increasing the size or complexity of the collection devices or structures, one hundred percent (100%) sediment removal cannot realistically be expected from temporary sedimentation controls. As a result, other measures aimed at reducing the initial erosion shall be included in the erosion and sedimentation control plan. The focus of these measures shall be all denuded and graded areas, soil stockpiles, water bodies, or otherwise exposed contaminated soils.

   C.   Storm water runoff from active construction sites can be effectively intercepted and collected using earthen berms, straw bales, silt fences, and other techniques described in the handbook of best management practices. For sites of less than five (5) acres, located more than one-quarter (1/4) mile from a designated surface water or environmentally sensitive area, temporary sediment traps are recommended and will likely be required (based on the potential for erosion, sedimentation, and soil contamination problems in the site area). While specific design details are presented in the handbook, some general, but important, considerations are given below:

      1.   Any barrier, berm, or structure forming a sediment trap shall be located to provide for maximum volume capacity for trapping sediment behind the structure and for greatest ease of cleanout.

      2.   Sedimentation traps shall provide a minimum of one foot (1') below the outfall (overflow) elevation of cleanout.

      3.   The sediment traps shall be located as close as possible to the disturbed area.

      4.   Runoff from undisturbed areas should be excluded from the sediment trap to the extent possible.

      5.   No contaminated soil shall be left exposed to a storm event of any kind. Work shall be done in such a fashion that soil exposed during the day is, at a minimum, covered with a synthetic membrane (6 mil poly tarp) at the end of the day if precipitation is expected.

      6.   Each sediment trap shall be inspected after significant runoff events to check for damage or operational problems. Sediment shall be removed from traps when it fills half (1/2) of the constructed capacity and disposed of in an appropriate fashion (at a PHD approved repository). All collected sediments will be considered, until demonstrated otherwise, to be contaminated and shall be disposed of at the Page site. Access to the Page site will be allowed via PHD.

      7.   Sediment traps should be used where they will see two (2) years or less of service. Once the contributing drainage area has been stabilized, the sediment traps should be removed.

      8.   For sites of greater than five (5) acres, or where the distance to a designated surface water is less than one-quarter (1/4) mile, or for permanent runoff control, sedimentation basins or detention ponds may be required based on local erosion/sedimentation and contaminant migration potential (see below).

      Many runoff controls described in the handbook are appropriated for use in controlling the initial detachment of soils or rock that occurs in the erosion process. One very basic technique which should be inherently obvious to all site developers and storm water control engineers is the preservation of existing vegetation. A stable plant cover, in a healthy condition, will preclude erosion where it provides seventy five to one hundred percent (75–100%) cover 1 . Preservation of natural and/or restorative vegetation shall be considered during the planning and design stages of a development and areas to be preserved shall be indicated on the erosion and sedimentation control plan (see below). Further, designated preserve areas must be fenced to protect them from damage by construction equipment.

   D.   Other runoff controls which are recommended for erosion control are mulches, pervious covers (jute/cotton woven netting), impervious covers (the most effective inexpensive means of protecting exposed contaminated soils from precipitation), and filter fabric silt fences. Whichever of these, or other, runoff and erosion controls are used, they should be maintained and left in place until the construction site or other disturbed area is permanently stabilized and all permanent barriers are in place.

   E.   Temporary or permanent soil stabilization of graded areas, denuded areas, and soil stockpiles must be applied within two (2) weeks after initial clearing 2 . If there is greater than a thirty percent (30%) chance of a precipitation event of more than 0.25 inch expected within that two (2) week period, interim measures such as plastic tarps and silt fences should be used. Although this is a nonenforceable requirement by virtue of the precipitation probability, it is incumbent upon the contractor or owner to make a reasonable attempt to stay abreast of weather forecasts and make an appropriate effort to ensure that the site is protected from any significant precipitation.

   Temporary or permanent soil stabilization shall be performed as soon as possible after initial soil disturbance. Applicable practices include vegetation establishment, mulching, and early application gravel base on areas to be paved. Soil stabilization measures should be appropriate for the time of year, site conditions, and estimated duration of use (for example winter grading with hydroseeding will not be allowed).

   F.   A final requirement which is primarily related to construction activities is the transportation or tracking of soils (mud) from work sites by vehicles, particularly in areas where heavy metals are present in the soil. Wherever construction vehicle access routes intersect paved public roads, provisions must be made (in the erosion and sedimentation control plan) for control of this. While daily street sweeping can be effective in limiting this movement of soils off site, temporary rock construction entrances are generally more effective and less labor intensive (see handbook). In the case of job sites with contaminated soils, all vehicular traffic which comes into contact with the contamination must be made free of mud, dust, or other incidental dirt before leaving the site in order to control migration of the contaminants to clean or remediated areas. (A detailed description of decontamination procedures is to be included in the BOP for PHD's review.) This process is simplified if a rock pad is established on a portion of the job site and is used as a staging area for employee parking, deliveries, personnel and equipment decontamination, fueling and any other general site access. The pad may be kept and incorporated later into the project as part of a road base or building pad (e.g., covered by a permanent barrier) or it must be removed and treated as contaminated material.

(Ord. 364, 4-5-1989; amd. Ord. Ord. 637, 4-9-2025)

   A.   General: When erosion and sedimentation control plans are prepared they must consider potential erosion for the development "in perpetuity". Fortunately for developers, the risk of erosion and resultant sedimentation, either on or off site, diminishes when the soils become stabilized. Soil stabilization can be accomplished by establishing vegetation or covering with pavement, structures or other permanent impervious barrier. The preferred method of soil stabilization (where structures are not to be placed) is by vegetation since this allows the soil to maintain its natural absorptive and purifying characteristics. This stabilization should take place as soon as possible following the initial disturbance.

      The following is a list of considerations that must be addressed, where appropriate, for adequate long term control of erosion on a site:

      1.   Permanent Vegetation: Establish perennial, herbaceous vegetation (grass) on disturbed area in the maximum extent possible. As shown in article F of this chapter, grassed areas are extremely beneficial in absorbing moisture in rainfall and snow melt and in filtering runoff as it percolates to the ground water. Grasses and other types of vegetation can be applied effectively as seed; with mulch, fertilizer and other additives recommended to protect and enhance the initial growth. Care should be taken with all fertilizer applications to prevent its wash off and transport to surface water bodies. Grass can also be applied as sod, which is appropriate on steep slopes, and for late season applications. It is important, however, to select the proper vegetation for the intended uses of an area, particularly in areas where the pH restricts growth for many species.

      2.   Vegetative Buffer Strips: In order to provide for short and long term protection of existing, adjacent watercourses, water bodies or wetlands, vegetated buffer (or filter) strips shall be created and/or protected and maintained. Buffer strips shall consist of grass or other close growing vegetation designed to receive overland flow. The width of the buffer shall be sufficient to prevent erosion, trap sediment in overland flow, provide access to water body and allow for periodic flooding without damage to structures. Generally, good performance for pollutant removal can be expected if the buffer is fifty (50) to seventy five feet (75') wide with an additional four feet (4') for each one percent (1%) slope at the site 1 . Specific proposed widths will be evaluated on a case by case basis.

      3.   Watercourses:

         a.   Natural watercourses (channels carrying natural drainage on an intermittent or infrequent basis) should be avoided during clearing and site development. However, if these areas do become impacted they must be restored and revegetated using natural plant species to the extent possible. Temporary erosion controls (jute netting for instance) will be required within one week after completion of work and during the establishment of a permanent vegetative cover. All soils disturbed during construction should be stabilized within fourteen (14) days of initial disturbance. Any contaminated soils within the naturally established stream environment, as defined by the handbook, which have been exposed during development shall not be left subject to any storm event in excess of 0.25 inch, to the extent that weather predictions can be followed, and shall be covered by a temporary impervious synthetic membrane (such as 6 mil poly tarp) if such an event is predicted. This practice is to be continued until all permanent barriers and erosion control measures have been constructed. Any contaminated soils which have been removed must be properly disposed of at a PHD approved soil repository.

         b.   Artificial watercourses such as storm water conveyance channels, grassed swales and gutters must be designed with short and long term erosion controls in mind. Generally it is recommended that these "structures" be grass lined as this provides for percolation and filtration of a portion of the runoff flow. Grassy vegetation also tends to slow the flow and this dampens the peak flows to downslope areas. Vegetative cover in channels and swales will naturally tend to reduce runoff velocities but suitable grade must be designed which is favorable to growth of the cover and which provides velocities not greater than four (4) to six feet (6') per second 2 . Rigid lining of any artificial watercourse such as gunite or concrete shall be constructed to ensure that it is not damaged by frost heave. Specific designs will be evaluated on a case by case basis by a professional engineer. The planning administrator may elect to share any engineering review to limit the cost. Review costs, however, will be borne by the applicant. Other design considerations are detailed in the handbook.

      4.   Detention And Retention Basins:

         a.   The most common "low structural" best management practice for storm water control is detention storage 1 . Storage in depressions or actual detention basins reduces peak runoff flows and treats the runoff by sedimentation and percolation. Detention refers to the temporary or partial storage of storm flows, so detention basins would have an overflow outlet structure to allow them to be drained either downslope or subsurface. When released to surface drainages off site, detained water shall be released in a manner approximating the predevelopment, natural flow of runoff. Retention on the other hand is the complete or permanent storage of storm water. Retention basins could be larger structures at the end of a runoff collection system or network, or they could be smaller depressions providing "off line" storage. The treatment mechanisms in retention systems are percolation and evapotranspiration. Evapotranspiration does not occur in the Silver Valley in the winter months; therefore, the designer must account for this when calculating storage volumes. Either detention or retention facilities will be required in areas of soil contamination to ensure that any incidental sediment picked up by storm water coming from the developed area is removed before the water is released to downstream areas. If the soil in the drainage area has been demonstrated to be clean either by testing or review of PHD data, detention and/or retention facilities may or may not be required depending on what the runoff and erosion calculations reveal. Runoff control is mandatory.

      b.   Both detention and retention facilities can be either wet or dry areas. Dry catchment basins can be developed to serve as dual purpose areas, providing dry weather recreational or aesthetic benefits along with wet weather runoff storage. Wet ponds or manmade wetlands can provide some recreation and aesthetic benefits and also a greater degree of pollutant removal. If the detention or retention facility is to be used for recreation, it must be demonstrated that the collected sediments have levels below the action level (1,000 ppm). If sediment levels exceed the action level, then access control (fencing) must be considered. The maintenance of healthy biological communities in wet basins can be extremely difficult, however. The banks of detention and retention basins shall be constructed with a gentle slope into the basin for safety reasons and to encourage growth of vegetation in wet ponds. Other design details, especially for embankments (berms) and overflow details shall be as required in the handbook. Note that natural water bodies and wetlands shall not be used for storm water runoff storage or treatment.

      5.   Maintenance: All temporary and permanent erosion and sedimentation control practices must be maintained and repaired as needed to assure the continued performance of their intended function. A long term maintenance program must also be outlined in the erosion and sedimentation control plan and the operation and maintenance plan (see article E of this chapter) which includes responsibility for annual inspections, postrunoff event inspections, access controls, repairs, and liability in the event of downslope damages or contamination.

      Maintenance efforts may include (but are not limited to) removal and disposal of sediment from detention or retention basins or ponds, replacement or vegetation in grassed infiltration areas, buffer strips or watercourses, repair (filling) of eroded rills or gullies and watering of established vegetation. Removal of sediments from control structures requires that the sediments be considered contaminated and disposed of at the Page site via PHD access or demonstrate by testing that the sediments area free of contaminants (below 350 ppm lead) and may be disposed of in some other location.

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)

   The erosion and sedimentation control plan mirrors in several respects the storm water management plan, the BOP, and contaminated soil disposal plan described in article C of this chapter. While the focus is somewhat different, there are several areas of overlap and duplication. In spite of this, it is the intention of the planning administrator that the erosion and sedimentation control plan be a separate document (a plan sheet with notes for example) which will receive a separate review and approval. This plan will, however, make reference to the overall site storm water management plan, the BOP, and the contaminated soil disposal plan where appropriate.

   Each erosion and sedimentation control plan which meets the review requirements of the planning administrator and all appropriate engineering reviews may be required to contain complete information on the following topics:

   A.   Existing site conditions (map at not more than one hundred feet (100') to the inch).

   B.   Site boundaries.

   C.   Location of existing roads, structures, general vegetation types, surface water bodies (streams, rivers, lakes, ponds, wetlands) and their buffers, utilities and easements within five hundred feet (500') of the site boundary.

   D.   Topography (2 foot intervals for 0–10 percent slope, 5 foot intervals for 10–30 percent slope, and approximate intervals for slope greater than 30 percent).

   E.   Existing storm water runoff patterns with known or anticipated flow (in cubic feet per second), volume (cubic feet) and velocity (feet per second) information.

   F.   Soil type distribution and profiles (with tabulated USDA soil conservation service "erodability" and "permeability" factors) and any soil test results showing lead levels present. Soil tests shall be performed per the soil analysis protocol (SAP) or performed by PHD and the cost borne by the applicant.

   G.   Proposed site conditions.

   H.   Location of all buildings, structures, and paving (including graveled walks and driveways). Location of all temporary changes to the land surface: clearing limits, vegetation preserve areas, soil stockpile areas, temporary erosion and sedimentation control features, and temporary IC barrier locations and types (with notes indicating type of facility, construction materials, dimensions, and holding capacity in gallons per cubic feet).

   I.   Location of all permanent changes to the land surface: cuts (excavations) and fills (with cross sections), revegetation areas (with type, size and extent), permanent erosion and sedimentation control areas (with notes indicating type of facility, construction materials, dimensions and holding capacity where appropriate), and barriers (types and locations).

   J.   Proposed storm water runoff patterns with anticipated flow, volume and velocity information.

   K.   Schedule of construction and revegetation work.

   L.   Anticipated starting and completion dates of each phase of work.

   M.   Maintenance/repair schedule and responsibility.

   N.   Routine inspection schedules for during construction and afterward.

   O.   Anticipated maintenance efforts.

   P.   Who will be responsible (legally bound) for long term maintenance repairs and liability after construction is complete.

   The erosion and sedimentation control plan shall be submitted and approved by the planning administrator prior to the commencement of any work on site. Failure to comply with this will be cause for enforcement actions as provided by the storm water ordinance and the PHD.

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)

   The BOP has overlapping information with other plans required in the overall storm water management plan but due to the levels of heavy metal contamination found in the soils in the site and the health problems that they are capable of creating, it is viewed as prudent, and therefore may be required by PHD, that this issue be given a plan of its own. This plan will, however, make reference to the overall site storm water management plan, the erosion and sedimentation control plan, and the contaminated soil disposal plan where appropriate.

   If the soil on the site tests out below the action levels, one thousand (1,000) ppm in individual residential yards and three hundred fifty (350) ppm average over the entire area of a proposed development, no BOP is required for submittal. There are two (2) areas of concern on a given site; the primary development location and the adjacent hillside property. Both of these need to be tested in accordance with the SAP on file with PHD.

   A.   The BOP shall include the following information:

      1.   Soil test results for the site itself showing lead levels at the required soil depths. Soil sampling and testing methods shall be in accordance with those laid out in the ICP and SAP and shall be performed by a qualified lab or provided in the tracking information held by PHD. The developer may use test results already on file with PHD if they reflect the present condition of the site.

      2.   Soil profiles and types found on the site as listed by the soil conservation service in their "Interim Soil Survey Of The Silver Valley Area, Idaho" in conjunction with a more in depth study by a qualified soil scientist of the on site if conditions warrant.

      3.   Locations, types, and present condition of all existing barriers on the site shall be shown on a map of the site (scale not to exceed 100 feet to the inch). Most or all of the existing conditions should be available through the PHD tracking system.

      4.   The map shall show all natural drainage features within five hundred feet (500') of the site including all environmentally sensitive areas downstream of the site. The map shall also show topographic intervals as specified in the erosion and sedimentation control plan.

      5.   Locations of all proposed barrier disturbances and/or the exposure of any previously unexposed contaminated soils.

      6.   Detail drawings, locations, descriptions of, and schedules for installation, maintenance, and removal of all temporary barriers to be used during construction.

      7.   Detail drawings, locations, descriptions of all permanent barriers to be installed. A long term maintenance and inspection plan shall also be included.

      8.   Personal protective measures, including apparel, equipment, sanitation, and safety zones to be taken by employees during all activities involving exposed contaminated soils.

      9.   Any other requirements mandated by the PHD or IDHW-DEQ regarding environmental safety.

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)

   A.   Disposal of contaminated soils must be done in an approved manner and location in order to minimize the migration of soil contaminants and maintain the integrity of the barrier system. If the excavation involves less than one cubic yard of residential soil, PHD will provide a container for the soil and will collect it for disposal at the Page soil repository or other PHD approved location. This service is free of charge. If the excavation exceeds one cubic yard, it becomes the responsibility of the property owner to remove the soil and transport it for disposal. There will be no charge for dumping. Access to the repository site will be arranged through the PHD.

   B.   The following elements are required in a submittal if any soils are to be excavated and removed. Some of these elements are required for other portions of the storm water plan and may be referred to instead of resubmitted.

      1.   Calculations determining the amount of contaminated soil to be removed.

      2.   A plan explaining the process to be used for soil removal, transporting, and disposal, including equipment to be used.

      3.   An equipment decontamination plan showing design details. It is recommended that this be done over a pad of crushed rock in an area which will later be paved (for road or driveway) and maintained as a barrier zone. All decontamination procedures must comply with those outlined in the ICP;BDC&PR.

      4.   A detailed dust control plan and schedule for the duration of the project per the ICP;BDC&PR.

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)

   A.   Soil Conservation Service. Urban Hydrology for Small Watersheds. Technical Release No. 55. U.S. Department of Agriculture, Washington, D.C. (1975).

   B.   Minnesota Pollution Control Agency. Protecting Water Quality in Urban Areas (1989).

   C.   Wanielista, M.P. Stormwater Management: Quantity and Quality. Ann Arbor Science, Ann Arbor, MI (1979).

   D.   Field, A., A.N. Tafuri and H.E. Masters. Urban Runoff Pollution Control Technology Overview. Report No. EPA-600/2-77-047. Office of Research and Development U.S. Environmental Protection Agency (1977).

   E.   Final Commercial Properties Remedial Design Report for the Bunker Hill Superfund Site, January 1994; prepared for ASARCO Inc., Hecla Mining Company, Sunshine Mining Company.

   F.   Final Residential yards Remedial Design Report for the Bunker Hill Superfund Site, January 1994; prepared for ASARCO Inc., Hecla Mining Company, Sunshine Mining Company.

   G.   Final Rights-Of-Way Remedial Design Report for the Bunker Hill Superfund Site, January 1994; prepared for ASARCO Inc., Hecla Mining Company, Sunshine Mining Company.

   H.   Institutional Controls Program for the Bunker Hill Superfund Site, Regulatory and Performance Standards Component, September 20, 1993; Murray Lamont & Associates, Inc.

   I.   Interim Soil Survey of Silver Valley Area, Idaho, Part of Shoshone County, June 1989; Soil Conservation Service.

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)

   A.   All land developed within the site shall incorporate storm water runoff treatment facilities to mitigate the potential for ground and surface water degradation. The preferred treatment mechanism is the infiltration of storm water runoff through a grassed area. When development of the land is in the planning stages, these types of controls can easily be included in the design of green area landscaping. The planning administrator understands that much of the terrain and soil types within the site do not lend themselves well to use of GIAs; however, many of the lower areas within the site basin are appropriate. The use of this technology wherever possible will limit the further migration of heavy metal contaminants towards the local ground water supply as well as the Coeur d'Alene River drainage system. Alternative systems may be used subject to the approval of the planning administrator.

TABLE 5-1: EXPECTED CONTAMINANT REMOVALS

FOR GRASSED INFILTRATION AREAS 1

 

   B.   The primary principle of storm water controls using natural vegetated areas (i.e., grassed infiltration areas) is, when runoff passes over or is held over grass covered permeable soils, the volume is reduced by percolation and the quality of this percolate is improved by biological, chemical and physical filtering. A design criterion which must be kept in mind is to provide enough grassed infiltration area to handle the runoff generated at the site so as not to overload the infiltration areas. An overload is defined as that volume or rate of runoff which could damage the grassed infiltration or swale area and reduce its effectiveness in future runoff events. Numerous possibilities exist to aid the process of storm water disposal utilizing grassed infiltration areas.

   C.   Grassed swales and gutters can be used instead of impervious passageways to transport storm flows 2 . These grassy areas will not only percolate and filter a portion of the flow, but also slow the flow and thus dampen the peak flows to downslope areas. Certain types of grasses, termed "high roughness", help to emphasize the flow delay 3 and may be found in the soil conservation service's interim soil survey of the Silver Valley area. Vegetative cover in swales and gutters will naturally tend to reduce runoff velocities but a suitable slope must be designed which is favorable to the growth of the cover and which provides velocities not greater than four feet (4') per second 4 (evaluated on a case by case basis).

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)

   A.   Standards:

      1.   Where the grassed infiltration concept is employed, infiltration of the required runoff volumes (first 1/2 inch from impervious surfaces) shall be assured by designing the system to retain the prescribed volume. This is necessary to ensure that various biological, chemical, and physical processes active within the root zone act on the required volume of runoff.

      2.   No GIA shall be designed to receive direct runoff from an area contaminated with heavy metals or low pH unless that area has had barriers installed or can otherwise be demonstrated not to be a contaminant migration source. All contaminated areas shall have an appropriate barrier(s) to prevent direct contact with storm water or all runoff from such areas shall be directed through a detention/containment facility with a sediment trap or other approved method of sediment detention.

      3.   The GIA needs to be situated on a good soil mantle which allows the grass to grow and should not have to be fertilized. The subsoil underlying the infiltration area should not be compacted during or subsequent to construction or its ability to pass water could be severely compromised. High permeability soils (silty sand to sandy soils are preferable) underlying the topsoil mantle are recommended of the GIA is to pass water at a reasonable rate.

      4.   The surface area required for GIA installation is dependent on the flooded depth. The design of the surface area of the grassed infiltration area is to be determined using the volume of the runoff, received from the developed areas (including, but not limited to, roofs and paved or gravel driveways, parking areas, and walkways) as well as precipitation falling on the GIA directly, which produces an average depth of six inches (6") within the GIA. Flexibility in depth is desirable but there are certain limits which must be observed. An average flooded depth on the order of four inches (4") is desirable for three (3) reasons:

         a.   At this flooded depth, the time the grassed area is under water allows the use of traditional turf grass rather than water tolerant varieties;

         b.   The amount of surface area involved should provide twenty to thirty (20–30) years of effective contaminant removal in areas which are not subject to concentrated contaminants. GIAs should be monitored on an annual basis for build up of heavy metals and the soil in these GIAs should be removed, disposed of, and replaced when the lead levels exceed the action level of one thousand (1,000) ppm. Access control (fencing) should be considered if heavy metal accumulation is expected;

         c.   Shallow grassed areas have a desirable aesthetic appearance.

      5.   Typically, best management practices dictate that the design of GIAs be sufficient to catch and treat the first one-half inch (1/2") of a storm event 1 . For storm events producing volumes of runoff in excess of one-half inch (1/2"), a drywell structure is located within the grassed infiltration area to handle the excess runoff. In Idaho, these structures are regarded as class V(e) injection wells and they are discussed in detail in subsection 11-13G-3F of this chapter.

      Regarding class V(e) injection wells, the Idaho Code (title 42, chapter 39), states: "the ground waters of this state to be a public resource which must be protected against unreasonable contamination or deterioration of quality to preserve such waters for diversion to beneficial uses". "Contamination" as defined by the statute means:

      6.   Permitting of class V(e) shallow injection wells (drywells) is not required; however, they must be inventoried (registered) with IDWR. They are also subject to IDHW rules for individual subsurface sewer disposal systems.

      7.   Because of these restrictions, other types of overflow controls may be required in conjunction with GIAs. Within the site it may be more desirable to divert any overflow into the natural surface drainage system than into an injection well in order to safeguard the quality of the ground water. This depends on the quality of the storm water entering the GIA. Factors such as ground level soil quality in the upstream areas, barrier condition and effectiveness, environmental sensitivity of the downstream areas, and soil drainage and filtration capabilities at drywell depth must be considered in determining which approach to take. PHD and IDWH-DEQ should be consulted to help resolve this question if it arises.

      8.   Within the site, GIAs shall be designed to store and treat the first one-half inch (1/2") (0.04 feet) of runoff. Following the requirements stated in article G of this chapter, however, the GIA and adjacent land and improvements shall not be damaged in any manner by the peak runoff flow from a 25-year storm event. In areas where barrier erodability and integrity is an issue (soil barriers especially), the design storm shall be a 50-year event.

      9.   Grassed infiltration areas shall be a maximum of eight inches (8") deep, calculated as the difference between the low point of the grassed infiltration area and the inlet of the overflow structure. The GIA's cross sectional area shall be flat bottomed, having a permissible average depth of six inches (6"). Use of GIAs with a triangular, elliptical, or rounded cross sections should be avoided. Swales with rounded shaped cross sections may be used for conveyance of the storm water provided the swale's area is not used in the calculation for the percolation area for the GIA. The maximum side slope for a GIA shall be three (3) horizontal to one vertical (3:1). Lower side slopes of four (4) horizontal to one vertical (4:1) or five (5) horizontal to one vertical (5:1) are preferred. Consideration should be given to integrating the GIA into the overall landscape design in an aesthetically pleasing manner.

      10.   The ICP may require soil tests to ensure that the soil is capable of assimilating the contaminants in the runoff and will percolate the runoff at an acceptable rate (i.e., that which will infiltrate the given depth of water in a 24 hour period under optimum, nonfrozen conditions).

      11.   One test each for cation exchange capacity (CEC) 1 and soil organic matter 2 may be required for each GIA of one thousand five hundred (1,500) square feet or less. As additional set of tests may be required for each additional two thousand (2,000) square feet or fraction thereof. Tests shall be performed on a well mixed composite sample consisting of the top six inches (6") of soil from at least four (4) cores uniformly distributed over the infiltration area. If the average cation exchange capacity is fifteen (15) milliequivalents per one hundred grams (100 g) or more, or the soil is two percent (2%) organic carbon or more, the soil will be considered suitable for storm water treatment. Test results shall be reported in the storm water management plan (see article C of this chapter).

      12.   Where existing data is sufficient to assure that the soil type on the site meets the requirements for contaminant removal and percolation (as described above), the planning administrator may waive the testing requirements.

      13.   To handle runoff volumes greater than that from the first one-half inch (1/2"), a drywell or overflow structure should be placed at a level equivalent to the ponded depth of the storm water runoff, in a static condition, received from the first one-half inch (1/2"). The overflow or drywell inlet should be located near the inlet of the ponded area, off to one side of the flow pattern. This will preclude the mixing of the high pollutant, "first flushed" ponded water with more dilute continued runoff in the infrequent case of a storm event above the design storm value. If an overflow diversion to the natural drainage system is being used, it should be located in a similar fashion within the GIA to avoid sending the "first flushed" water downstream. When the inlet structure is located within the GIA, the slopes around the inlet shall be no greater than two horizontal to one vertical (2:1). The drywell or structural system constructed to handle the excess volume of runoff shall be designed in conformance with the standards in article G of this chapter (and the handbook). When drywell structures are used in combination with GIA storage, a simplified routing technique, known as the bowstring method for detention basin design, can sometimes be used to reduce the number of drywells required 1 .

      14.   If additional ponded depth over four inches (4") is designed, it should be recognized that the life of the grass in the infiltration area may be shortened. The developer may want to increase the surface area (lower the flooded depth) to extend the life span of the infiltration area. Properly designed, a GIA should provide twenty to thirty (20–30) years of effective contaminant removal in areas where there are no concentrated levels of contaminants. Effective GIA contaminant removal life span within the site may be much shorter and should be monitored regularly.

   B.   Maintenance:

      1.   Sediment Removal: Proper maintenance of GIAs is critical. The infiltration areas should be inspected by the owner (or assignee approved by the planning administrator, see article E of this chapter) semiannually as a minimum and after major storm events which approach or exceed the design storm event. Sediment removal should be accomplished when the sediment is dry enough to crack and readily separates from the floor of the basin. Proper handling and disposal (at the Page site) methods for the collected sediments shall be adhered to.

      2.   Grass Maintenance: The sod or grass should be maintained as needed to control weed growth and maintain the health of the grass. This will include mowing and minimal amounts of fertilizer. (Chemical fertilizers or pesticides shall not be used within vegetated buffer zones or otherwise within 100 feet of permanent surface waters.) It is preferable that the soil mantle provide the nutrients necessary for the health of the grass to avoid the potential for leaching from the fertilization process. Selection of the grass in the design process should include consideration of those varieties of grass which are suited to the site's climatological and soil conditions, providing low maintenance, drought resistance, low pH tolerance, or other desirable characteristics (i.e., red top, Timothy, Canada bluegrass, carrex spp.) 1 .

      3.   Renovation:

         a.   When the GIA has reached the end of its useful life, it shall be renovated. The useful life will be considered to have ended when water remains standing in the GIA more than seventy two (72) hours following the end of a runoff event or when the cover material dies, whether due to the build up of toxic materials in the soil or any other cause, or if soil tests reveal lead concentrations in excess of one thousand (1,000) ppm. Access control may be needed if contaminant levels are expected to reach this level frequently or before soil removal is effected.

         b.   When infiltration fails, renovation shall be accomplished by any needed action from standard soil aeration to removal of the sod layer, scarification of the underlying soil, removal and replacement of soil and replacement of the sod layer.

         c.   When the cover material dies, the sod and at least six inches (6") of soil shall be removed from the affected area and disposed of at an approved site. The soil shall be replaced, graded, and a new cover material planted, or sod applied.

   C.   Roof Runoff: Runoff from the roofs and buildings shall be discharged to GIAs when possible, rather than into drywells or off site. Expected volumes of runoff generated from roofs shall be considered in GIA design.

   D.   Sand And Silt Traps: If gravel roads, driveways, or parking areas are utilized within the development, it will be necessary to install a sand and silt trap upstream of the GIA. Storm water studies have indicated large quantities of sand and silt are washed off gravel structures. This phenomenon could significantly decrease the life of grass percolation areas by covering grass and filling low areas which would otherwise hold runoff.

   E.   Grassed Swales And Gutters: The use of grassed swales and gutter areas as a method of conveyance of storm water runoff would, in most storm events, serve as an additional infiltration area, reducing the volume which would need treatment in the GIA. Under certain circumstances though, these conveyance systems could lose their ability to percolate water overloading the GIAs downstream. In late fall or early spring, frozen soil conditions may not allow the grassed swales or gutter areas to percolate runoff. In this instance, the downslope grassed infiltration area will have to be designed with additional capacity to serve as a storage area for runoff. The use of grassed swale conveyance should receive additional attention during design and construction if their area and storage capacity is a factor in the overall GIA. Only those swales which are flat bottomed and meet the other design criteria for grassed infiltration areas shall be considered as a part of percolation area.

   F.   Spill Control Requirements: Special considerations need to be addressed for storm water runoff from areas within commercial and industrial developments where chemical spills may occur. Specific problem areas include outdoor loading docks and outdoor chemical storage facilities. Design of facilities for these situations will be addressed on a site specific basis by the planning administrator.

      In the event of a toxic material(s) spill within a GIA, the polluted grass and soil shall be replaced and disposed of in an approved manner. This accomplishes two (2) goals:

      1.   Removal of the toxic material from the site.

      2.   Removal and replacement of the dead grass to maintain an effective infiltration area.

(Ord. 364, 4-5-1989)

   A.   Obviously, design of the GIA is site specific. In one development, the site topography and layout may readily allow the use of regular geometric shapes for the design of the impervious surface areas or GIAs. In other cases, the site topography may not allow design using standard geometric shapes or patterns. The equations listed below or a combination of these equations should provide a basis for the developer to evaluate the characteristics for acceptable GIAs given a defined impervious surface on a site specific basis. See figure 1 for an example of a uniform rectangular GIA.

   B.   As mentioned above, the GIA shall be designed to store and treat the first one-half inch (1/2") of runoff draining from a developed, impervious surface area (ISA) from any storm event. Impervious surface areas shall include paved or graveled roads, driveways, parking area, and walkways, and roof areas. Using the concept of storage volume as the basis for the GIA design yields the following equation:

 

 

      1.   The volume of a GIA, with a flat bottom and uniform width, can be determined by multiplying the basin cross sectional area by the length of the basin. In this case, the above equation becomes:

 

 

UNIFORM RECTANGULAR GRASSED INFILTRATION AREA AND IMPERVIOUS SURFACE AREA

TRAPEZOIDAL CROSS SECTION

Figure 1 - Example of an impervious surface area and grassed infiltration area showing characteristics used in design equations.

      2.   If the impervious area and the GIA are rectangular in area and the GIA has a trapezoidal cross section, the relationship in the above equation becomes:

 

   C.   This will work for a basic calculation of the area. For the final design the developer should calculate the area based on the actual area of the circular sector.

   D.   In most cases where GIAs are used along roadsides, the slope limitation will be the controlling factor in design of the GIA width.

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)

   A.   Spokane County Engineers, Guidelines for Stormwater Management, Spokane, WA (1984).

   B.   Wanielista, M.P., Y.A. Yousef, and J.G. Taylor. Stormwater Management to Improve Lake Water Quality. Report No. EPA 600/2-82-048. Office of Research and Development, U.S. Environmental Protection Agency, Washington, D.C. (1982).

   C.   Field, R., A.N. Tafuri, and H.C. Masters. Urban Runoff Pollution Control Technology Overview. Report No. EPA 600/2-77-047. Office of Research and Development, U.S. Environmental Protection Agency, Washington, D.C. (1977).

   D.   Berwick, R., M. Schapiro, J. Kuhner, D. Leucke and J.J. Winerman. Selected Topics in Stormwater Management Planning for Residential Developments. Report No. EPA/2-80-013. Office of Research and Development, U.S. Environmental Protection Agency, Washington, D.C. (1980).

   E.   Soil Conservation Service, Interim Soil Survey of Silver Valley Area, Idaho. U.S. Department of Agriculture Soil Conservation Service, Washington, D.C. (1989).

   F.   Wanielista, M.P. Stormwater Management: Quantity and Quality. Ann Arbor Science, Ann Arbor, MI (1979).

   G.   Page, A.L. (editor). Methods of Soil Analysis, Part II (Chemical Measurements) Second Edition. American Society of Agronomy, Soil Science Society of America (1982).

   H.   Bunker Hill Superfund Site Hillside Area Remedial Design Report. Pintlar Corporation and Jasberg Technical Services, Kellogg, ID (1993).

   I.   Final Commercial Properties Remedial Design Report for the Bunker Hill Superfund Site, January 1994; prepared for ASARCO Inc., Hecla Mining Company, Sunshine Mining Company.

   J.   Final Residential Yards Remedial Design Report for the Bunker Hill Superfund Site, January 1994; prepared for ASARCO Inc., Hecla Mining Company, Sunshine Mining Company.

   K.   Final Rights-Of-Way Remedial Design Report for the Bunker Hill Superfund Site, January 1994; prepared for ASARCO Inc., Hecla Mining Company, Sunshine Mining Company.

   L.   Institutional Controls Program for the Bunker Hill Superfund Site, Regulatory and Performance Standards Component, September 20, 1993; Murray Lamont & Associates, Inc.

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)

   As the population density and land values increase, the effects of uncontrolled runoff become an economic burden and a serious threat to the health and well being of a community and its citizens. This is especially true in an area with the environmental circumstances of the site. Therefore, the focus of storm water management is to protect the health, safety, and welfare of the general public, and mitigate property damage by reducing the frequency and severity of flooding, and limit, insofar as possible, the migration of heavy metal contaminants and their direct contact with the public. Estimating the frequency and severity of future flooding events makes systematic planning and installation of structural and nonstructural measures to reduce these hazards to an acceptable level available to the designer. Additionally, treatment of storm water to improve its quality is now required at federal, state, and local levels to control contaminants and prevent degradation of public water sources (GIAs and sediment removal).

   Studies in other parts of the country indicate that for a basin of one square mile that is completely storm sewered and whose surface is completely (or 100 percent) impervious, the mean annual flood (approximately the two (2)-year flood) is about eight (8) times larger than for the natural basin. The mean annual flood from a basin of one square mile that is completely storm sewered but zero percent (0%) impervious is about 1.7 times as large as the natural basin. The mean annual flood for a basin which is completely impervious but not sewered is about two and a half (2.5) times as large as for the natural basin. Information of this type is unavailable regarding discharges from urbanized areas in Idaho 1 .

   A.   "Precipitation", whether it occurs as rain or snow, is the source of surface runoff in watersheds. The extent of the storm and the distribution of rainfall during the storm are two (2) major factors which affect the rate of peak runoff.

      1.   The storm distribution can be thought of as a measure of how the rate of rainfall, or "intensity", varies within a given time interval. For example, within a given twenty-four (24) hour period, a measurable amount of precipitation may have been recorded. However, this precipitation may have occurred over the entire twenty-four (24) hour period or just within one hour. These two (2) scenarios represent entirely different storm distributions.

      2.   The size of the storm is described by the length of time over which the precipitation occurs, the total amount of precipitation occurring and how often this same size of storm would be expected to occur (its "frequency"). A ten years (10)-year, twenty-four (24)-hour storm is a storm producing a measured intensity of rain in twenty four (24) hours with a ten percent (10%) chance of occurrence in any given year. Thus a one hundred (100)-year, twenty-four (24)-hour storm is a storm producing a measured intensity of rain in twenty-four (24) hours with a one percent (1%) chance of occurrence in any year.

      3.   The "time of concentration" is that time it takes for water to travel from the most distant part of the watershed to the point of interest. The time of concentration affects the peak rate of runoff and shape of the hydrograph.

      4.   A streamflow or discharge "hydrograph" is a graph or table showing the flow rate as a function of time at a given location on the stream or for a watershed. In effect, the hydrograph is "an integral expression of the physiographic and climatic characteristics that govern the relations between rainfall and runoff of a particular drainage basin" 2 .

      5.   "Antecedent moisture content" is the existing soil moisture content resulting from the amount of precipitation occurring during the preceding five (5) days. This content, especially when close to saturation, is a very important feature in the process of runoff generation.

      6.   The "infiltration and percolation rates" of soils indicate their potential to absorb precipitation and thereby reduce the volume of runoff. In general, the higher the rate of infiltration, the lower quantity of storm runoff. Fine textured soils such as clay produce a higher rate of runoff than do coarse textured soils such as gravelly sand.

      7.   The "type of cover" and its hydrologic condition affects the runoff volume. Vegetation, including the ground litter, forms numerous IC barriers along the path of the land which slows the water down and reduces the volume of runoff. Fallow land yields more runoff than forests or grassland for a given soil type. Development reduces the natural storage and infiltration increasing the amount of runoff.

      8.   Plants transpire moisture into the atmosphere creating a moisture deficiency in the soil which must be replaced by some portion of the precipitation before runoff occurs. The canopy provided by the foliage and ground litter allow the soil to retain its infiltration potential. Some moisture is retained on the surface of the foliage, evaporating back to the atmosphere. Other portions of the intercepted precipitation take so long to drain from the plants to the soil that it is withheld from the initial period of runoff.

      9.   "Surface depression storage" begins when the precipitation exceeds infiltration. "Overland flow" starts when surface depressions are full. The water in depression storage is not available as direct runoff.

      10.   In wetlands, or on very flat areas where ponding occurs within a watershed, a considerable amount of surface runoff may be retained in temporary storage, thus reducing the runoff.

   B.   "Steep and unstable slopes" present additional hydrologic problems. It should be recognized that while a certain site may be stable under natural conditions, the same site with steeper slopes may have increased erosion potential and an unstable slope when the site is developed. Slope and soils are in balance with vegetation, underlying geologic structure, and the ground and surface hydrologic environment in its natural, predeveloped state. Any development will permanently alter this natural equilibrium and may affect the stability of the slope.

      1.   Slope stability is a relative measure of the earth's resistance to physical change. Natural processes such as wind, rain, runoff, ground water conditions, or human influenced processes such as the removal of surface vegetation or the soil mantle result in erosion, downcutting, slumping, and sliding of the slope.

      2.   Development design on slopes is influenced by the potential for damage which result from such processes. The relative risk to life or property may have no influence on the design or may be so great that development on a particular site should be abandoned. In all cases, special engineering considerations should be integrated into development of the site to provide an acceptable level of risk.

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)

   A.   The general climate of the Idaho Panhandle is transitional between a north Pacific coastal type and a continental type. The Pacific influence is particularly noted by an autumn and winter maximum in low type cloudiness and precipitation. These are enhanced by the mountainous topography and its forced uplift of moist airflow 1 . Precipitation increases may be expected from this uplift. In Shoshone County, this orographic effect is strongly evident with average annual precipitation in the lower valleys of thirty to thirty five inches (30–35") increasing to fifty five to sixty five inches (55–65") in the adjacent mountainous area 2 . The cloudiest, wettest months are usually November, December, and January. (See figure 6-1, the mean annual precipitation map attached to ordinance 364, appendix B, on file in the office of the city clerk.)

      1.   a.   In the city of Kellogg between the years of 1905 and 1992, there were four (4) storm events which exceeded two inches (2") of precipitation in a twenty-four (24) hour period with the extreme being four inches (4") on December 9, 1987. The high monthly total in Kellogg was 12.28 inches in December 1933.

         b.   Wallace recorded ten (10) events between 1931 and 1992 which exceed two inches (2") in a twenty-four (24) hour period with a high of 3.18 inches on November 26, 1960, and a high monthly total of 14.56 inches during January of 1974.

         c.   Mullan recorded two (2) events between 1972 and 1992 which exceeded two inches (2") in a twenty four (24) hour period with a high of 3.17 inches on February 16, 1986, and a high monthly total of 10.2 inches in the month of November 1990. These precipitation totals are in inches of water but may have come in the form of snow or rain.

      The following twenty-four (24)-hour duration storm event frequencies shall be used for design purposes in the site:

 

 

      These event frequencies are from the national oceanic and atmospheric administration (NOAA) 1973, "Precipitation-Frequency Atlas Of The Western U.S.", NOAA atlas 2, volume V-Idaho, National Weather Service, Silver Spring, Maryland. The data is from Kellogg, ID 1933-1954 daily precipitation data, state climatologist, University of Idaho, Moscow, ID.

      2.   In Idaho, most of the extremely high rates of runoff of record are caused by intense thunderstorms. Portions of Shoshone County area considered to be susceptible to flooding from intense thunderstorms and meltoff events. The south fork of the Coeur d'Alene River creates a substantial floodplain as it passes through the site and areas of it are designated as such by the national flood insurance program (NFIP). These storm cells are often small and may be confined to a small portion of the basin, but produce rates of runoff which are much higher than typical precipitation events 1 . June, July, and August are normally the peak months for thunderstorm activity in the Idaho Panhandle with an average of three (3) thunderstorms per month 2 .

      3.   Occasionally, in the spring climatic conditions (frozen ground, rising air temperatures, increasing wind velocities, abundant snowpack, and rain) may combine to produce high rates of runoff. As the temperature warms, the snow melts, when temperatures drop below freezing again the melt water freezes in place forming an impermeable frozen barrier. Even the most permeable alluvial deposits or basaltic surfaces are no longer able to infiltrate runoff. If additional snowfall or rain follow, high rates of runoff may result (see figure 6-2 attached to ordinance 364, appendix B, on file in the office of the city clerk).

   B.   Given the varied topography and precipitation rates within Shoshone County, the environmental concerns, the susceptibility to flooding from intense thunderstorms, the possibility of spring flooding from abundant snowfall and climatic conditions, a peak runoff design storm with a twenty-five (25)-year reoccurrence level is considered to be prudent for storm water management designs in areas of the site which do not have soil barriers. All areas within the site which do have soil barriers must be designed to withstand a storm of fifty (50)-year reoccurrence.

(Ord. 364, 4-5-1989)

   The standards established by this chapter are intended to represent the minimum design standards for the construction of storm drainage facilities and stream channel improvements. Compliance with these standards does not relieve the designer of the responsibility to apply conservative, sound professional judgment to protect the health, safety, and welfare of the general public. These are minimum design standards and should be considered the lowest acceptable limits in design. Special site conditions and environmental constraints may require a greater level of protection than would normally be required under these standards.

   The design policies and standards contained herein are intended to serve as a basic guide in design work; however, they are not to be considered inflexible. They are also not intended to be a substitute for engineering knowledge, experience or judgment. When it is deemed necessary or desirable to deviate from these design policies and standards, the developer shall submit plans which show compliance with at least the minimum design standards for approval from the planning administrator.

   A.   Storm Water Control: The rate of runoff from any proposed land development shall not exceed the rate of runoff for the design storm prior to the proposed land development.

      1.   Restriction of storm water runoff on the proposed land development shall be controlled by structural measures and best management practices (BMPs). Structural measures include collection systems, conveyance systems and storage (retention/detention) systems with treatment processes. BMPs are low structural or nonstructural alternatives designed to control the runoff volume from the source and improve the runoff water quality. Grassed infiltration areas are recognized as a nonstructural, low cost BMP, with high levels of contaminant removal (see article D of this chapter).

      2.   Runoff cannot be diverted in the proposed development and released to downstream property at locations not receiving runoff prior to the proposed development unless an easement, consent, and on site retention is provided by the downstream property owner and approval is granted by the planning administrator.

      3.   a.   For sites with tributary basins ten (10) acres or less and slopes of zero to ten percent (0–10%), an allowable runoff rate shall be limited to the predevelopment peak runoff for a twenty-five (25)-year storm. If limiting the runoff to this level is unfeasible for some reason, it must be demonstrated to the planning administrator in detail in the storm water control plan. Other alternatives shall be discussed and analyzed in detail for possible downstream locations and repercussions of increased runoff before any permit is issued to the developer. Construction of structures for controlling any increase of runoff from a site may be required of the developer by the planning administrator. The peak runoff rate shall be computed using the rational method (see subsection 11-13G-4B of this article).

         b.   For sites with tributary basins ten (10) acres or less and slopes of ten to forty percent (10–40%), an allowable runoff rate shall be limited to the predevelopment peak runoff for a 50-year storm. Other alternatives shall be discussed and analyzed in detail for possible downstream locations and repercussions of increased runoff before any permit is issued to the developer. Construction of structures for controlling any increase of runoff from a site may be required of the developer by the planning administrator. The peak runoff rate shall be computed using the soil conservation service TR-55 method (see subsection 11-13G-4C of this article).

         c.   For sites with tributary basins greater than ten (10) acres and slopes of ten to forty percent (10–40%), an allowable runoff rate shall be limited to the predevelopment peak runoff for a 50-year storm. Other alternatives shall be discussed and analyzed in detail for possible downstream locations and repercussions of increased runoff before any permit is issued to the developer. Construction of structures for controlling any increase of runoff from a site may be required of the developer by the planning administrator. The peak runoff rate shall be computed using the soil conservation service TR-55 method.

      4.   The rainfall intensity shall be obtained from the Idaho transportation department's intensity-duration-frequency charts or table 6-1. These charts are based on U.S. weather bureau records. The state of Idaho has been divided into different intensity-duration-frequency zones (IDF zones). For the site, the IDF zone E or F may be applicable depending on the geographic location of the site. A map of Idaho delineating the IDF zones and the IDF charts for zones E and F are shown in figures 6-3, 6-4, 6-5, 6-6, and 6-7, respectively.

      5.   Other hydrologic methods may be appropriate for determination of runoff rate; however, it is strongly recommended that the designer consult with the planning administrator and all federal, state, or local authorities prior to beginning hydrology studies for the project if an alternate hydrologic method is selected.

      6.   Developments shall be engineered and constructed to provide control of the quality and quantity of storm water runoff during and after construction. The on site drainage system including conveyance systems, detention/retention systems, pollution control, and emergency overflow elements must be properly designed and sized to handle runoff from the site and conveyance through the site.

      7.   Appropriate easements and rights of way shall be established by the developer to the satisfaction of the planning administrator through federal, state and local agencies.

      8.   a.   Off site storm water runoff passing through the site shall be conveyed by hydraulically adequate conveyance systems and shall pick up any sediments from the site during its passage through it.

         b.   Off site surface water (streams) entering the site shall be received and discharged at the existing locations with adequate energy dissipaters to minimize downstream damage. There shall be no diversion at either of these points.

      9.   Storm water detention/retention systems and culverts shall be designed to permit storage and conveyance capacity as required in these standards. In addition, these structures shall be designed to provide a satisfactory level of protection for the 100-year storm. This design standard will assure a level of protection for all habitable structures and assure reasonable access is available to all habitable structures during the 100-year storm event.

      10.   Outfalls must conform to the requirements of the planning administrator and all federal, state, and local authorities.

      11.   For any future developments, class V(e), shallow injection wells shall be authorized for disposal of storm water runoff provided the effluent from the well meets the water quality standards set by the department of water resources under the rules and regulations for "Construction And Use Of Injection Wells". Attention shall be paid to the surrounding upstream soil types and the possibility of heavy metal contaminants entering the ground water through any injection well.

      12.   a.   Any and all stream work shall be consistent with the requirements of the planning administrator and any or all other regulatory agencies including, but not limited to, the environmental protection agency, Idaho department of water resources, Idaho department of lands, Idaho department of fish and wildlife, Idaho department of environmental quality, Panhandle health district, and army corps of engineers.

         b.   For purposes of these standards, "stream" shall mean any and all surface water routes generally consisting of a channel having a bed, banks, and/or sides in which surface waters flow in draining from higher to lower land, both perennial and intermittent; the channel, banks, and intervening artificial components.

      13.   Lands that lie within "flood hazard zones" as delineated on appropriate maps prepared by the federal insurance administrator, shall comply with the regulations of the national flood insurance program (NFIP).

      14.   The planning administrator shall be consulted on the availability and applicability of computer programs for hydraulic design. The Idaho transportation department maintains a list of applicable computer programs for hydrologic applications. Programs identified or provided by the planning administrator or Idaho transportation department may be used for design. Responsibility for the use of these programs and their accuracy will be borne by the designer. Programs which have been used to develop the remedial studies for the site will be considered as acceptable and are as follows:

         a.   U.S. army corps of engineers, "HEC-1 Flood Hydrograph Package", 1990. (Haestad Methods, Inc.)

         b.   U.S. army corps of engineers, "HEC-2 Water Surface Profiles", 1990. (Haestad Methods, Inc.)

         c.   Haestad Methods, Inc., "Flowmaster Model For Open Channels And Pressure Pipes".

         d.   Haestad Methods, Inc., "Pondpack Model For Detention Pond Design".

         e.   Intelisolve, "Hydraflow, Vol. 1 Hydrographs, Vol. II Open Channels And Vol. IV Sanitary For Piping System Analysis".

         f.   McDonald and Harbaugh, 1988, "MODFLOW, 3-Dimensional Groundwater Flow Model", USGS.

         g.   Dames & Moore, 1985, "TARGET, Dames & Moore Mathematical Model Of Groundwater Flow And Solute Transport", October 1985.

         h.   U.S. army corps of engineer waterways experiment station, 1989, "Hydrologic Evaluation Of Landfill Performance (HELP)", version 2.04.

         i.   Civil Software Design, 1989, "SEDCAD, Version 3.0, Flood Hydrograph And Sedimentgraph Package" (used for hillside runoff/sedimentation study).

      Table 6-1 provides the minimum storm design frequency for hydraulic structures as approved by the planning administrator. The designer may find that the federal, state, or local ordinances will require more restrictive design frequencies.

   B.   Storm Sewers: A storm sewer is a system of drainage conduits that carries surface drainage or street wash, from catch basins or surface inlets, to an outfall. All storm sewer designs will be based on engineering analysis which takes into consideration total drainage areas, runoff rates, pipe capacity, foundation conditions, soil characteristics, pipe strength, potential construction problems, and any other factors pertinent to design.

      Storm sewer design is based on the calculation of peak runoff from an area and design of a pipe system to carry the runoff. The peak runoff design storm shall be the 25-year storm except in areas where soil IC barriers are present, where the peak runoff design storm shall be the 50-year storm.

      1.   Minimum design standards for storm sewers:

         a.   Future Expansion Of The Site: If it is anticipated that the site or storm sewer system may be expanded in the future, provisions for the expansion shall be incorporated into the current design.

         b.   Location Of The System: The location of the storm sewer system as it relates to the overall site design and delineation of boundaries for differing regulatory agencies (federal, state, or local) shall be clearly defined.

         c.   Pipe Alignment: No storm sewer pipe alignment in a drainage easement shall have its centerline closer than five feet (5') to a private property line.

         d.   Soil Conditions: Surface and subsurface drainage shall be provided to assure stable soil conditions necessary for adequate soil bearing capacity and protection of fills or cuts. Soil contaminant migration due to surface or subsurface drainage shall be avoided in all circumstances.

         e.   Profile Of Grade For The Storm Sewer: A profile of the pipe system shall be provided showing invert elevations, manholes, or catch basins showing top and bottom elevations, existing and finished groundline or grade elevations, etc.

         f.   Minimum Pipe Slope: Minimum pipe slope shall be used to maintain minimum velocities of flow. All storm sewers shall be designed and constructed to give mean velocities, when flowing full, of not less than two and a half feet (2.5') per second.

         g.   Minimum Pipe Diameters: The minimum pipe diameter shall be twelve inches (12") for storm sewers and irrigation systems, except that single laterals less than fifty feet (50') long may be eight inches (8") in diameter. Pipe carrying drainage from irrigated lands shall be considered as culverts and the appropriate minimum size used.

         h.   Inlet Spacing And Capacity: Inlet spacing on roads and highways shall be designed and constructed according to the requirements of the planning administrator, and all federal, state, and local authorities.

         i.   Junction Spacing For Catch Basins And Manholes: Catch basins or manholes shall be provided for breaks in grade, or alignment or along pipe runs. For pipe runs, junctions shall not exceed three hundred feet (300') for pipes smaller than forty eight inches (48") or five hundred feet (500') for pipes greater than forty eight inches (48").

         j.   Grades Through Junctions: Grades through junctions shall be consistent with the standards of the planning administrator.

         k.   Minor Head Loss Calculations: Minor head loss calculations shall be included in the storm water management plan. In some cases, head losses occurring at pipe junctions may not be negligible. The design shall provide calculations for the head losses in the following cases:

            (1)   Very high velocities with the inlet and outlet pipes forming a sharp or acute angle at the junction.

            (2)   Changes in pipe diameter.

            (3)   Abrupt changes in slope.

         l.   Outfalls: Outfalls must conform to the requirements of the planning administrator and all federal, state, and local authorities.

         m.   Siltation Basins: Siltation basins shall be used to prevent clogging or silting of the storm sewer lines or the migration of silts which have been contaminated by heavy metals or other substance in the soil and shall be incorporated with the use of cast metal inlets, concrete inlets, or any other inlet which does not have a silt basin.

         n.   Project Life: Project life of materials for the purpose of selecting storm sewer materials shall be one hundred (100) years.

   C.   Culverts: A culvert is a conduit used as an artificial channel under a roadway or embankment to maintain flow from a natural channel or drainage ditch. A properly designed culvert will carry the flow without causing damaging backwater, excessive constriction, or excessive outlet velocities. Designing culverts involves the determination of design flow, hydraulic performance, economy, pipe materials, horizontal and vertical location, environmental considerations, and end designs.

      The culvert design should include the profile of the culvert flow line (invert), culvert length, allowable headwater depths for the design flood and the 100-year flood, roadway or embankment cross sections, and a roadway or embankment profile showing the height of the fill. The hydraulic features of the downstream controls, tailwater or backwater must be given.

      There are certain cases where hydraulic capacity is not the only consideration for determining the size of a waterway opening. Fish passage requirements may dictate a different type of crossing. Wetland preservation may require a culvert be upsized. The designer shall follow the regulations and recommendations of the appropriate authority (federal, state, local) and document the information in the storm water management plan.

      1.   Minimum Design Standards For Culverts:

         a.   Minimum Culvert Size: The minimum diameter of culvert pipes under the main roadway shall be eighteen inches (18") until a length of seventy feet (70') is reached. All culverts over seventy feet (70') shall be twenty four inches (24") or more in diameter or shall conform to the requirements of the federal, state, or local authority, whichever is applicable. Culvert pipes from grated inlets or catch basins in the roadway have a minimum diameter of eight inches (8"). Culvert pipe under driveways, roadways, approaches, and median drains shall have a minimum diameter of eight inches (8"). Pipe draining irrigated land shall be sized using the minimum culvert sizes.

         b.   Allowable Headwater: All culverts shall be designed to carry the design frequency flood with a headwater (HW) depth not greater than 1.25 times the culvert depth (D). The intent of this limitation is to prevent the headwater from materially increasing the size of the flooded upstream area under normal conditions. It must be determined in the field what headwater depths can be permitted for the design flood and the 100-year flood. Allowable headwater depths are determined by the field conditions and are not associated with design criteria.

         c.   Level Of Protection: This design standard shall assure a level of protection for all habitable structures and assure reasonable access is available to all habitable structures during the one hundred (100)-year storm event (i.e., the culvert should pass the 100-year storm without damaging or eroding the roadway or embankment).

         d.   Depth Of Tailwater: Depth of tailwater is important in determining the hydraulic capacity of culverts flowing with outlet control. When the downstream water surface elevation is controlled by a downstream obstruction or backwater and the tailwater at the outlet of the culvert exceeds the height of the water outlet for the culvert barrel, the capacity of the culvert is reduced. Field inspection of all major culvert locations should be made during project design to evaluate the downstream controls and to assess the potential discharge capacity of the culvert or the resulting headwater when a culvert is flowing under the outlet control.

         e.   Velocity In Culverts: Due to its hydraulic characteristics, a culvert will increase the velocity of flow over that in the natural channel. High velocities (abrasive velocities) are most critical just downstream from the culvert outlet where the erosion potential from the energy in the water is enormous.

         f.   Placement Of Culverts: Culverts shall be placed on grades that produce a nonsilting or nonabrasive velocity, three (3) to ten feet (10') per second. Silting velocities may be overcome by raising the inlet of the culvert above the stream invert to increase the grade and provide a siltation basin at the inlet. Siltation basins upstream of the inlet may also be provided by excavating a basin or using a concrete basin. Siltation basins must be large enough to decrease velocities to two feet (2') per second but shall not short circuit the flow over the top or along the sides of the embankment. Oversized culverts may be used where silting will occur to prevent blocking and to facilitate cleaning.

         g.   Abrasive Velocities: Abrasive velocities may be overcome by raising the outlet to decrease the grade. Structural plate pipe may be provided with extra thickness in the bottom plates to account for possible abrasion. Concrete box culverts and concrete bases for arches may have steel inverts of rails or beams, or extra slab thickness to resist abrasion.

         h.   Alignment And Grade: Culverts shall generally be placed on the same alignment and grade as the natural streambed or channel to maintain the natural drainage system. This avoids creating unnatural ponding at the inlet or drops at the outlet.

         i.   Embankment Material: The embankment for the culvert shall consist of nonerodable material and riprap or other outlet protection is essential. The embankment material should be free draining and capable of withstanding ponding.

         j.   Culvert Material: Concrete pipe may be used for any grade up to ten percent (10%). Corrugated metal pipe may be used for grades up to twenty percent (20%). For grade in excess of twenty percent (20%), the design and material shall be approved by the planning administrator.

         k.   Minimum Cover: Minimum cover for a culvert within any roadway embankment shall be one foot (1').

         m.   Project Life: Project life of culverts for the purposes of selecting culvert materials shall be one hundred (100) years.

         o.   Camber: If a large amount of settlement is anticipated due to high fills, it may be necessary to add some camber to the culvert profile to correct for settlement. A soils engineer shall be consulted at the planning administrator's request as to the amount of camber which should be placed in a culvert.

         p.   Moderate/High Fill Culverts: Culverts installed under moderate to high fills can experience differential settlement. At the pipe ends, there is no fill, at the centerline of the embankment the depth of the fill is at maximum. The difference in surcharge pressure at the elevation of the culvert causes the differential settlement. Consult a soils engineer for design of a culvert under these conditions.

         q.   Culvert End Considerations: The type of end treatment used on a culvert depends on many interrelated and sometimes conflicting considerations. The designer must consider safety, aesthetics, debris capacity, hydraulic efficiency, scouring, and economics.

            (1)   Projecting Ends: Projecting ends are the simplest and most economical of all end treatment designs; however, projecting ends provide no transition to prevent scouring at the outlet and buoyancy may become a problem at the entrance. Projecting ends shall not be used in a highway "clear zone" as designated by Idaho transportation department.

            (2)   Beveled End: For culverts thirty inches (30") or less in diameter, a beveled end section shall be the preferred end treatment. The standard bevel end section shall not be used on pipes laid on a skew of more than thirty degrees (30°) from the perpendicular to the centerline of the roadway.

            (3)   Flared End: A flared metal end section is a manufactured culvert end that provides a simple transition from the culvert to the streambed or drainage channel. It provides a hydraulic transition which allows the flow to spread out and slow down before it is discharged into the watercourse. Flared ends shall not be used for culverts greater than forty eight inches (48") in diameter. Flared ends shall not be used in a highway "clear zone" as designated by Idaho transportation department.

         r.   Headwalls: All pipes larger than thirty inches (30") in diameter shall be beveled at the entrance to conform to the fill slope and provided with a concrete headwall. Pipe culverts larger than seventy two inches (72") in diameter shall have their inlet headwalls beveled, tapered or rounded to improve the inlet flow characteristics. In addition to the headwall, riprap may be required by the planning administrator.

         s.   Wingwalls: Wingwalls shall be used on reinforced concrete box culverts. However, they may be modified for use on circular culverts in areas with severe scour problems. The purpose is to retain and protect the embankment and to provide a transition from the channel to the culvert.

         t.   Improved Inlets: When head losses in the culvert become critical, the designer shall consider the use of the hydraulically improved inlet. These inlets provide side transitions as well as top and bottom transitions that have been designed to maximize the culvert capacity. However, it should be noted it is generally less expensive to increase the size of the culvert by one or two (2) sizes to achieve the same or greater benefits.

         u.   Energy Dissipaters: When the outlet velocities of a culvert or storm sewer outfall are excessive for the site conditions, the designer shall provide energy dissipaters for culvert outlets shall be used to prevent severe scouring at the outlet. Debris and maintenance problems shall be addressed in the design.

            (1)   Riprap: Riprap placed at the outlet of a culvert is the simplest method of handling outlet velocities when the soils are unstable.

            (2)   Special Energy Dissipating Structures: Special energy dissipating structures, including impact basins and stilling basins, shall meet the requirements of the planning administrator and any or all federal, state, or local authorities and shall be approved on a site specific basis.

         v.   Culvert Debris: Debris problems can cause even an adequately designed culvert to experience hydraulic capacity problems. The culvert site is a natural location for these materials to settle and accumulate. Debris may consist of anything from limbs, sticks, orchard prunings to logs and trees. Silt, sand, gravel, and boulders can also be classified as debris.

            (1)   There is no method to accurately predict debris problems. Examining maintenance history for similar sites is probably the most reliable way of determining potential problems. Requirements for debris deflectors, racks, basins, or spillways will be addressed on a site specific basis by the planning administrator.

   D.   Open Channel Flow: Open channel flow is encountered naturally in rivers and streams. Artificial channels include irrigation channels, drainage ditches, swales, or gutter flow for pavement drainage. Proper design requires that open channels have sufficient hydraulic capacity to convey the magnitude of runoff from the design storm. In the case of grassy swales, bank protection is also required if the velocities are high enough to cause erosion or scouring.

      1.   Minimum Design Standards For Open Channel Flow:

         a.   Stream Velocities: Stream velocities are useful in determining the hydraulic capacity of a channel and the need for erosion protection along the banks. As the depth of the water in an open channel increases, the velocity also increases. Maximum velocity in open channels shall be four feet (4') per second.

         b.   Manning's Equation: Manning's equation shall be the method of analysis for open channel flow. The trapezoid is the most often used for a channel cross section. Manning's coefficients are provided in table 6-4.

         c.   Critical Depth: Occasionally, it is necessary to determine critical depth in design of open channels. This will occur when the level of the headwater is controlled by the entrance of the pipe or channel rather than by the pipe or channel itself. Critical depth should always be calculated since it may control (when critical depth is less than normal depth) the capacity of the channel on a relatively steep slope (0.5–1.0 percent).

         d.   River Backwater Analysis: Natural river channels tend to be highly irregular in shape so as simple analysis using Manning's equation, while helpful for making an approximation, is not sufficiently accurate to determine a river water surface profile. The planning administrator shall be consulted on the availability and applicability of computer programs and data useful in calculating a backwater profile. The Idaho transportation department maintains a list of applicable computer programs. Programs provided by the planning administrator and Idaho transportation department may be used for design. Responsibility for the use of these programs and their accuracy will be borne by the designer.

         e.   Stream Bank Stabilization: Stream banks shall be stabilized to the planning administrator's satisfaction where degraded or impacted by the development. Bank stabilization is required when design flow velocities of new or existing channels exceed three feet (3') per second or erosive velocity for the channel bed, whichever is smaller.

         f.   Existing Ditch Modification: Existing ditches shall be provided with riprapped bottoms and side slopes at the discharge points of storm sewers or culverts. The rock shall extend for a minimum of ten feet (10') downstream from the end of the storm sewer or culvert.

         g.   Channel Modification: All channel sides and bottoms shall be seeded, sodded, or riprapped as directed by the planning administrator.

         h.   Velocity Control: Check dams, pools, nonerosive drop structures, or other means shall be used as necessary to control velocity.

   E.   Detention/Retention Facilities: The peak rate of runoff from a site shall not be increased due to the proposed development for the design storm unless it has been sufficiently demonstrated to the planning administrator that no detrimental downstream effects will occur as a result and any structural requirements to limit said effects have been met. Therefore, retention or detention facilities will be required on the developed site.

      In certain situations, it is possible and even desirable to store a volume of water before allowing it to discharge to the downstream drainage courses. When this storage effect is properly considered, the design can reduce the amount of peak flow released to the downstream drainage facility or eliminate it altogether. Detention/retention facilities include GIAs, earthen basins, shallow injection wells, and buried tanks or large diameter pipes.

      1.   Open or closed storage detention systems are desirable in the overall management plan for the following reasons:

         a.   Prevention of storm water runoff in excess of predevelopment runoff to downstream properties.

         b.   Reduction of flow rates in downstream drainage structures.

         c.   Reduction of frequency and capacity of infiltration, conveyance and drainage systems on the site as a whole.

         d.   The size of pump stations could be designed with smaller pumps if storage for the runoff was provided.

         e.   Existing culverts or storm sewers on site which would be undersized may be made adequate if upstream storage were provided.

         f.   Allow any contaminated silts to settle out before the runoff is released into local surface waters.

      Detention/retention facilities which are subject to collecting contaminated sediments must either be cleaned on a frequent regular basis or have some form of access control (fencing) around them. As maintenance often has a tendency to be put off, access control is the preferred method of dealing with the issue. Detention and retention facilities should be cleaned after all storm events approaching the design intensity, bimonthly during the wet season, and at the end of the winter as a minimum. Testing of collected sediments should be performed annually for all such control facilities to determine the level of contamination expected in the sediments collected in each structure. All materials with lead levels in excess of three hundred fifty (350) ppm must be disposed of at a PHD approved repository. All sediments collected in a given control structure are to be considered as contaminated if no testing has been performed at that location. If the sediments from a structure test less than one hundred (100) ppm for two (2) consecutive years, then the sediments from that structure shall be considered as clean and no further testing will be required.

      2.   Liabilities and potential problems associated with detention systems must also be considered:

         a.   There must be adequate space available for a storage basin.

         b.   The potential for damage from overtopping the design level of the storage basin must be considered.

         c.   There is a potential safety hazard to the public for storage basins which could become covered with several feet of water.

         d.   Basins subject to frequent flooding may lose natural vegetation from the excessive amounts of water.

         e.   Accumulated sediments which may be contaminated by heavy metals or have a low pH, posing a public health hazard.

      3.   Minimum design standards for detention/retention facilities:

         a.   An overflow type storage detention system which regulates the rates of flow shall be the preferred storage facility. Landscaped facilities such as parks and playfields are recommended for use as storm water detention facilities unless the sediments are likely to contain heavy metals or have a low pH.

         b.   All storage detention facilities shall be accessible for maintenance and a regular maintenance schedule shall be submitted with the operation and maintenance plan. Access for maintenance vehicles (generally 8 to 10 feet wide travel lane) shall be included in the design.

         c.   Storage detention facilities with side slopes steeper than five (5) horizontal to one vertical shall be fenced as approved by the planning administrator.

         d.   The reservoir routing method is straightforward and shall be used for design of the storage detention facility. This method involves plotting an inflow hydrograph for the future storm event, then superimposing an outflow hydrograph over the inflow hydrograph. The area inscribed between the two (2) graphs represents the volume to be stored in the facility. The inflow hydrograph will be developed using the rational method (see below).

         e.   The outflow hydrograph will be dependent upon the particular control structure selected as well as the depth of the water in the storage detention basin. The actual shape of the outflow hydrograph can be developed by an application of a technique known as reservoir routing.

         f.   Any detention or retention facility which is likely to accumulate contaminated sediments shall have an impervious bottom and access control (fence) to eliminate public contact with the sediments.

   F.   Shallow Injection Wells:

      1.   Shallow injection wells are any excavation or artificial opening into the ground which meets the following four (4) criteria:

         a.   It is a bored, drilled, or excavated hole, a driven mine shaft;

         b.   It is deeper than its largest straight line surface dimension;

         c.   It is used for or intended to be used for injection; and

         d.   The well is less than or equal to eight (8) vertical feet of depth below ground level. Catch basins with lateral subsurface infiltration piping are also considered to be injection wells under subsection F1c of this section.

      2.   Class V (e) shallow injection wells are used for the disposal of nonhazardous, nonradioactive wastes such as clean storm water runoff and irrigation wastewater. Regarding class V (e) injection wells the Idaho Code, title 42, chapter 39, states: "the ground waters of this state to be a public resource which must be protected against unreasonable contamination or deterioration of quality to preserve such waters for diversion to beneficial uses". Contamination as defined by the statute means:

      3.   Minimum design standards for shallow injection wells:

         a.   The capacity of a site's subsurface soils to infiltrate storm water from a shallow injection well shall be determined based on SCS soil permeability criteria. Soil gradation (sieve analysis) determinations of specific soils shall be required at the option of the planning administrator to confirm SCS soil class criteria prior to injection well construction.

         b.   Class V (e), shallow injection wells are authorized by rule for the life of the facility provided that the required inventory information is furnished to the department of water resources and use of the well does not contaminate a drinking water source.

         c.   The owner or operator of a proposed class V (e) injection well shall submit the following inventory information to the director of the department of water resources as a condition of authorization:

            (1)   Facility name and location;

            (2)   County in which the injection well(s) is (are) located;

            (3)   Location of the well(s) by legal description to the nearest ten (10) acre tract, or by highway milepost where the well is owned or operated by a state or local entity;

            (4)   Ownership of the wells;

            (5)   Name, address, and phone number of legal contact;

            (6)   Type or function of the well(s);

            (7)   Number of wells of each type; and

            (8)   Operation status of the well(s).

         d.   For any future developments, class V (e), shallow injection wells shall be authorized for disposal of storm water runoff provided the effluent from the well meets the water quality standards set by the department of water resources under the rules and regulations for "Construction And Use Of Injection Wells".

         e.   If the operation of a class V (e) injection well is causing or may cause contamination of a drinking water source, the planning administrator, or director of the department of water resources, shall require immediate cessation of the injection activity.

   G.   Constructed Wetlands 1 : Manmade wetlands can provide significant storage capacities for runoff and provide effective pollutant removal. Other benefits include recreation and aesthetic possibilities. Studies have found that controlled storm water retention in marshes has resulted in better vegetative conditions which in turn enhanced storm water nutrient removal. Other reports indicate removal of storm generated pollution has been achieved by detention and natural treatment in small urban lakes.

      The use of wetlands, whether they be lakes, ponds, or marshes, is straightforward from an engineering design standpoint. However, the disadvantage of these types of systems is a considerable effort may be required to maintain them in a healthy state. Wetlands can be seen to remove waterborne pollutants principally through physical and chemical processes which are substantially improved by biological processes associated with aquatic vegetation. This vegetation is generally resistant to the pollutants found in storm water; however, the interactions of various plant and animal species are not completely understood and changes in this community structure may affect the overall health, aesthetic character, and pollutant removal capacity of the wetland.

      1.   Minimum design standards for constructed wetlands:

         a.   Constructed wetlands shall be approved on a site specific basis by the planning administrator.

         b.   The designer shall be responsible for obtaining approval through all federal, state, and local authorities.

         c.   Wetland design should utilize a team approach with expertise in hydrology, water quality, soils, botany, wildlife ecology, landscape architecture as well as design and construction engineering.

         d.   Water retention time (approximated by volume divided by outflow rate) for particulate removal should be approximately one week. For removal of nutrients and soluble pollutants, greater than two (2) weeks retention time should be allowed.

         e.   Surface area (wet pool area divided by watershed or drainage basin area) should be 0.01 minimum, but preferably greater than 0.025.

         f.   Optimum water depth distribution within a wetland is fifty percent (50%) less than six inches (6") (a bench form), twenty five inches (25") to one foot (1') and twenty five percent (25%), two (2) to three feet (3').

         g.   Ideally a forebay would be constructed in the inlet channel prior to the wetland area proper. The depth of this forebay should be approximately three feet (3') and its surface area approximately twenty five percent (25%) of the total wetland area.

         h.   Consider a liner if needed to maintain summer water and to prevent ground water contamination.

         i.   To prevent flow short circuiting, place the outlet remote from the inlet, minimize flow velocities, construct multicelled configuration, and design a length to width rates of three to one (3:1) minimum, greater than five to one (5:1) preferably.

         j.   For inlet design, use techniques that will minimize entrance velocity and momentum such as level spreaders, enlarged inlet conveyance, reduced inlet slope, and baffle islands or peninsulas.

         k.   Soils to be used for wetland construction should have moderate to fine texture (loams) with relatively high muck. (This can be an important source of plant "propagules".)

         l.   Soil and water pH should be circumneutral (6 to 8) with slightly alkaline (7 to 8) best. Note that many of the soils located within the site have low to very low pHs. As such, care should be taken to study the use of wetlands as a contaminant removal technology in the site.

         m.   Plant selection principles center not around water quality goals, but:

            (1)   Prospects for effective establishment and continued survival under site conditions;

            (2)   Using native species;

            (3)   Avoiding invasive or nuisance species; and

            (4)   Selection to fulfill multiple objectives (if any).

         n.   Best pollutant removal plants are those dense, fine herbaceous plants with good winter viability. Choices for specific pollutants (NW native plants) include:

            (1)   For Metals Removal: Oenanthe sarmentosa (water parsley), Spirea douglasii (hardback), Carex species (sedges), Elodea canadensis lemna species (duckweeds), Nuphar species (water lilies), Scirpus species (bulrushes), Typha species (cattails).

            (2)   For Oils And Other Organics: Plants that grow from basal meristem- Juncus species (rushes), or Scirpus species (bulrushes).

         o.   Best results for establishing plantings are by using live plants or dormant rhizomes from nurseries; obtaining plants from natural wetlands is discouraged. Seeding offers lower prospects of success.

         p.   The designer shall provide a monitoring program for ensuring the wetland functions as is intended in the design.

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)

   A.   Drainage Basin: The size and shape of the drainage basins are important factors regardless of which method is used to determine the hydrologic characteristics of the basin. Determination of the basin area should only proceed after procurement of the best available topographic maps of the entire area contributing surface runoff to the site. Outline the area on the map(s) and determine the size in acres or square miles. Any areas that are known to be noncontributing to surface runoff should be subtracted from the total drainage area and justification given for this. The slope, length, and surface roughness of the drainage basin or watershed affect the travel times for runoff.

   B.   Rational Method:

      1.   Purpose: The rational method is used to predict peak flows for small drainage areas which can be either natural or developed. The greatest accuracy is obtained for smaller drainage basins or for developed conditions with large areas of impervious surfaces.

      2.   Drawbacks: Drawbacks of the rational method are:

         a.   It gives only the peak discharge and provides no information about the time distribution of the storm runoff.

         b.   Selection of the variables for the formula is more an art of judgment than a precise account of the antecedent moisture content or an aerial distribution of rainfall intensity 1 .

      3.   Facility Design: Given these limitations, the rational method shall only be used for design of storm water management facilities for drainage basins ten (10) acres or less. However, for these areas, the formula is accurate in determining the total runoff volume as well as peak discharges. These two (2) factors will provide the designer with a basis for designing GIAs, storm sewers, culverts, outfalls, open channels, etc.

      4.   Formula: The formula for the rational method is as follows:

 

      When several subareas within a drainage basin have different runoff coefficients, the rational formula can be modified as follows:

 

 

      5.   Runoff Coefficients: The runoff coefficient represents the ratio of runoff to rainfall. The rational method implies that this ratio is fixed for a given drainage basin. In reality, the coefficient may vary with respect to prior wetting and seasonal conditions.

      The coefficients in table 6-2 are applicable for peak storms of 10-year frequency. Less frequent, higher intensity storms will require the use of higher coefficients because infiltration and other losses have proportionally smaller effect on the runoff. Conversely, long duration storms with low average intensities would require the use of lower coefficients. Until such time as coefficients are available for 25-year frequency storms, this table of coefficients shall be used for Shoshone County areas.

      6.   Rainfall Intensity:

         a.   The rainfall intensity shall be obtained from the Idaho transportation department's intensity-duration-frequency charts. These charts are based on U.S. weather bureau records, and the state of Idaho has been divided into different intensity-duration-frequency zones (IDF zones). For Shoshone County IDF zone E, F, or H may be applicable depending on the geographic location of the site.

         b.   When using these graphs, it should be noted that the data from which they are derived is sporadic and much more information is needed for short duration storms in order to obtain more accurate estimates.

         c.   A map of Idaho delineating the IDF zones and the IDF charts for zones E, F, and H are shown in figures 6-3, 6-4, 6-5, 6-6, and 6-7, respectively.

      7.   Time Of Concentration:

         a.   The time of concentration is that time it takes for water to travel from the most distant part of the watershed to the point of interest. The time of concentration affects the peak rate of runoff and shape of the hydrograph.

         b.   Overland flow, storm sewer flow, and channel flow are three (3) phases of runoff flow commonly used in computing travel time. In a given watershed or drainage basin some areas would be dominated by overland (sheet) flow while other areas would be dominated by channel flow. Developed drainage basins may be further complicated by significant flows from storm sewers. The time of concentration is equal to the sum of the travel times computed for each subarea.

         c.   Special attention should be given to the computation of concentration and travel time. Once storm drains are installed the flow patterns may be changed so significantly that flow retardence cannot be represented by factors based on runoff curve numbers or overland flow. Velocities of flow through culverts and channels should be computed using hydraulic procedures that take into consideration the characteristics of flow paths.

         d.   An exception to this procedure may occur when a complex drainage pattern exists. The designer should be cautious when two (2) or more subbasins have different types of cover (i.e., different runoff coefficients). The most common case would be a large paved area together with a long narrow flat strip of natural area. In this case the designer should check the paved area and a fraction of the natural area to determine if this combination would produce a greater peak than the peak produced by using the longest time of concentration.

         e.   The procedure for determining the time of concentration is based on the U.S. department of agriculture, soil conservation service method which was originally developed from the Manning formula. It is sensitive to slope, type of ground cover, and the size of channels.

 

         (1)   The ground cover coefficient is provided in table 6-3.

         (2)   Manning's roughness coefficient is provided in table 6-4 (see "Open Channel Flow").

   C.   Soil Conservation Service Method:

      1.   For watersheds or drainage basins greater than ten (10) acres, the SCS, TR-55 1 method shall be used to calculate runoff volume and peak rates of discharge. The SCS uses three (3) standard rainfall distributions, types I, IA, and II. Type II distribution applies to all areas of the United States except for parts of the Pacific coast states (California, Oregon, and Washington). The type II distribution shall apply to the state of Idaho. The amount of runoff from a given watershed is solved using the following equations:

 

      2.   Runoff Curve Numbers: CN represents and indicates the runoff potential of a watershed. A higher CN results in a higher runoff potential. The factors affecting the CN, called the soil cover complex are a combination of the hydrologic soil group, the land use and treatment class (ground cover).

      3.   Hydrologic Soil Groups: The hydrologic soil groups describe a soil's potential to produce runoff from a precipitation event. Soils not protected by vegetation are placed in one of four (4) groups on the basis of the intake of water after the soils have been wetted and have received precipitation from long duration storms. The four (4) standard hydrologic soil groups 1 are:

         a.   Group A soils, having a high infiltration rate when thoroughly wet. These consist chiefly of deep, well drained to excessively drained, sands or gravels. These soils have a high rate of water transmission.

         b.   Group B soils, having a moderate infiltration rate when thoroughly wet. These consist chiefly of moderately deep or deep, moderately well drained or well drained soils that have moderately fine texture to moderately course texture. These soils have a moderate rate of water transmission.

         c.   Group C soils, having a slow infiltration rate when thoroughly wet. These consist chiefly of soils that have a layer that impedes the downward movement of water or soils that have a moderately fine texture or fine texture. These soils have a slow rate of water transmission.

         d.   Group D soils, having a very slow infiltration rate (high runoff potential) when thoroughly wet. These consist chiefly of clay soils that have a high shrink-swell potential, soils that have a permanent high water table, soils that have a claypan or clay layer near the surface, and soils that are shallow over a nearly impervious material. These soils have a very slow rate of water transmission.

      4.   Factors For Computing CN: Several factors should be considered when computing the anticipated future CN for urban areas. The types of structural or nonstructural methods used within the development for controlling the runoff will vary the quantity and flow paths of the runoff.

         a.   Consideration should be given to construction methods. Soils compacted by heavy equipment beyond natural compaction conditions, the extent of vegetative cover or sod versus barren soil within a pervious area, the impact of grading on surface and subsurface soils (to the extent that the mixed soils may create a completely different hydrologic condition) all must be factored into the CN determination.

         b.   Irrigation in farming communities or heavily watered lawns in suburban areas may significantly increase the moisture content in the soil above that which would be expected with natural rainfall conditions.

         c.   Table 6-5 gives CNs for agricultural, suburban, and urban land use classifications. The suburban and urban CNs are based on typical land use relationships that exist nationwide. The CNs should only be used when it has been determined that the area under study meets the criteria for which the CN was developed.

         d.   There will be areas to which the values in table 6-5 do not apply. The percentage of impervious area for the various types of residential areas or the land use condition for the pervious areas may vary from the conditions assumed in table 6-5. A curve for each pervious CN can be developed to determine the composite CN for any density of impervious area. Figure 6-8 was developed assuming a CN of ninety eight (98) for the impervious area. The curves in the figure can help estimate the increase in runoff as the impervious area increases for a given development.

         e.   The percentage of impervious to pervious area should be determined accurately and be representative of the site so the composite CN will assist the designer in accurately predicting the volume of runoff. The designer's assumptions and calculations shall be included in the hydraulic analysis to justify the selection runoff curve number.

      5.   Peak Runoff Rates:

         a.   A quick and reliable method of computing peak runoff rates from agricultural drainage areas, one to two thousand (2,000) acres in size, is through the use of the figures developed by the soil conservation service. These figures were prepared for the solution of the general relationships described above, are based on type II rainfall distribution and are applicable to the state of Idaho.

         b.   The figures are only to be used for watersheds within the one to two thousand (2,000) acre range. They provide a basic peak discharge rate for a 24-hour duration storm associated with the watershed in a natural condition. Figure 6-9, flat slope is based on a one percent (1%) slope. Figure 6-10, moderate slope is based on a four percent (4%) slope. Figure 6-11, steep slope is based on a sixteen percent (16%) slope. For slopes other than one percent (1%), four percent (4%), sixteen percent (16%), use the factors in table 6-6 to modify the peak discharge.

         c.   The following equations relate a method of adjusting the peak runoff rates from the charts to reflect the increase in the runoff due to urbanization:

            (1)   Modification of peak runoff rates due to urbanization:

 

            (2)   Figure 6-12 and 6-13 give the factors IMP and HLM for adjusting peaks calculated from the charts on the same parameters that affect watershed lag and time of concentration.

         d.   There are watersheds that deviate considerably from these relationships. The peaks can be modified for other shape factors using the following procedure:

            (1)   Determine the amount of runoff in inches for the watershed.

            (2)   Determine the hydraulic length of watershed and compute an "equivalent drainage area" using figure 6-14.

            (3)   Using the "equivalent drainage area" determine the rate of runoff from figure 6-9, 6-10, or 6-11 in cubic feet per second per inch.

            (4)   Using the equivalent area, obtain the slope adjustment factor from table 6-8 and multiply by the rate of runoff obtained in subsection C5d(3) of this section to obtain the equivalent peak discharge.

            (5)   Multiply the result from subsection C5d(4) of this section by the actual area of the drainage basin divided by the equivalent area obtained in subsection C5d(2) of this section.

            (6)   Compute the "actual peal discharge" for the watershed by multiplying the result in subsection C5d(5) of this section by the amount of runoff in subsection C5d(1) of this section.

      6.   Tabular Method Of Determining Peak Runoff:

         a.   An alternate method for determining the peak runoff rate is the tabular method. This method computes peak discharges from urban areas using time of concentration (Tc) and travel time (Tt). This method approximates the more detailed hydrograph analysis, SCS, Tr-20 "Computer Program For Project Formulation - Hydrology" developed by the engineering division of the soil conservation service.

         b.   The tabular method can be used to develop composite hydrographs at any point within a watershed by dividing the watershed into subareas and computing the time of concentration for each subarea and the travel time through each reach. The above factors also apply to the tabular method: twenty four (24) hour rainfall amount, a given rainfall distribution, hydrologic soil-cover complexes (CNs), time of concentration, travel time, and drainage area.

         c.   The tabular method is most effective when applied to watersheds where hydrographs are needed to measure nonhomogeneous runoff; i.e., the watershed is divided into subareas. It is especially useful in developed watersheds and can be used to determine the effects of structures, combinations of structures, and channel modifications at different locations within the watershed.

         d.   Table 6-7 shows the tabular discharge values for the type II rainfall distribution used in this procedure. Tabular discharges, in terms of cubic feet per second per square mile (csm) per inch of runoff, are given for a range of Tc from 0.1 to two (2) hours, and Tt from zero to four (4) hours. The table was developed by computing hydrographs for one square mile of drainage area for a range of time of concentration, routing them through stream reaches with a range of travel times. A constant runoff curve number of seventy five (75) and a rainfall volume sufficient to yield three inches (3") of runoff were used.

         e.   The tabular method should not be used when large changes in the runoff curve number occur among subareas within a watershed and when runoff volumes are less than about 1.5 inches for curve numbers less than sixty (60). For most watershed conditions, this procedure is adequate to determine the effects of urbanization on peak rates of discharge for subareas up to approximately twenty (20) square miles in size.

         f.   The computed values of time of concentration and travel time can be rounded to the nearest value in table 6-7 or, if more refinement is warranted, the discharges can be computed using the calculated time of concentration and travel time through interpolation between the Tc and Tt shown in the table. The information needed to calculate the peak discharge at a point in the watershed is:

            (1)   The drainage area of each subarea.

            (2)   Tc for each subarea.

            (3)   Tt for each routing reach.

            (4)   The runoff curve number for each subarea.

         (5)   The twenty four (24) hour rainfall for a selected frequency.

         (6)   The runoff in inches for each subarea.

      g.   For examples of soil conservation service design method consult "Urban Hydrology For Small Watersheds, Technical Release No. 55" published by the soil conservation service, USDA, January 1975.

      Additionally, the Idaho department of transportation offers self-taught courses in the following areas:

         (1)   Drainage design I hydrology.

         (2)   Drainage design II cross drainage.

         (3)   Drainage design III open channels.

         (4)   Drainage design IV roadway.

         (5)   Urban storm sewer design for Idaho highways.

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)

   A.   Brach, John, Wayne P. Anderson, Patricia Engelking, Protecting Water Quality in Urban Areas, Minnesota Pollution Control Agency, Division of Water Quality, the Soil Conservation Service, and the Environmental Protection Agency, St. Paul, MN (1989).

   B.   State of Idaho Transportation Department Design Manual, Section IV - Preliminary Design, 14-450 to 14-461, Idaho Department of Transportation Department, Boise, ID (1982).

   C.   Finklin, Arnold I. and William C. Fischer. Climate of the Deception Creek Experimental Forest, Northern Idaho, General Technical Report INT-226. United States Department of Agriculture, Forest Service, Intermountain Research Station, Ogden, UT (1987).

   D.   Berwich, R., M. Schapiro, J. Kuhner, D. Leucke and H.H. Winerman. Selected Topics in Storm Water Management Planning for Residential Developments. Report No. EPA/2-80-013. Office of Research and Development, U.S. Environmental Protection Agency, Washington, D.C. (1980).

   E.   Soil Conservation Service, Soil Survey of Kootenai County Area, Idaho. U.S. Department of Agriculture Soil Conservation Service, Washington, D.C. (1975).

   F.   Conservation Service, Urban Hydrology for Small Watersheds. Technical Release No. 55, U.S. Department of Agriculture Soil Conservation Service Engineering Division, Washington, D.C. (1975).

   G.   Spokane County Engineers, Guidelines for Stormwater Management 1981 (with addenda). Spokane, WA (1990).

   H.   Cow, Ven Te., Maidment D.R., Mays L.W., Applied Hydrology. McGraw- Hill Book Company, Series in water resources and environmental engineering. New York, NY (1988).

   I.   Richard R. Horner, Ph.D., Biofiltration for Stormwater Runoff Quality Enhancement, Course Spokane Community College, Spokane, WA (March 18, 1992).

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)

   The majority of the listed required contents for the storm water management plan can and should be shown on the drawings. Where additional text is desired to describe design features or functioning for example, this information shall be included in the project summary and design calculation section.

   A.   Project Summary And Design Calculations:

      1.   Project Summary:

         Size of site (acres or square feet).

         Structures to be constructed, including roof areas.

         Size (area) of planned roads, parking areas, and sidewalks.

         Changes in drainage.

         Changes in vegetative cover.

         Proposed construction schedule.

         Design problem.

         Constraining environmental conditions.

         Related soils, geologic, water quality, stream flow, or other data (as available).

         Analysis of ICP needs (yes/no).

      2.   Design Calculations:

         Hydrologic model used (SCS, rational method).

         Assumption made.

         Data used.

         Off site runoff flowing on site for design storm (peak flow, and total volume).

         Existing runoff generated on site for design storm (peak flow, and total volume).

         Postconstruction runoff generated on site for design storm (peak flow, and volume).

         Storm water contents planned to treat and dispose of additional flow and volume.

         Anticipated loss rates (evaporation, infiltration) for each specific control feature.

         Anticipated flow capacity and velocity in swales, ditches, and pipe systems.

         Any other pertinent design consideration which will help.

         Describe the appropriate functioning of the system.

         Any other pertinent design consideration which will help.

         Describe appropriate functions of the system.

         Postconstruction runoff which will move off site.

   B.   Vicinity Drainage Plan: Vicinity drainage plan (not greater than 2,000 feet to the inch) showing:

         Storm water drainage patterns within one mile of site.

         Existing surface water bodies (streams, rivers, lakes, wetlands) within one mile.

         Environmentally sensitive areas within one mile.

         Contaminated areas within one mile (particularly upstream) including known types and concentrations of contaminants.

   C.   Site Plan: Site plan (not greater than 100 feet to the inch) showing:

      1.   Existing Features:

         Development site boundaries.

         Roads, sidewalks, and parking areas (indicating paved, gravel, dirt, etc.).

         Structures (described dimensions, construction).

         Water sources (springs, wells).

         Surface water bodies and designed buffer zones.

         Water drainage channels (with flow, velocity, and volume information).

         Storm water runoff flow patterns.

         Utilities.

         Easements.

         Location and type of existing IC barriers.

         Topography (2 foot contour intervals with reference datum).

         Location of soil types (from SCS soils survey, or other available information).

         Location of vegetative cover types (grassland, scrubs, trees, wetland).

      2.   Proposed Features:

         Specific vegetation preserve areas.

         Areas to be cleared of vegetation.

         Areas where topsoil is to be removed and/or stockpiled.

         Areas to be graded, filled, and/or excavated (with proposed final contours).

         Areas to be revegetated (indicating lawn, landscaping details).

         New structures (describe dimensions, construction).

         New roads, parking areas, sidewalks (indicating paved, gravel, dirt, etc.).

         Utilities.

         Easements.

         Postconstruction storm water drainage patterns.

         Storm water control features:

            a.   For swales and open ditches show cross section(s), bottom elevations, slope(s), length(s).

            b.   For GIAs and detention/retention basins show area, volume cross section(s), bottom elevation(s), and inlet and overflow capacity, location and elevation.

            c.   For culverts, and pipe systems show size, type, invert elevation, horizontal (vertical angle points and lengths).

            d.   For any other storm water control feature show sufficient detail to convey the design parameters and method of functioning.

         Cross section dimensions and bottom elevations of any off site drainage channel which will either contribute runoff to the site or into which on site runoff will pass.

   D.   Erosion And Sedimentation Control Plan: All details of erosion and sedimentation controls shall be shown on one drawing with not greater than one hundred feet (100') to the inch.

      1.   Existing Site Condition:

         Site boundaries.

         Roads, sidewalks, and parking areas (indicating paved, gravel, dirt, etc.).

         Structures.

         Water sources (springs, wells).

         Surface water bodies, and designated buffer zones.

         Surface water drainage channels (with flow, velocity, and volume information).

         Storm water runoff patterns.

         Utilities.

         Easements.

         Topography (2 foot contour intervals with reference datum).

         Location of soil types (include any contaminants present).

      2.   Proposed Site Conditions:

         New roads, sidewalks, and parking areas.

         New structures (describe dimension, construction).

         Areas to be cleared of vegetation.

         Vegetation preserve areas.

         Areas where topsoil is to be removed and/or stockpiled.

         Areas to be graded, filled, or excavated (with proposed final contours).

         Utilities.

         Easements (including new drainage system areas).

         Postconstruction drainage patterns.

         Location of temporary erosion and sedimentation control features (jute netting, mulching, straw bales, etc.).

         Location of permanent erosion and sedimentation control features (grassed swales, detention basins, etc.).

         Locations of any IC barriers.

      3.   Proposed Construction And Revegetation Schedule:

         Anticipated start and completion dates for all phases of the work.

         Anticipated installation of temporary and permanent IC barriers.

      4.   Maintenance And Repair Schedule And Responsibility:

         Routine inspection schedule for during construction and afterward.

         Routine IC barrier inspection for during and after construction.

         Anticipated maintenance efforts.

         Person responsible for long term maintenance after construction is complete.

   E.   Barrier Option Plan (BOP): All details of the BOP shall be shown on one drawing with not greater than one hundred feet (100') to the inch. Any information which is pertinent but is better conveyed in written form may be submitted as part of the calculations for the BOP.

      1.   Existing Site Conditions:

         Soil test results for the site listing the concentrations of lead.

         Locations of all contaminants.

         Soil profiles of all existing soil types on the site.

         Locations, types, and conditions of all existing IC barriers.

         Hydrologic map (not less than 2,000 feet to the inch) showing all drainage features within five hundred feet (500') and all environmentally sensitive features within one mile including any contaminated areas upstream of the site.

      2.   Proposed Site Conditions:

         Locations and details of all proposed disturbances of existing IC barriers.

         Locations and types of all temporary IC barriers to be used during construction.

         Maintenance schedule for all IC barriers on the site.

         Schedule for all IC barrier disturbances, repairs, and construction.

         Detailed dust control plan.

         Temporary and permanent access controls.

   F.   Contaminated Soil Disposal Plan: If a specific site has lead levels exceeding one thousand (1,000) ppm (per SAP or PHD) or a development has an average lead level exceeding three hundred fifty (350) ppm then a contaminated soil disposal plan is required. The following elements shall be submitted:

         Calculations determining the quantity of contaminated soil to be removed.

         Detailed plan for handling, transporting, and disposal of all contaminated soils per BHSS BMP handbook and the appropriate RDR.

             1.   If the soil volume is less than one cubic yard then PHD will provide container, transport, and dump site.

            2.   If the soil volume exceeds one cubic yard then the contractor or agency performing the work is required to transport the soil to the Page soil repository for disposal (no charges for dumping but access is via the PHD).

            3.   A proposed travel route must be submitted for hauling the soil in a covered vehicle. No free liquid is permitted to be transported to the Page site (soil must have no free liquid).

         Detailed personnel and equipment decontamination plan per BHSS BMP handbook and the appropriate RDR.

         Personnel protection plan for all workers on site and the public in general.

         Detailed dust control plans per BHSS BMP handbook and the appropriate RDR.

         Access control details for the site while construction is in progress.

   G.   Operation And Maintenance Plan:

      1.   Inspection schedule.

      2.   Contact person.

      3.   Maintenance activities.

(Ord. 364, 4-5-1989; amd. Ord. 637, 4-9-2025)