(a)

Purpose. The City of Gahanna, State of Ohio, adopts these floodplain regulations, as permitted by Article XVIII, Section 3, of the Ohio Constitution, in order to promote the health, safety, and general welfare of its citizens and to minimize public expenditures and private losses due to flood conditions.

(b)

Findings of Facts. The flood hazard areas of the City are subject to periodic inundation which may result in loss of life and property, health and safety hazards, disruption of commerce and governmental services, extraordinary public expenditures for flood protection and relief, and impairment of the tax base, all of which adversely affect the public health, safety and general welfare. These flood losses are caused by the cumulative effect of obstructions in areas of special flood hazards which increase flood heights and velocities and, when inadequately anchored, damage uses in other areas. Uses that are inadequately flood proofed, elevated, or otherwise protected from flood damage also contribute to the flood loss.

(c)

Methods. In order to accomplish its purposes, these floodplain regulations include methods and provisions for:

(1)

Restricting or prohibiting uses which are dangerous to health, safety and property due to water hazards, or which result in damaging increases in flood heights or velocities;

(2)

Requiring that uses vulnerable to floods, including facilities which serve such uses, be protected against flood damage at the time of initial construction;

(3)

Controlling the alteration of natural floodplains, stream channels and natural protective barriers, which help accommodate or channel floodwaters;

(4)

Controlling filling, grading, dredging and other development which may increase flood damage; and,

(5)

Preventing or regulating the construction of flood barriers which will unnaturally divert floodwaters or which may increase flood hazards in other areas.

(d)

Applicability. These floodplain regulations shall apply to all areas of special flood hazards within the jurisdiction of the City of Gahanna, Ohio, as identified by the Federal Emergency Management Agency, including any additional flood hazard areas annexed by the City of Gahanna that are not identified on the effective Flood Insurance Rate Map.

The areas of special flood hazard have been identified by the Federal Emergency Management Agency in a scientific and engineering report entitled "Flood Insurance Study, Franklin County, Ohio and Incorporated Areas". This study, with accompanying Flood Insurance Rate Maps with an effective date of August 2, 1995, and any revisions thereto, is hereby adopted by reference and declared to be a part of this Zoning Ordinance. The Flood Insurance Study is on file with the City.

(e)

Exemption from Filing a Floodplain Use Permit. An application for a floodplain use permit shall not be required for maintenance work such as roofing, painting, and basement sealing, or for small non-structural development activities (except for filling and grading) valued at less than $1,000.00. Any proposed action exempt from filing for a floodplain use permit is also exempt from the standards of this chapter.

(f)

Interpretation. In the interpretation and application of this chapter, all provisions shall be:

(1)

Considered as minimum requirements;

(2)

Liberally construed in favor of the governing body; and,

(3)

Deemed neither to limit nor repeal any other powers granted under State statutes. Where a provision of this chapter is in conflict with State law, such State law shall take precedence over this chapter.

(g)

Disclaimer of Liability. The degree of flood protection required by these floodplain regulations is considered reasonable for regulatory purposes and is based on scientific and engineering considerations. Larger floods can and will occur on rare occasions. Flood heights may be increased by man-made or natural causes. These floodplain regulations do not imply that land outside the areas of special flood hazard or uses permitted within such areas will be free from flooding or flood damages. These floodplain regulations shall not create liability on the part of the City of Gahanna, any officer or employee thereof, or the Federal Emergency Management Agency, for any flood damage that results from reliance on this chapter or any administrative decision lawfully made thereunder.

(h)

General Floodplain Standards. In all areas of special flood hazards the following standards are required:

(1)

Anchoring. All new construction and substantial improvements shall be anchored to prevent flotation, collapse or lateral movement of the structure resulting from hydrodynamic and hydrostatic loads, including the effects of buoyancy.

(2)

Construction materials and methods.

A.

All new construction and substantial improvements shall be constructed with materials resistant to flood damage.

B.

All new construction and substantial improvements shall be constructed using methods and practices that minimize flood damage; and

C.

All new construction and substantial improvements shall be constructed with electrical, heating, ventilation, plumbing and air conditioning equipment and other service facilities that are designed and/or located so as to prevent water from entering or accumulating within the components during conditions of flooding.

D.

Where permitted, fill or other materials shall be protected against erosion by vegetative cover, riprap or bulkheading.

(3)

Utilities. The following standards apply to all water supply, sanitary sewerage and waste disposal systems not otherwise regulated by the Ohio Revised Code:

A.

All new and replacement water supply systems shall be designed to minimize or eliminate infiltration of floodwaters into the system.

B.

New and replacement sanitary sewerage systems shall be designed to minimize or eliminate infiltration of floodwaters into the systems and discharge from the systems into floodwaters; and,

C.

On-site waste disposal systems shall be located to avoid impairment to them or contamination from them during flooding.

(4)

Subdivision proposals.

A.

All subdivision proposals shall be consistent with the need to minimize flood damage.

B.

All subdivision proposals shall have public utilities and facilities such as sewer, gas, electrical and water systems located and constructed to minimize flood damage.

C.

All subdivision proposals shall have adequate drainage provided to reduce exposure to flood damage; and,

D.

All subdivision proposals shall meet the specific standards of Section 1113.01(j), Subdivisions and Large Developments.

(i)

Standards in Areas of Special Flood Hazard Without Base Flood Elevation Data. In all areas of special flood hazard identified as Zone A on the Flood Insurance Rate Map where base flood elevation data are not available from any source, the following provisions apply:

The applicant shall be required to supply the following:

(1)

Calculation of water surface elevations and flood protection elevations based upon a hydraulic analysis of the portions of the stream channel and other areas inundated by the base flood. Flood protection elevations shall be one foot above the water surface elevations of the base flood plus the increase in flood heights caused by the proposed development.

(2)

Computation of the floodway required to convey this flood without increasing flood heights to an extent which would cause substantial upstream or downstream damage to existing or reasonably anticipated future development. Computation of increases in flood heights caused by any encroachment shall be based upon the reasonable assumption that there will be an equal degree of encroachment on both sides of the stream within that reach. Any increase in flood stages attributable to encroachments on the floodplain of any river or stream shall not exceed five-tenths foot at any point along the river or stream.

(j)

Specific Floodplain Standards. In all areas of special flood hazards where base flood elevation data have been provided as set forth in Chapter 1113, the following provisions are required.

(1)

Residential Construction. New construction and substantial improvement of any residential structure shall have the lowest floor, including basement, elevated to the flood protection elevation.

(2)

Nonresidential Construction. New construction and substantial improvement of any commercial, industrial or other nonresidential structure shall either have the lowest floor, including basement, elevated to the level of the flood protection elevation; or, together with attendant utility and sanitary facilities, shall:

A.

Be flood proofed so that the structure is watertight with walls substantially impermeable to the passage of water to the level of the base flood elevation. In order to be eligible for lower flood insurance rates, the structure should be flood proofed at least one foot above the base flood elevation.

B.

Have structural components capable of resisting hydrostatic and hydrodynamic loads and effects of buoyancy; and,

C.

Be certified by a registered professional engineer or architect that the design and methods of construction are in accordance with accepted standards of practice for meeting the standards of this subsection. Such certification shall be provided to the official as set forth in Section 1117.10(d).

(3)

Accessory Structures. A relief to the elevation or dry flood proofing standards may be granted for accessory structures (e.g., sheds, detached garages) containing 576 square feet or less in gross floor area. Such structures shall meet the encroachment provisions of Section 1113.01(k) and the following additional standards:

A.

They shall not be used for human habitation;

B.

They shall be designed to have low flood damage potential;

C.

They shall be constructed and placed on the building site so as to offer the minimum resistance to the flow of floodwaters;

D.

They shall be firmly anchored to prevent flotation; and,

E.

Service facilities such as electrical and heating equipment shall be elevated or flood proofed.

(4)

Enclosures Below the Lowest Floor. The following standards apply to all new and substantially improved residential and nonresidential structures which are elevated to the base flood elevation using pilings, columns, or posts. Fully enclosed areas below the lowest floor that are usable solely for parking of vehicles, building access or storage in an area other than a basement and which are subject to flooding shall be designed to automatically equalize hydrostatic flood forces on exterior walls by allowing for the entry and exit of floodwaters. Designs for meeting this requirement must:

A.

Be certified by a registered professional engineer or architect; or B. Shall meet or exceed the following criteria:

1.

A minimum of two openings having a total net area of not less than one square inch for every square foot of enclosed area shall be provided;

2.

The bottom of all openings shall be no higher than one-foot above grade; and

3.

Openings may be equipped with screens, louvers, valves or other openings provided that they permit the automatic entry and exit of floodwaters.

(k)

Subdivisions and Large Developments. In all areas of special flood hazard where base flood elevation data have not been provided in accordance with this chapter, the following standards apply to all subdivision proposals, including manufactured home subdivisions and other proposed developments containing at least 50 lots or five acres (whichever is less):

(1)

The applicant shall provide base flood elevation data performed in accordance with standard engineering practices; and,

(2)

If division (j)(1) above is satisfied, all new construction and substantial improvements shall comply with all applicable flood hazard reduction provisions of Section 1113.01(g), General Floodplain Standards, and Section 1113.01(i), Specific Floodplain Standards.

(l)

Floodways. The Flood Insurance Study referenced in Section 1113.01(d) identifies a segment within areas of special flood hazard known as a floodway. Floodways may also be delineated in other sources of flood information as specified in Section 1113.01(d). The floodway is an extremely hazardous area due to the velocity of floodwaters which carry debris, potential projectiles, and erosion potential. The following provisions apply within all delineated floodway areas:

(1)

Prohibit encroachments, including fill, new construction, substantial improvements, and other development unless a hydrologic and hydraulic analysis performed in accordance with standard engineering practices demonstrates that the proposed encroachment would not result in any increase in flood levels during the occurrence of the base flood discharge.

(2)

If subsection (1) hereof is satisfied, all new construction and substantial improvements shall comply with all applicable flood hazard reduction provisions of Section 1113.01(i), Specific Floodplain Standards.

(3)

Any encroachment within the floodway that would result in an increase in base flood elevations can only be granted upon the prior approval by the Federal Emergency Management Agency. Such requests shall be submitted by the City's Designee to the Federal Emergency Management Agency and shall meet the requirements of the National Flood Insurance Program.

(m)

Floodways—Prohibited Uses. The following structures and uses are hereby prohibited in the floodway:

(1)

Structures designed or used for human habitation;

(2)

The storage or processing of materials that are pollutants, buoyant, flammable, poisonous, explosive or could be injurious to human, animal or plant life in time of flooding, or that have a high flood damage potential;

(3)

Garbage and waste disposal facilities including any further encroachment upon the floodway at existing sites; and

(4)

Any use which will adversely affect the efficiency or unduly restrict the capacity of the channels or floodways of any tributary to the main stream, drainage ditch or any other drainage facility or system.

(n)

Floodways—Permitted Uses. The following uses may be permitted within the floodway provided that they comply with the provisions of this section and other standards established in this Zoning Ordinance, and any conditions attached by the City's Designee to the issuance of the floodplain permit:

(1)

Agricultural uses such as general farming, pasturing, grazing, outdoor plant nurseries, horticulture, viticulture, truck farming, forestry, sod farming and wild crop harvesting;

(2)

Industrial-commercial uses such as loading areas and parking areas;

(3)

Recreational uses such as golf courses, tennis court, driving ranges, soccer fields, baseball diamonds, archery ranges, picnic grounds, boat launching ramps, swimming areas, basketball courts, parks, wildlife and nature preserves, game farms, fish hatcheries, shooting preserves, target ranges, hunting and fishing areas and hiking and horseback riding trails;

(4)

Residential uses such as lawns, gardens, parking areas and play areas;

(5)

Circuses, carnivals and similar transient amusement enterprises;

(6)

Roadside stands;

(7)

Extraction of sand, gravel and other materials;

(8)

Navigational and drainage aids, marinas, boat rentals, docks, piers, wharves, and water measuring and control devices;

(9)

Railroads, streets, bridges, utility transmission lines, underground culverts, pipes and pipelines;

(10)

Stables;

(11)

Storage of material or equipment other than that prohibited by these floodplain regulations, provided that such material or equipment is not subject to major damage by floods, is firmly anchored to prevent flotation, is readily removable from the area within the time available after flood warning and before flooding, and does not present and ecological quality risk;

(12)

Public utility facilities and water-oriented industries which must be adjacent to watercourses, provided that the development is located so that it shall not substantially alter flood flows, heights or velocities of the 100-year flood. Whenever necessary, compensating measures shall be required to be undertaken to offset any adverse effects of allowing the use within the floodway and to keep increases in stages of the 100-year flood within the limits specified in these regulations;

(13)

Structural works for flood control such as dams, levees, dikes and floodwalls, subject to the issuance of a floodplain use permit. In addition, any proposed structural work in the beds of public waters as defined in Ohio R.C. 1521.06, which shall change the course, current or cross section of the waters shall be subject to the provision of Ohio R.C. 1521.06 and 1521.07, and other applicable statutes;

(14)

Other uses similar in nature to uses described in this subsection which are consistent with the provisions set out in the purpose clause; and

(15)

Structures, temporary or permanent, accessory to permitted uses, provided that:

A.

They have a low flood damage potential;

B.

They are constructed and placed on the building site so as to offer the minimum obstruction to the flow of floodwaters. Whenever possible, structures shall be constructed with the longitudinal axis parallel to the direction of flood flow; and so far as practicable, structures shall be placed approximately on the same flood flow lines as those of adjoining structures;

C.

They are firmly anchored to prevent flotation which may result in damage to other structures, restriction of bridge openings and other narrow sections of the stream or river; and

D.

Service facilities such as electrical and heating equipment are placed at or above the regulatory flood protection elevation for the particular area or adequately flood proofed.

(o)

Nonconforming Uses. A structure or the use of a structure or premises which was lawful before June 2, 1973, but which is not in conformity with the provisions of this chapter may be continued subject to the following conditions:

(1)

No nonconforming use shall be expanded, extended, replaced, reconstructed, substituted, changed, enlarged, or altered in the floodway.

(2)

If such use is voluntarily discontinued for two years or more, any future use of the building premises shall conform to this chapter.

(3)

If any nonconforming use, created by this chapter, is destroyed by any means, including floods, to an extent of 50 percent or more of its assessed value, it shall not be reconstructed except in conformity with the provisions of this chapter. The City's designee may permit reconstruction if the use is located outside of the floodway and, upon reconstruction, is adequately flood proofed, elevated, or otherwise protected in conformity with this chapter.

(4)

Repairs and maintenance work required to keep a nonconforming use in sound condition may be made.

(5)

Any alteration or addition to any nonconforming use which would results in substantially increasing its flood damage potential shall be protected by measures in accordance with chapter 1117.10(b)(8).

(6)

Whenever a nonconforming use has been changed to a conforming use, such use shall not thereafter be changed to a nonconforming use.

(7)

Uses or adjuncts thereof or accessory uses which are found by a Court of competent jurisdiction as nuisances shall not be entitled to continue as nonconforming uses.

(8)

The provisions in this section are not intended and shall not be construed to permit greater expansion, extension, enlarging or replacement than is permitted under any existing Zoning Code sections relating to nonconforming uses.

(Ord. No. 0007-2024, § 2(Exh. A), 4-1-24; Ord. No. 0076-2024, § 1(Exh. A), 12-2-24)

(a)

Purpose. These stormwater management regulations are adopted to implement best management practices that minimize the impact to public waters caused by earth disturbance activities, subsurface drainage, and land use changes connected with activities within a development area, and shall include, at a minimum, all requirements of the Ohio Environmental Protection Agency general permit for construction activities.

(b)

References. The stormwater management regulations rely on the following sources of data:

(1)

The design standards contained in the Ohio Department of Natural Resources (ODNR) Rainwater and Land Development Manual, latest edition, shall be used to determine the technical acceptability of land development stormwater management methods as applicable and shall be used as technical guidance. The City's Designee shall determine the acceptability of all hydraulic and hydrologic engineering and design.

(2)

The United States Department of Agriculture Natural Resource Conservation Service (NRCS) soil classification mapping of the City shall be used to determine soil classification for the purpose of all stormwater management design unless more detail data is prepared by a competent authority and accepted by the City's Designee.

(3)

The condition of property prior to earth disturbing activity shall be used to determine predevelopment runoff coefficients and runoff curve numbers based on the most recent NRCS values.

(c)

Applicability. The stormwater management regulations apply to all land development, except for land preparation for active agricultural areas, orchards, sod farms and nursery operations; and land grading or leveling for erosion control under direction of the local soil conservation district.

(d)

Approval. The Planning Commission shall not approve the final plat of any development or subdivision over which it has jurisdiction without certification from the City that such development or subdivision shall be in full compliance with the design requirements of the stormwater management regulations.

(e)

Waivers.

(1)

A waiver may be granted for development situations that have none of the harmful effects associated with increases in runoff rates and volume. The waiver applies only to the requirement that runoff be controlled, and does not in any way imply a relaxation in the requirement for adequate on-site drainage or the ability to accept runoff from land tributary to the development. The waiver application shall request in writing that such requirements for stormwater runoff control be waived. The application shall include sufficient detail to determine that granting a waiver shall not result in increased flooding and that the added volume of runoff shall not damage the receiving stream. A condition of the waiver shall be that any addition, extension, or modification of a development for which a waiver has been granted shall be required to provide stormwater runoff control for the entire site if preceding limitations are exceeded by subsequent additions, extensions, or modifications.

(2)

The following land uses and developments are eligible to apply for a waiver on stormwater runoff control requirements contained in this chapter:

A.

Development areas abutting and tributary to Big Walnut Creek on which surface watershed flows directly into Big Walnut Creek.

(3)

All waiver applications shall be recommended by the City and the Planning Commission and approved by Council.

(4)

In no case shall a waiver eliminate or replace any other local, state or federal permit or compliance requirements.

(f)

Stormwater Management Standards.

(1)

Stormwater management shall address both peak rate of runoff and total volume of runoff.

(2)

The peak rate of runoff from an area after development shall not exceed the peak rate of runoff from the same area before development for all return period storms from one year up to a 100-year return period; 24-hour storm.

(3)

In addition, if it is found a proposed development shall increase the volume of runoff from an area, the peak rate of runoff from certain more frequent storms shall be controlled further.

(4)

The permissible peak rates shall be determined as follows:

A.

Determine the total volume of runoff from a one-year return period, 24-hour storm, occurring over the area before and after development; and,

B.

Using TR-55 Methodology, or other volume-based methodology acceptable to the City (the rational method is not acceptable), determine the percentage of increase in volume due to development, and, using this percentage, pick the critical storm from the following table:

(5)

The peak rate of runoff from the critical storm occurring over the development shall not exceed the peak rate of runoff from a one-year return period storm occurring over the same area under predevelopment conditions. Storms of less frequent occurrence (longer return period) than the critical storm, shall have a peak rate of runoff not greater than the same storm under predevelopment conditions. As an example, if the total volume is to be increased by 35 percent, the critical storm is a five-year storm. The peak rate of runoff for all storms up to this intensity shall be controlled so as not to exceed the peak rate of runoff from a one-year return period storm under predevelopment conditions in the area. The runoff from a more intense storm, up to a 100-year return period storm must be controlled so as not to exceed the predevelopment peak rate from the same return period storm.

(6)

Appropriate stormwater runoff control addresses both peak rate and total volume of runoff. In addition to peak rates of stormwater runoff, methods for minimizing post construction increases in stormwater runoff volumes are strongly encouraged. Methods for reducing runoff volumes may include those listed below.

A.

Retarding flow velocities by increasing friction; for example, grassed road ditches rather than paved street gutters where practical; discharging roof water into vegetated areas; or grass and rock lined drainage channels;

B.

Grading and construction of terraces and diversions to slow runoff and use of grade stabilization structures to provide a level of control in flow paths and stream gradients;

C.

Induced infiltration of terraces and diversions to slow runoff and use of grade stabilization structures to provide a level of control in flow paths and stream gradients;

D.

Provisions for retention and detention; for example, permanent ponds and lakes with stormwater basins provided with proper drainage, multiple use areas for stormwater detention and recreation, wildlife, transportation, fire protection, aesthetics, or subsurface storage areas;

E.

Other methods for controlling post construction water quality as approved by the City's Designee;

F.

Attenuation of runoff rates from upstream areas is not required to be provided. Flow from such areas will be routed through the drainage system in the development under consideration at a rate determined in the same manner as the on-site system. Anticipated future development, however, must be considered in the designers' hydrologic analysis. Off-site land uses prior to development, or anticipated to be constructed in the future, shall be considered as the pre-development condition for the purpose of calculating changes in runoff; and,

G.

All stormwater management and sediment control practices shall be designed, constructed and maintained with consideration for the proper control of mosquitoes and other vectors. Practices may include, but are not limited to:

1.

There should be no depressions in a normally dry detention facility where water might puddle when the water level is receding. Underdrains are encouraged where appropriate.

2.

All structures shall be designed in accordance with the Ohio Department of Natural Resource's rainwater and land development handbook (most recent edition).

(7)

Numerous methods of rainfall-runoff computation are available on which the design of storm drainage and flood control system may be based. The rational method, the NRCS hydrologic methods (available in TR-20, TR-55 and HEC-1) are accepted as adequate for determining peak runoff rates for drainage areas. USGS methodologies are also acceptable. Other methods may be accepted with the concurrence of the City.

(8)

The total watershed that produces stormwater runoff across the site proposed to be developed shall be included when estimating flood discharge runoff. Dependent on watershed size, the following principal methods shall be considered acceptable to estimate design discharge.

A.

For small watersheds of 25 acres or less, the design runoff may be determined by the rational method. This method may also be used for catch basin hydrology. The rational method shall not be used for sizing attenuation basins or any other facility that requires estimation of runoff volumes.

B.

For five 100 acres of an urbanizing watershed, the design runoff may be estimated by using the method as Published, Urban Hydrology for Small Watersheds (TR55). This method may be applicable to areas up to 300 acres depending upon the topography. TR-20 and HEC-1 may be acceptable for larger areas. The designer is encouraged to meet with the City prior to selecting a methodology for large watershed.

(g)

System Design.

(1)

Initial drainage system. The initial drainage system is the part of the storm drainage system which is used regularly for collecting, transporting and disposing of stormwater runoff, snowmelt and miscellaneous minor flows. The capacity of the initial drainage system should be equal to the maximum rate of runoff expected from a design storm of established frequency.

A.

All new storm sewer systems must be adequate to convey anticipated runoff of a watershed from a five-year storm at just full flow. Pressure flows for five-year design storms are unacceptable.

B.

The storm sewer hydraulic grade line shall be determined for the ten-year storm event. The hydraulic grade line at the ten-year storm shall be below the grate and/ or cover of all structures. The hydraulic grade line should never be below the normal depth of flow in the conduit. If calculations illustrate this condition, then the designer shall use the normal depth of flow elevation as the hydraulic grade line (HGL) elevation.

C.

Discharge outlets must be adequate to accept additional runoff from the proposed development without overloading. If the existing outlet is inadequate for such additional flow, an improved outlet or some time-release method of discharge (detention), satisfactory to the City, must be provided. A tailwater analysis must be completed and used as part of all hydraulic design.

D.

Culverts shall be designed to easily convey the ten-year design storm. Headwater depth shall not be within 12 inches of the final pavement (lowest point in road) elevation for the 50-year storm.

E.

All culverts shall be designed with a uniform barrel cross section throughout their length. Location alignment, material specifications, and end treatments (e.g., headwalls, wingwalls, riprap, apron slabs), shall be approved by the City.

(2)

Major drainage system. The major drainage system is that part of the storm drainage system which carries the runoff which exceeds the capacity of the initial drainage system. The major drainage system shall have the capacity to carry runoff from a storm with a return period of not less than 100 years without posing significant threat to property or public safety.

A.

Major system design shall be considered and accomplished for all development to alleviate potential adverse impact from flooding due to infrequent storms. Sufficient easements shall be provided to protect the major flow paths from being filled, blocked or otherwise disturbed.

B.

Where a system outlets to an erodible channel, measures shall be taken to lessen potentially destructive velocities. Maximum velocities for discharge into erodible channels shall flow ODOT standards.

(h)

Initial Storm: Physical Design Criteria for On-Site Improvements.

(1)

Depth of flow in natural channels shall not exceed bank full stage with backwater effects considered.

(2)

Depth of flow in artificial channels shall not exceed 0.8 bank full stage. Velocity of flow shall be determined in accordance with the design criteria for open channels and shall not exceed seven feet per second. Where flows exceed this rate, special channel lining and erosion protection shall be provided. Design approach shall follow the guidelines of the ODNR Rainwater and Land Development Manual.

(3)

Depth of flow in road side ditch swales shall not exceed one foot or be of such depth that flow would extend out of the right-of-way if the side ditch is less than one foot in depth. Velocity at this depth shall not exceed six feet per second with grass swales or ten feet per second with paved ditches.

(4)

Depth of flow in streets with curb and gutter shall not exceed the curb height. Velocity of flow in the gutter at design depth shall not exceed ten feet per second. In addition to the above, the following are maximum encroachments of the minimum five-year initial design storm onto the pavement.

A.

For minor streets carrying traffic from the individual residence to collector and secondary streets, the flow may spread to the crown of the street.

B.

For collector or secondary streets, one lane shall be free from water.

C.

For primary streets, one lane in each direction shall be free from water.

D.

For freeways, no encroachment is allowed on traffic lanes.

(5)

In the design of the conduit, the conduit may be designed on the basis of flowing full with surcharge to gutter line. Backwater effects shall be considered.

(i)

Major Storm: Physical Design Criteria for On-Site Improvements.

(1)

The major storm floodway and floodway fringe for natural streams shall be as defined by the Federal Emergency Management Agency (FEMA).

(2)

Many of the drainageways associated with the major storm system are in areas beyond those designated as floodway or floodway fringe. For these areas, the major storm flood limits shall be determined by the U. S. Corps of Engineers HEC-2 method or other accepted methods of determining water surface profiles using the major design storm runoff. One-half foot elevation shall be added to the flood profile as freeboard for protection in the event of future encroachments into the floodway fringe or in the drainageway.

(3)

Where the street is designed as the major drainageway, the depth of flow shall not exceed 18 inches of width at gutter line for local and collector streets and shall not exceed six inches depth at roadway crown for primary streets and freeways. The same maximum depth criteria shall apply where a major drainageway crosses the street. Where a major drainageway is located outside a street, dedicated stormwater easements shall be provided.

(4)

In determining the required capacity of surface channels and other drainageways provided for the major storm runoff, the street storm inlets and conduit provided for the initial design storm shall be assumed to be carrying not more than one-half their design capacity. This is a safety factor to allow for the surcharged outlets, obstructed inlets or other malfunctions.

(j)

Stormwater Detention/Retention. Stormwater detention or retention is required for all developers unless a waiver is granted for areas designed for storage of stormwater by detention or retention incorporated into the natural features of the general area. Cooperative planning and joint owner construction of detention discharge control or retention facilities and use of natural land contours is strongly encouraged. No such facilities shall be permitted which may become aesthetically unpleasing, construction or maintenance problems. The City encourages such facilities which are designed as multipurpose spaces such as open space, recreation and/or scenic areas. Detention/retention areas shall also comply with all post construction runoff requirements, including those of the Ohio EPA construction general permit.

(k)

Public Notification of Watercourse. In all sales offices, land developers shall place copies of the land development grading plan with graphic and written descriptive information clearly showing and describing the purpose of all drainage easements, floodway routing, flood hazard areas and other watercourses contained on or designed into the land development.

(Ord. No. 0007-2024, § 2(Exh. A), 4-1-24)

(a)

Purpose. This section is adopted for the purpose of controlling the pollution of public waters by sediment from accelerated stormwater runoff caused by earth-disturbing activities and land use changes connected with developing urban areas. Control of such pollution shall promote and maintain the health, safety and general well-being of life and inhabitants within the city.

(b)

Applicability. A Stormwater Pollution Prevention Plan (SWPPP) shall be submitted to the City prior to any earth disturbing activity on property of one acre or more for residential development or re-development, and on all land disturbances of one acre or more for commercial, manufacturing, multi-family development or redevelopment and public utility construction. Any Stormwater Pollution Prevention Plan developed for a site with an associated land disturbance of one acre or more shall be prepared in full compliance with the current Ohio EPA General Permit for Construction Activity. For earth disturbing activity on residential property of less than one acre, the need for erosion and sediment control activity shall be determined by the City's Designee.

(c)

Process. The Stormwater Pollution Prevention Plan may be submitted as a part of the detailed engineering plans. A copy of the approved Stormwater Pollution Prevention Plan and Notice of Intent (NOI) must be posted on site, submitted to the City for review, and available for review throughout the entire construction process.

The City shall indicate its approval or disapproval (status of compliance or noncompliance) to the person who filed the Stormwater Pollution Prevention Plan. Indication of disapproval (noncompliance) shall include the plan deficiencies and the procedures for filing a revised plan. Pending preparation and approval (determination of compliance) of a revised plan, earth-disturbing activities shall proceed only in accordance with conditions outlined by the City's Designee.

(d)

Stormwater Pollution Prevention Plan Content. A Stormwater Pollution Prevention Plan for a proposed development, with maps drawn to a scale of one inch equals 50 feet, shall be submitted to the City containing the following information:

(1)

Location of the area and its relation to its general surroundings including but not limited to:

A.

Off-site areas susceptible to sediment deposits or to erosion caused by accelerated runoff; and,

B.

Off-site areas affecting potential accelerated runoff and erosion control.

(2)

Existing and proposed topography and drainage of the development area and adjacent land within 100 feet of the boundaries. A topographic map should contain an appropriate contour interval to clearly portray the conformation and drainage pattern of the area:

A.

Existing and proposed drainage watersheds, including the size of each watershed in acres (entire watershed area, not just the individual site);

B.

Surface water locations including springs, wetlands, streams, water bodies, etc. on or within 200 feet of the site. This should include the locations of riparian, or wetlands setbacks, floodway, and the 100-year floodplain.

(3)

The location of existing buildings, structures, utilities, water bodies, drainage facilities, vegetative cover, paved areas (streets, roads, driveways, sidewalks, etc.) and other significant natural or man-made features on the development area and adjacent land within 100 feet of the boundaries;

(4)

A general description of the predominant soil types, their location and their limitations for the proposed use. Special attention should be given to hydric or highly erodible soils;

(5)

Proposed use of the development area including present development and ultimate utilization with detail on soil cover, both vegetative and impervious;

(6)

Section/phasing limits of the development area;

(7)

All proposed earth disturbance including:

A.

Areas of excavation, grading and filling;

B.

The finished grade, stated in feet horizontal to feet vertical, of cut and fill slopes; C. Kinds of utilities and proposed areas of installation;

D.

Proposed paved and covered areas in square feet or to scale on a plan map;

E.

Makeup of proposed surface soil (upper six inches) on areas not covered by buildings, structures or pavement. Description shall be in such terms as original surface soil, subsoil, sandy, heavy clay, stony, etc.; and,

F.

Proposed kind of cover on areas not covered by buildings, structures or pavement. Description shall be in such terms as: Lawn, turf grass, shrubbery, trees, forest cover, rip-rap, mulch, etc.

(8)

Provisions for temporary and permanent erosion control shall follow the latest edition of the Ohio Department of Natural Resources (ODNR), Rainwater and Land Development Manual and shall be depicted on the plan and shall also comply with OEPA CGP;

(9)

Provisions for the management of stormwater, derived both on-site and from upper watershed areas, including the control of accelerated on-site runoff, to a stable receiving outlet;

(10)

Location of designated construction entrances where vehicles will enter/exit site. A detail for this entrance must be included on the plan;

(11)

Provisions for maintenance of control facilities including easements to ensure short as well as long term erosion and sediment pollution control and stormwater management;

(12)

Proposed construction sequence and time schedule for all earth disturbing activities and installation of provisions for erosion and stormwater management;

(13)

Design computations and applicable assumptions for all structural measures for erosion and sediment pollution control and water management. Volume and velocity of flow shall be given for all surface water conveyance. This information shall also be provided for surface water outlets;

(14)

Seeding mixtures and rates, lime and fertilizer application rates, and kind and quantity of mulching for both temporary and permanent vegetative control measures;

(15)

Location of any in stream activities, including stream crossings;

(16)

Location of permanent stormwater management practices to be used to control pollutants after construction operations are complete;

(17)

Estimate of cost of erosion and sediment control and water management structures and features;

(18)

Title, scale, direction, legend and date of all plan maps;

(19)

Names and addresses of the person(s) preparing the plan, the owner and the person responsible for the development area;

(20)

Certification that all earth disturbance, construction and development shall be done pursuant to the plan;

(21)

Total area of the site, and the area of the site that is expected to be disturbed, including off-site borrow areas;

(22)

A calculation of the runoff coefficients for both the pre-construction and post construction site conditions;

(23)

An estimate of the existing percentage based on the site's predevelopment condition as well as proposed percentage based on the site post-development condition impervious area created by the construction activity; and,

(24)

Location of stormwater and sediment and erosion control BMP's. A detail specification for each must be included in the plan.

The City may waive specific requirements for plan detail or may require additional information to show that work shall conform to basic requirements of this chapter, solely at their discretion.

(e)

Erosion and Sediment Control Standards. All sediment and erosion control best management practices (BMPs) as depicted in the Stormwater Pollution Prevention Plan and construction drawings shall be designed, constructed and maintained in accordance with the current Ohio EPA General Permit for Construction Activity. This includes, but is not limited to, the timing of BMP installations, permanent / temporary stabilization, BMP selection, surface water protection, dewatering operations, etc.

(f)

Post-Construction Runoff Controls.

(1)

All post-construction stormwater management BMPs as depicted in the Stormwater Pollution Prevention Plan and construction drawings shall be designed and constructed in accordance with the current Ohio EPA General Permit for Construction Activity. Owner/developer must also provide detailed maintenance plans for all Post-Construction BMPS. Maintenance plans shall also be provided by the permittee to the post-construction operator of the site (including homeowner associations). The permittee, land owner or other entity with legal control over the property shall be required to develop and implement a maintenance plan to comply with local MS4 requirements. The use of innovative and/or emerging stormwater management post-construction technologies shall be at the discretion of the City's Designee and could require monitoring to ensure compliance with OEPA'S General Permit for Construction Activity.

(2)

The post-construction portion of the stormwater pollution prevention plan shall include the following:

A.

Description of post-construction BMPS to be installed during construction, including estimated installation schedule and sequencing plan (including post-construction sediment removal and installation of final outlets);

B.

Rationale for selection - to address anticipated downstream impacts (on the channel and floodplain, morphology, hydrology and water quality);

C.

Detailed post-construction BMP drawings and specifications; and,

D.

MP maintenance plan for all BMPs selected and presented to post-construction operator. This maintenance plan shall include a disposal statement for structural BMPs to ensure pollutants collected with structural BMPs are disposed of in accordance with local, state and federal regulations.

(3)

Post-Construction Runoff Control Design. Selected structural BMPs shall be sized for protection of watercourses from erosion (quantity) and include water quality volumes for controlling sediment volumes.

WQv = Volume of runoff from A 0.75 inch rain event WQv is determined according to following method:

WQv = C * P * A/12

where:

WQv = Channel protection and water quality volume in acre-feet C = Runoff coefficient appropriate for storm less than one inch.

P = 0.75 inch precipitation depth

A = Area, in acres, draining into the BMP

I = Watershed imperviousness ratio (percent total impervious divided by 100)

NOTES:

• Where the land use will be mixed, the runoff coefficient should be calculated using a weighted average.

• An additional volume equal to 20 percent of the WQv shall be incorporated into the BMP for sediment storage and/or reduced infiltration capacity during construction.

• BMPs shall be designed such that the drain time is long enough to provide settlement treatment, but short enough to provide storage available for successive rain events.

* Provide both a permanent pool and an extended detention volume above the permanent pool, each sized for 75 percent WQV.

(4)

Recommended Post-Construction Best Management Practices. The post-construction best management practice controls, on the following page, are identified in the current Ohio EPA's General Permit for Construction Activity and shall be incorporated in project development and design. The City will also consider non-structural practices in combination with these structural practices in reviewing site plans. Supporting documentation of non-structural BMP estimated pollutant removal information, map of on-site BMP locations, description of BMP type, and frequency with which the BMPs include: Site impervious area sweeping, natural buffers, pervious pavements, etc. All BMPs shall be designed and constructed per the Ohio Department of Natural Resources Rainwater and Land Development Manual.

(5)

Post Construction Runoff BMP Drawdown Curve. A drawdown curve (Volume vs. Drawdown Time) shall be calculated and drawn for each BMP and submitted to the City.

(6)

Ponds. Ponds are one of the most widely used BMPs for meeting water quantity requirements and providing water quality treatment. A pond improves water quality by detaining stormwater for an extended period of time in a permanent pool to allow pollutants to settle. Pollutants removed include suspended solids, organic matter, dissolved metals, and nutrients. Pond BMPs must specifically follow the guidelines below; BMPs other than ponds shall include consideration for all of the guidelines below and shall address each item to the greatest extent practical.

A.

Land area. Land constraints, such as small sites or highly developed areas, may preclude the installation of a pond. The wet pond BMP is most useful for large subdivisions, or development sites. Owners of small properties may cooperatively construct a single large wet pond to serve several sites. Fewer ponds are preferred to numerous individual small ponds. Wet detention ponds are effective in meeting stormwater quantity and water quality (post construction runoff control) goals. Well designed wet ponds can also provide an aesthetic amenity.

B.

Soils and minimal water supply. Wet detention ponds must maintain a permanent pool of water. Wet ponds are recommended for medium to large drainage areas (generally greater than ten acres).

C.

Retrofit. Wet ponds provide opportunities for retrofit coverage for existing development. With minor excavation and/or modification of the outlet, existing dry ponds can be converted to wet ponds. The retrofits can generate greater water quality benefits for the receiving stream, helping the (community) meet water quality goals. Retrofit opportunities are encouraged in the City of Gahanna.

D.

Maintenance. Adequate maintenance access and easements from public or private right-of-way to the basin shall be reserved. The access shall be on a slope of 5:1 (vertical to horizontal) or shallower, stabilized to support heavy construction equipment, and provide direct access to both the forebay and the riser outlet. An adequate area for temporary staging of spoils, prior to ultimate disposal shall be provided. This area shall be protected such that no sediment laden runoff will be directed back into the stormwater management system or onto private property. An easement must be provided over the disposal area.

E.

Basin inlet/outlet design. Velocity dissipation measures shall be incorporated into basin designs to minimize erosion at inlets and outlets, and to minimize the resuspension of pollutants. Inverts for inlet pipes shall discharge at the elevation of the permanent pool and perpendicular to the surface to allow the pool to dissipate the energy of the inflow. Stone riprap shall extend from the pipe invert to the pond bottom to prevent erosion. Antiseep collars should be installed on any piping passing through the sides or bottom of the basin to prevent leakage through the embankment. To the extent feasible, the distance between inlet and outlet shall be maximized. The length and depth of the flow path across basins can be maximized by:

1.

Increasing the length-to-width ratio of the entire design.

2.

Increasing the dry weather flow path within the system to attain maximum sinuosity.

F.

Dual orifices, or other designs should be used to assure an appropriate detention time for all storm events. Where a pipe outlet or orifice plate is to be used to control discharge, it should have a minimum diameter of six inches. If this minimum orifice size permits release rates greater than those specified in these rules, alternative outlet designs will be utilized that incorporate self-cleaning flow restrictors, such as perforated risers, that provide the required release rate. The outlet should be well-protected from clogging.

A reverse-slope-submerged orifice or hooded, broad crested weirs are recommended options. If a reverse-slope pipe is used, an adjustable valve may be necessary to regulate flows. Orifices used to maintain a permanent pool level should withdraw water at least one foot below the surface of the water.

G.

Backwater (tailwater) on the outlet structure from the downstream drainage system shall be evaluated when designing the outlet. All outlets shall be designed to be easily accessible for heavy equipment required for maintenance purposes.

H.

All basins shall include provisions for a defined emergency spillway constructed on undisturbed soil.

I.

The emergency spillway should be set at the elevation of the 100-year or more frequent storm. Where feasible, a drain for completely dewatering wet ponds should be installed for sediment removal and other maintenance purposes.

J.

Riser design. Hoods or trash racks should be installed on the riser to prevent clogging. Grate openings should be a maximum of three inches. The riser should be placed near or within the embankment, to provide for ready maintenance access. Inlet and outlet barrels and risers should be constructed of materials that will reduce future maintenance requirements. The riser pipe should be a minimum of 24 inches in diameter for riser pipes up to four feet in height. Riser pipes greater than four feet in height should be 48 inches in diameter. Riser pipes shall be constructed with poured-in-place concrete bottoms.

K.

The height of water in detention facilities shall not be excessive and shall comply with the ODNR dams and reservoir safety requirements.

L.

A table of elevations (stage-storage-discharge table with storage volume and discharge rates) shall be provided with the final engineering and construction plans for all basins. Profile view drawings of the outlet structure, with elevations, shall also be included.

M.

Prior to final acceptance of the pond improvements, as-built data shall be submitted to the City verifying that the structure(s) have been built as designed and will function accordingly. The as-built information shall be prepared and stamped by a registered professional Civil Engineer or a registered Landscape Architect.

(g)

Stormwater Management Maintenance and Operation Plans.

(1)

All stormwater treatment practices shall have an enforceable operation and maintenance agreement to ensure the system functions as designed.

(2)

This agreement shall include any and all maintenance easements required to access and inspect the stormwater treatment practices, and to perform routine maintenance as necessary to ensure proper functioning of the stormwater treatment practice.

(3)

A legally binding covenant specifying the parties responsible for the proper maintenance of all stormwater treatment practices shall be secured prior to issuance of any permits for land disturbance activities. An executed copy of the agreement shall be provided to the City of Gahanna, prior to project acceptance.

(h)

Stormwater Management Enforcement.

(1)

All development areas may be subject to external inspections by the City of Gahanna, its authorized agents, or the Ohio Environmental Protection Agency to ensure compliance with the approved Stormwater Pollution Prevention Plan.

(2)

After each external inspection, the City or its authorized agents shall prepare and distribute a status report to the applicant.

(3)

If an external inspection determines that operations are being conducted in violation of the approved Stormwater Pollution Prevention Plan, the City is authorized to take action as detailed in Section 1113.03(j).

(i)

Violations.

(1)

No person shall violate, cause, or knowingly permit to be violated any of the provisions of Section 1113.03, or fail to comply with any of such provisions or with any lawful requirements of any public authority made pursuant to this regulation. No person shall knowingly use, cause, or permit the use of any lands in violation of this regulation or in violation of any permit granted under this regulation.

(2)

Upon notice, the City's Designee may suspend any active soil disturbing activity for a period not to exceed 90 days, and may require immediate erosion and sediment control measures when a determination has been made that such activity is not meeting the intent of this regulation. Such notice shall be in writing, shall be given to the applicant, and shall state the conditions under which work may be resumed. In instances where the City's Designee finds that immediate action is necessary for public safety or the public interest, work may be stopped upon verbal order pending issuance of the written notice.

(Ord. No. 0007-2024, § 2(Exh. A), 4-1-24)