1007.09 PERFORMANCE STANDARDS.
(a) General: The stormwater system, including SCMs for storage, treatment and control, and conveyance facilities, shall be designed to prevent structure flooding during the 100-year, 24-hour storm event; to maintain predevelopment runoff patterns, flows, and volumes;
and to meet the following criteria:
(1) Integrated practices that address degradation of water resources. The SCMs shall function as an integrated system that controls flooding and minimizes the degradation of the physical, biological, and chemical integrity of the water resources receiving stormwater discharges from the site. Acceptable practices shall:
A. Not disturb riparian areas, unless the disturbance is intended to support a Streambank: Stabilization project and complies with Section 1001.07 Uses Permitted in Wetland and Riparian Setbacks.
B. Maintain predevelopment hydrology and groundwater recharge on as much of the site as practicable.
C. Only install new impervious surfaces and compact soils where necessary to support the future land use.
D. Compensate for increased runoff volumes caused by new impervious surfaces and soil compaction by reducing stormwater peak flows to less than predevelopment levels.
E. Be designed according to the methodology included in the most current edition of the Rainwater and Land Development Manual. SCMs that meet the criteria in this regulation, and additional criteria required by the Engineer, shall comply with this regulation.
(2) Practices designed for final use: SCMs shall be designed to achieve the stormwater management objectives of this regulation, to be compatible with the proposed post-construction use of the site, to protect the public health, safety, and welfare, and to function safely with routine maintenance.
(3) Stormwater management for all lots: Areas developed for a subdivision, as defined in Chapter 1101, shall provide stormwater management and water quality controls for the development of all subdivided lots. This shall include provisions for lot grading and drainage that prevent structure flooding during the 100-year, 24-hour storm; and maintain, to the extent practicable, the predevelopment runoff patterns, volumes, and peaks from each lot.
(4) Stormwater facilities in water resources: SCMs and related activities shall not be constructed within a surface water of the state unless the applicant shows proof of compliance with all appropriate permits from the Ohio EPA, the U.S. Army Corps, and other applicable federal, state, and local agencies as required in Section 1007.07 of this regulation, and the activity is in compliance with Section 1007.08 Comprehensive Stormwater Management Plan and Chapter 1001 Riparian and Wetland Setbacks, all as determined by the Engineer.
(5) Stormwater ponds and surface conveyance channels: All stormwater pond and surface conveyance designs must provide a minimum of one (1) foot freeboard above the projected peak stage within the facility during the 100-year, 24-hour storm. When designing storm water ponds and conveyance channels, the applicant shall consider public safety as a design factor and alternative designs must be implemented where site limitations would preclude a safe design.
(6) Exemption: The site where soil-disturbing activities are conducted shall be exempt from the requirements of Section 1007.09 if it can be shown to the satisfaction of the City that the site is part of a larger common plan of development where the stormwater management requirements for the site are provided by an existing SCMs, or if the stormwater management requirements for the site are provided by practices defined in a regional or local stormwater management plan approved by the City. A regional storm water BMP may be used to meet the post-construction requirement if: (1) the BMP meets the design requirements for treating the WQv; and (2) a legal agreement is established through which the regional BMP owner or operator agrees to provide this service in the long term. Design information for such facilities such as contributing drainage areas, capacities, elevations, outlet details and drain times shall be included in the SWP3.
(7) Small Construction Activities: For all construction activities authorized under this permit which result in a disturbance greater than one (1) acre, a post-construction practice shall be used to treat storm water runoff for pollutants and to reduce adverse impacts on receiving waters. The applicant must provide a justification in the SWP3 why the use of Table 4a and 4b practices in the current Ohio EPA General Construction Permit are not feasible. The justification must address limiting factors which would prohibit the project going forward should Table 4a and 4b practices be required. Please note that additional practices selected will require approval from the regulated MS4. The use of green mfrastructure BMPs such as runoff reducing practices is also encouraged.
(8) Transportation Projects: The construction of new roads and roadway improvement projects by public entities (i.e., the state, counties, townships, cities, or villages) may implement post-construction BMPs in compliance with the current version (as of the effective date of this Ordinance) of the Ohio Department of Transportation's "Location and Design Manual, Volume Two Drainage Design" that has been accepted by Ohio EPA as an alternative to the conditions of this permit.
(9) Runoff Reduction Practices: The size of structural post-construction practices used to capture and treat the WQv can be reduced by incorporating runoff reducing practices into the design of the site's drainage system. The approach to calculate and document runoff reduction is detailed in the Rainwater and Land Development Manual. BMP-specific runoff reduction volumes are set by specifications in the Rainwater and Land Development Manual for the following practices: impervious surface disconnection, rainwater harvesting, bioretention, infiltration basin, infiltration trench, permeable pavement with infiltration, underground storage with infiltration, grass swale, sheet flow to filter strip, and sheet flow to a conservation area.
Runoff reducing practices may be used for areas of the site not draining into a common drainage system of the site (e.g., sheet flow from perimeter areas such as the rear yards of residential lots, low density development scenarios, or where the permittee can demonstrate that the intent of pollutant removal and stream protection is being addressed through non-structural post-construction BMPs) based upon review and approval by Ohio EPA.
(10) Maintenance: All SCMs shall be maintained in accordance with the Inspection and Maintenance Plan and Agreements approved by the City as detailed in Section 1007.08.
(11) Ownership: Unless otherwise required by the City, SCMs serving multiple lots in subdivisions shall be on a separate lot held and maintained by an entity of common ownership. SCMs serving single lots shall be placed on these lots, protected within an easement, and maintained by the property owner.
(12) Preservation of Existing Natural Drainage: Practices that preserve and/or improve the existing natural drainage shall be used to the maximum extent practicable. Such practices may include minimizing site grading and compaction; protecting and/or restoring water resources, riparian areas, and existing vegetation and vegetative buffer strips; phasing of construction operations in order to minimize the amount of disturbed land at any one time, and designation of tree preservation areas or other protective clearing and grubbing practices; and maintaining unconcentrated stormwater runoff to and through these areas. Post-construction stormwater practices shall provide perpetual management of runoff quality and quantity so that a receiving stream's physical, chemical and biological characteristics are protected and ecological functions are maintained.
(13) Preservation of Wetland Hydrology: Concentrated stormwater runoff from SCMs to wetlands shall be converted to diffuse flow before the runoff enters the wetlands. The flow should be released such that no erosion occurs downslope. Level spreaders may need to be placed in series, particularly on steep sloped sites, to ensure non-erosive velocities. Other structural BMPs may be used between storm water features and natural wetlands, in order to protect the natural hydrology, hydroperiod, and wetland flora. If the applicant proposes to discharge to natural wetlands, a hydrologic analysis shall be performed. The applicant shall attempt to match the pre-development hydroperiods and hydrodynamics that support the wetland. The applicant shall assess whether their construction activity will adversely impact the hydrologic flora and fauna of the wetland. Practices such as vegetative buffers, infiltration basins, conservation of forest cover, and the preservation of intermittent streams, depressions, and drainage corridors may be used to maintain wetland hydrology.
(14) Soil Preservation and Post-Construction Soil Restoration: To the maximum extent practicable leave native soil undisturbed and protect from compaction during construction. Except for areas that will be covered by impervious surface or have been incorporated into an SCM, the soil moisture-holding capacity of areas that have been cleared and graded must be restored to that of the original, undisturbed soil to the maximum extent practicable.
(b) Stormwater Conveyance Design Criteria: All SCMs shall be designed to convey stormwater to allow for the maximum removal of pollutants and reduction in flow velocities. This shall include but not be limited to:
(1) Surface water protection: The Engineer may allow modification to streams, rivers, lakes, wetlands or other surface waters only if the applicant shows proof of compliance with all appropriate permits from the Ohio EPA, the U.S. Army Corps, and other applicable federal, state, and local agencies as required in Section 1007.07 of this regulation, and the activity is in compliance with Section 1007.06 and Chapter 1001 Riparian and Wetland Setbacks, all as determined by the Engineer. At a minimum, stream relocation designs must show how the project will minimize changes to the vertical stability, floodplain form, channel form, and habitat of upstream and downstream channels on and off the property.
(2) Off-site stormwater discharges: Off-site stormwater runoff that discharges to or across the applicant's development site shall be conveyed through the stormwater conveyance system planned for the development site at its existing peak flow rates during each design storm. Off-site flows shall be diverted around stormwater quality control facilities or, if this is not possible, the stormwater quality control facility shall be sized to treat the off-site flow. Comprehensive Stormwater Management Plans will not be approved until it is demonstrated to the satisfaction of the Engineer that off-site runoff will be adequately conveyed through the development site in a manner that does not exacerbate upstream or downstream flooding and erosion.
(3) Sheet flow: The site shall be graded in a manner that maintains sheet flow over as large an area as possible. The maximum area of sheet flow shall be determined based on the slope, the uniformity of site grading, and the use of easements or other legally-binding mechanisms that prohibit re-grading and/or the placement of structures within sheet flow areas. In no case shall the sheet flow length be longer than three hundred (300) feet, nor shall a sheet flow area exceed 1.5 acres. Flow shall be directed into an open channel, storm sewer, or other SCMs from areas too long and/or too large to maintain sheet flow, all as determined by the Engineer.
(4) Open channels: Unless otherwise allowed by the Engineer, drainage tributary to SCMs shall be provided by an open channel with vegetated banks and designed to carry the 10-year, 24-hour stormwater runoff from upstream contributory areas.
(5) Open drainage systems: Open drainage systems shall be preferred on all new development sites to convey stormwater where feasible. Storm sewer systems shall be allowed only when the site cannot be developed at densities allowed under City zoning or where the use of an open drainage system affects public health or safety, all as determined by the Engineer. The following criteria shall be used to design storm sewer systems when necessary:
A. Storm sewers shall be designed such that they do not surcharge from runoff caused by the 10-year, 24-hour storm, and that the hydraulic grade line of the storm sewer stays below the gutter flow line of the overlying roadway, or below the top of drainage structures outside the roadway during a 25-year, 24-hour storm. The system shall be designed to meet these requirements when conveying the flows from the contributing drainage area within the proposed development and existing flows from offsite areas that are upstream from the development.
B. The minimum inside diameter of pipe to be used in public storm sewer systems is twelve (12) inches. Smaller pipe sizes may be used in private systems, subject to the approval of the Engineer.
C. All storm sewer systems shall be designed taking into consideration the tailwater of the receiving facility or water resource. The tailwater elevation used shall be based on the design storm frequency. The hydraulic grade line for the storm sewer system shall be computed with consideration for the energy losses associated with entrance into and exit from the system, friction through the system, and turbulence in the individual manholes, catch basins, and junctions within the system.
D. The inverts of all curb inlets, manholes, yard inlets, and other structures shall be formed and channelized to minimize the incidence of quiescent standing water where mosquitoes may breed.
E. Headwalls shall be required at all storm sewer inlets or outlets to and from open channels or lakes.
(6) Water Resource Crossings: The following criteria shall be used to design structures that cross a water resource in the City:
A. Water resource crossings other than bridges shall be designed to convey the stream's flow based on the most recent Cuyahoga County Engineer Drainage Manual Supplement to ODOT Location and Design Manual Volume 2, Drainage design Section 1000 and 1100.
B. Bridges, open bottom arch or spans are the preferred crossing technique and shall be considered in the planning phase of the development. Bridges and open spans should be considered for all State Scenic Rivers, coldwater habitat, exceptional warmwater habitat, seasonal salmonid habitat streams, and Class III headwater streams. The footers or piers for these bridges and open spans shall not be constructed below the ordinary high water mark.
C. If a culvert or other closed bottom crossing is used, twenty-five (25) percent of the cross-sectional area or a minimum of 1 foot of box culverts and pipe arches must be embedded below the channel bed. The conduit or conveyance must to be sized to meet the most recent Cuyahoga County Engineer Drainage Manual Supplement to ODOT Location and Design Manual Volume 2, Drainage design Section 1000 and 1100.
D. The minimum inside diameter of pipes to be used for crossings shall be 12 inches.
E. The maximum slope allowable shall be a slope that produces a 10-fps velocity within the culvert barrel under design flow conditions. Erosion protection and/or energy dissipaters shall be required to properly control entrance and outlet velocities.
F. All culvert installations shall be designed with consideration for the tailwater of the receiving facility or water resource. The tailwater elevation used shall be based on the design storm frequency.
G. Headwalls shall be required at all culvert inlets or outlets to and from open channels or lakes.
H. Streams with a drainage area of 5 square miles or larger shall incorporate floodplain culverts at the bankfull elevation to restrict head loss differences across the crossing so as to cause no rise in the 100-year storm event.
I. Bridges shall be designed such that the hydraulic profile through a bridge shall be below the bottom chord of the bridge for either the 100-year, 24-hour storm, or the 100-year flood elevation as determined by FEMA, whichever is more restrictive.
(7) Overland flooding: Overland flood routing paths shall be used to convey stormwater runoff from the 100-year, 24-hour storm event to an adequate receiving water resource or SCM such that the runoff is contained within the drainage easement for the flood routing path and does not cause flooding of buildings or related structures. The peak 100-year water surface elevation along flood routing paths shall be at least one foot below the finished grade elevation of all structures. When designing the flood routing paths, the conveyance capacity of the site's storm sewers shall be taken into consideration.
(8) Compensatory flood storage mitigation: In order to preserve floodplain storage volumes and thereby avoid increases in water surface elevations, any filling within floodplains approved by the City must be compensated by providing an equivalent storage volume. First consideration for the location(s) of compensatory floodplain volumes should be given to areas where the stream channel will have immediate access to the new floodplain within the limits of the development site. Consideration will also be given to enlarging existing or proposed retention basins to compensate for floodplain fill if justified by a hydraulic analysis of the contributing watershed. Unless otherwise permitted by the City, reductions in volume due to floodplain fills must be mitigated within the legal boundaries of the development. Embankment slopes used in compensatory storage areas must reasonably conform to the natural slopes adjacent to the disturbed area. The use of vertical retaining structures is specifically prohibited.
(9) Velocity dissipation: Velocity dissipation devices shall be placed at discharge locations and along the length of any outfall to provide non-erosive flow velocity from the structure to a water resource so that the natural physical and biological characteristics and functions of the water resource are maintained and protected.
(c) Stormwater Quality Control:
(1) Direct runoff to an SCM: The site shall be designed to direct runoff to one or more SCMs provided in the most current Ohio EPA NPDES Construction Permit or Rainwater and Land Development Manual and shall be designed to meet the following general performance standards:
A. SCMs are required for sites greater than one (1) acre, or part of a larger common plan of development or sale which will disturb one (1) or more acres, all commercial and industrial sites development and sites disturbing less than one (1) acre for which a Comprehensive Management Plan is required by the Engineer.
B. Extended detention facilities that detain stormwater; settle or filter particulate pollutants; and release the controlled stormwater to a water resource.
C. Infiltration facilities that retain stormwater; promote settling, filtering, and biodegradation of pollutants; and infiltrate captured stormwater into the ground. The City may require a soil engineering report to be prepared for the site to demonstrate that any proposed infiltration facilities meet these performance standards.
D. The Engineer may approve other SCMs if the applicant demonstrates to the Engineer's satisfaction that these SCMs meet the objectives of this regulation as stated in Section 1007.09(c)(5), and has prior written approval from the Ohio EPA.
E. For the construction of new roads and roadway improvement projects by public entities (i.e. the state, counties, townships, cities, or villages), the Engineer may approve SCMs not included in Table 4a and 4b of the Ohio EPA NPDES General Construction Permit, but must show compliance with the current version of the Ohio Department of Transportation "Location and Design Manual, Volume Two Drainage Design".
F. Offsite Mitigation of Post-Construction: Ohio EPA may authorize the offsite mitigation of the post-construction requirements on a case by case basis provided the permittee clearly demonstrates the BMPs listed in Tables 4a and 4b as listed in the current Ohio EPA General Construction Permit are not feasible and the following criteria are met: (1) a maintenance agreement or policy is established to ensure operations and treatment long-term; (2) the offsite location discharges to the same HUC-12 watershed unit; and (3) the mitigation ratio of the WQv is 1.5 to 1 or the WQv at the point of retrofit, whichever is greater.
(2) Criteria applying to all SCMs: SCMs shall be designed to meet the current requirements in the OEPA NPDES Construction Permit and ODNR Rainwater and Land Development Manual to treat the water quality volume (WQV).
(3) Additional criteria applying to infiltration facilities:
A. All runoff directed into an infiltration basin must first flow through a pretreatment practice such as a grass channel or filter strip to remove coarser sediments that could cause a loss of infiltration capacity.
B. During construction, all runoff from disturbed areas of the site shall be diverted away from the proposed infiltration basin site. No construction equipment shall be allowed within the infiltration basin site to avoid soil compaction.
(4) Additional criteria for extended detention facilities:
A. The outlet shall be designed to not release more than the first half of the water quality volume per the Ohio EPA NPDES Construction Permit. The outlet shall be designed to minimize clogging, vandalism, maintenance, and promote the capture of floatable pollutants.
B. The basin design shall incorporate the following features to maximize multiple uses, aesthetics, safety, and maintainability:
1. Basin side slopes above the permanent pool shall have a run to rise ratio of 3:1 or flatter. A run to rise ratio of 3:1 is the minimum if site conditions are limiting.
2. The perimeter of all permanent pool areas deeper than four (4) feet shall be surrounded by an aquatic bench that extends at least eight (8) feet and no more than fifteen (15) feet outward from the normal water edge. The eight (8) feet wide portion of the aquatic bench closest to the shoreline shall have an average depth of six (6) inches below the permanent pool to promote the growth of aquatic vegetation. The remainder of the aquatic bench shall be no more than fifteen (15) inches below the permanent pool to minimize drowning risk to individuals who accidentally or intentionally enter the basin, and to limit growth of dense vegetation in a manner that allows waves and mosquito predators to pass through the vegetation. The maximum slope of the aquatic bench shall be 10 (H) to 1 (V). The aquatic bench shall be planted with native plant species comparable to wetland vegetation that are able to withstand prolonged inundation. The use of invasive plant species is prohibited.
3. A forebay designed to allow larger sediment particles to settle shall be placed at basin inlets. The forebay and micropool volume shall be equal to at least ten percent (10%) of the water quality volume (WQv).
4. Detention basins shall be provided with an emergency drain, where practicable, so that the basin may be emptied if the primary outlet becomes clogged and/or to drain the permanent pool to facilitate maintenance. The emergency drain should be designed to drain by gravity where possible.
(5) Criteria for the Acceptance of Alternative post-construction BMPs: The applicant may request approval from the Engineer for the use of alternative structural post-construction SCMs if the applicant shows to the satisfaction of the Engineer that these SCMs are equivalent in pollutant removal and runoff flow/volume reduction effectiveness to those listed in ODNR Rainwater and Land Development Manual. Alternative post-construction BMPs may include, but are not limited to: vegetated swales, vegetated filter strips, hydrodynamic separators, high-flow media filters, cartridge filters, membrane filters, subsurface flow wetlands, multi-chamber treatment trains, road shoulder media filter drains, wetland channels, rain barrels, green roofs, rain gardens, or other non-structural post-construction approaches. Prior approval from the Ohio EPA must also be obtained on a case-by-case basis for use of alternative post-construction BMPs and it must be demonstrated that the use of a BMP listed in the Ohio EPA NPDES Construction Permit or the ODNR Rainwater and Land Development Manual is not feasible.
(d) Stormwater Quantity Control: The Comprehensive Stormwater Management Plan shall describe how the proposed SCMs are designed to meet the following requirements for stormwater quantity control for each watershed in the development:
(1) The peak discharge rate of runoff from the Critical Storm and all more frequent storms occurring under post-development conditions shall not exceed the peak discharge rate of runoff from a 1-year, 24-hour storm occurring on the same development drainage area under pre-development conditions.
(2) Storms of less frequent occurrence (longer return periods) than the Critical Storm, up to the 100-year, 24-hour storm shall have peak runoff discharge rates no greater than the peak runoff rates from equivalent size storms under predevelopment conditions. The 1, 2, 5, 10, 25, 50, and 100-year storms shall be considered in designing a facility to meet this requirement.
(3) The Critical Storm for each specific development drainage area shall be determined as follows:
A. Determine, using a curve number-based hydrologic method; or other hydrologic method approved by the Engineer, the total volume (acre-feet) of runoff from a 1-year, 24-hour storm occurring on the development drainage area before and after development. These calculations shall meet the following standards:
1. Calculations shall include the lot coverage assumptions used for full build out as proposed.
2. Calculations shall be based on the entire contributing watershed to the development area.
3. Drainage area maps shall include area, curve number, time of concentrations. Time of concentration shall also show the flow path and the separation in flow type.
4. Rainfall Depth - For the most accurate, up-to-date, location-specific rainfall data for stormwater design, use the Precipitation-Frequency Atlas of the United States, NOAA Atlas 14, Vol 2(3). [available online: http://hdsc.nws.noaa.gov/hdsc/pfds/.]
5. Temporal Distribution- Use the SCS Type II rainfall distribution for all design events with a recurrence interval greater than 1 year. Include lot coverage assumptions used for full build out of the proposed condition.
6. Curve numbers for the pre-development condition shall reflect the average type of land use over the past 10 years and not only the current land use.
i. Pre-development Curve Numbers - For all areas, use listed values from TR-55 NRCS USDA.
ii. Post-development Curve Numbers - For all areas, use listed values from TR-55 NRCS USDA.
7. Time of Concentration - Use velocity based methods from (TR-55 NRCS USDA Urban Hydrology in Small Watersheds, 1986) to estimate travel time (Tt) for overland (sheet) flow, shallow concentrated flow and channel flow.
i. Maximum sheet flow length is 100 ft.
ii. Use the appropriate "unpaved" velocity equation for shallow concentrated flow from Soil Conservation Service National Engineer Handbook Section 4- Hydrology (NEH-4).
8. The volume reduction provided by permeable pavement, bioretention, or other LID SCMs may be subtracted from the post development stormwater volume. Volume reductions for these practices may be demonstrated using methods outlined in Rainwater and Land Development Manual or a hydrologic model acceptable to the Engineer.
B. To account for future post-construction improvements to the site, calculations shall assume an impervious surface such as asphalt or concrete for all parking areas and driveways, regardless of the surface proposed in the site description except in instances of engineered permeable pavement systems. From the volume determined in Section 1007.09(d)(3)A., determine the percent increase in volume of runoff due to development. Using the percentage, select the 24-hour Critical Storm from Table 3.
Table 3: 24-Hour Critical Storm
If the Percentage of Increase in Volume of Runoff is: | The Critical Storm will be: | |
Equal to or Greater Than: | and Less Than: | |
--- | 10 | 1 year |
10 | 20 | 2 year |
20 | 50 | 5 year |
50 | 100 | 10 year |
100 | 250 | 25 year |
250 | 500 | 50 year |
500 | --- | 100 year |
For example, if the percent increase between the pre- and post-development runoff volume for a 1-year storm is 35%, the Critical Storm is a 5-year storm. The peak discharge rate of runoff for all storms up to this frequency shall be controlled so as not to exceed the peak discharge rate from the 1-year frequency storm under pre-development conditions in the development drainage area. The post-development runoff from all less frequent storms need only be controlled to meet pre-development peak discharge rates for each of those same storms. | ||
(e) Stormwater Management on Previously Developed Areas.
(1) Comprehensive Stormwater Management Plans for previously developed areas must accomplish one or a combination of the following two conditions:
Comprehensive Stormwater Management Plans for redevelopment projects must accomplish one of the following options:
A. Reduce existing site impervious areas by at least 25 percent, a one-for-one credit towards the 25 percent net reduction of impervious area can be obtained through the use of green roofs.
B. Treatment of 25 percent of the WQv for the previously developed areas using a SCM from Table 4a and 4b.
(2) Where there is a combination of redeveloped areas and new development, a weighted approached shall be used with the following equation available in the most current Ohio EPA NPDES Construction Permit or the ODNR Rainwater and Land Development Manual.
(3) Post-construction practices shall be located to treat impervious areas most likely to generate the highest pollutant load, such as parking lots or roadways, rather than areas predicted to be cleaner such as rooftops.
(4) Where conditions prevent impervious area reduction or on-site stormwater management for redevelopment projects, practical alternatives as detailed in Section 1007.10 shall be implemented.
(Ord. 2020-59. Passed 4-20-20.)