(a) General: The stormwater system, including SCMs for storage, treatment and control, and conveyance facilities, shall be designed to prevent structure flooding during the one hundred (100)-year, twenty-four (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 watercourse restoration project.
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 Rainwater and Land Development.
(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, 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 one hundred (100)-year, twenty-four (24)-hour storm; and maintain, to the extent practicable, the pre-development 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 1183.07
.
(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 one hundred (100)-year, twenty-four (24)-hour storm. When designing stormwater 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 1183.09
if it can be shown to the satisfaction of the Village 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 Village. 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 but less than two (2) acres, 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 2a and 2b practices are not feasible. The justification must address limiting factors which would prohibit the project going forward should Table 2a and 2b practices be required. Please note that additional practices selected will require approval from the regulated MS4. The use of green infrastructure BMPs such as runoff reducing practices is also encouraged.
(8) 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.
(9) Maintenance: All SCMs shall be maintained in accordance with the inspection and maintenance plan and agreements approved by the Village Engineer as detailed in Section 1183.08
.
(10) Ownership: Unless otherwise required by the Village, SCMs serving multiple lots in subdivisions shall be on a separate lot held and maintained by an entity of common ownership or, if compensated by the property owners, by the Village. SCMs serving single lots shall be placed on these lots, protected within an easement, and maintained by the property owner.
(11) 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 (1) 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.
(12) 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.
(13) 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 Village 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 1183.07
. 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 Village 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 one and one-half (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 Village Engineer.
(4) Open channels: Unless otherwise allowed by the Village Engineer, drainage tributary to SCMs shall be provided by an open channel with vegetated banks and designed to carry the ten (10)-year, twenty-four (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 Village zoning or where the use of an open drainage system affects public health or safety, all as determined by the Village 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 five (5)-year, twenty-four (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 ten (10)-year, twenty-four (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 Village 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 Village:
A. Water resource crossings other than bridges shall be designed to meet the minimum requirements of the Ohio Department of Transportation.
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. The minimum inside diameter of pipes to be used for crossings shall be twelve (12) inches.
D. The maximum slope allowable shall be a slope that produces a ten (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.
E. 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.
F. Headwalls shall be required at all culvert inlets or outlets to and from open channels or lakes.
G. Streams with a drainage area of five (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 one hundred (100)-year storm event.
H. Bridges shall be designed to meet the minimum standards of the Ohio Department of Transportation.
(7) Overland flooding: Overland flood routing paths shall be used to convey stormwater runoff from the one hundred (100)-year, twenty-four (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 one hundred (100)-year water surface elevation along flood routing paths shall be at least one (1) 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 Village 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 Village, 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 (1) or more of the following SCMs. These practices are listed in Table 2 of this regulation and shall be designed to meet the following general performance standards:
A. Extended detention facilities that detain stormwater; settle or filter particulate pollutants; and release the controlled stormwater to a water resource.
B. Infiltration facilities that retain stormwater; promote settling, filtering, and biodegradation of pollutants; and infiltrate captured stormwater into the ground. The Village Engineer may require a soil engineering report to be prepared for the site to demonstrate that any proposed infiltration facilities meet these performance standards.
For sites less than two (2) acres, but required to create a comprehensive stormwater management plan, the Village Engineer may approve other SCMs if the applicant demonstrates to the Village Engineer satisfaction that these SCMs meet the objectives of this regulation as stated in division (c)(6) of this section.
C. For sites greater than two (2) acres, or less than two (2) acres but part of a larger common plan of development or sale which will disturb two (2) or more acres, the Village Engineer may approve other SCMs if the applicant demonstrates to the Village Engineer satisfaction that these SCMs meet the objectives of this regulation as stated in division (c)(6) of this section, and has prior written approval from the Ohio EPA.
D. For the construction of new roads and roadway improvement projects by public entities (i.e. the state, counties, townships, cities, or villages), the Village Engineer may approve SCMs not included in Table 2 of this regulation, but must show compliance with the current version of the Ohio Department of Transportation Location and Design Manual, Volume Two Drainage Design.
E. Offsite mitigation of post-construction. Ohio EPA may authorize the off site mitigation of the post-construction requirements on a case by case basis provided the permittee clearly demonstrates the BMPs listed in Tables 2a and 2b 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 chosen must be sized to treat the water quality volume in conformance with Ohio EPA General NPDES Permit # OHC000005.
(3) Criteria for the acceptance of alternative post-construction BMPs: The applicant may request approval from the Village Engineer for the use of alternative structural post-construction SCMs if the applicant shows to the satisfaction of the Village Engineer that these SCMs are equivalent in pollutant removal and runoff flow/volume reduction effectiveness to those listed in Tables 2a and 2b. 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 Tables 2a and 2b is not feasible. To demonstrate the equivalency, the applicant must show:
A. The alternative SCM has a minimum total suspended solid (TSS) removal efficiency of eighty percent (80%) for influent concentrations equal to or greater than one hundred (100) mg/L TSS. If the influent concentration to the proposed alternative BMP is less than one hundred (100) mg/L TSS in the field, then the BMP must achieve an average effluent concentration less than or equal to twenty (20) mg/L TSS. Alternative post-construction practices must be tested using a defined particle size distribution and protocol comparable to the New Jersey Department of Environmental Protection (NJDEP)'s testing requirements for stormwater Manufactured Treatment Devices or Washington State Department of Ecology's Technology Assessment Protocol - Ecology (TAPE), and testing must be overseen by a qualified, independent, third-party testing organization.
1. For laboratory testing, the alternative BMP shall be tested using sediment with a specific gravity of 2.65, a particle size distribution closely matching the distribution shown in Table 3, and total suspended sediment (TSS) concentrations within ten percent (10%) of two hundred (200) mg/L (one hundred eighty (180) mg/L - two hundred twenty (220) mg/L TSS). For an alternative BMP to be acceptable, the test results must demonstrate that the minimum treatment rate is eighty percent (80%) TSS removal at the design flow rate for the tested BMP.
Table 3: Particle Size Distribution for Testing Alternative Post-Construction BMPs
Particle Size (microns) | Percent Finer (%) |
Particle Size (microns) | Percent Finer (%) |
1,000 | 100 |
500 | 95 |
250 | 90 |
150 | 75 |
100 | 60 |
75 | 50 |
50 | 45 |
20 | 35 |
8 | 20 |
5 | 10 |
2 | 5 |
2. For field testing, the alternative BMP shall be tested using storm water runoff from the field, not altered by adding aggregate or subjecting to unusually high sediment loads such as those from unstabilized construction disturbance. The storm water runoff used for field testing shall be representative of runoff from the proposed installation site for the alternative BMP after all construction activities have ceased and the ground has been stabilized. The influent and effluent TSS concentrations of storm water runoff must be collected in the field.
3. Testing shall demonstrate the maximum flow rate at which the alternative post-construction BMP can achieve the necessary treatment efficacy, including consideration for the potential of sediment resuspension.
4. Testing shall demonstrate the maximum volume of sediment and floatables that can be collected in the alternative post-construction BMP before pollutants must be removed to maintain eighty percent (80%) treatment efficacy.
5. Testing shall indicate the recommended maintenance frequency and maintenance protocol to ensure ongoing performance of the alternative post-construction BMP.
B. The water quality volume discharge rate from the selected alternative practice is reduced to prevent stream bed erosion and protect the physical and biological stream integrity, unless there will be negligible hydrologic impact to the receiving surface water of the state. The discharge rate from the SCM will have negligible impacts if the applicant can demonstrate one (1) of the following conditions:
1. The entire water quality volume is recharged to groundwater.
2. The larger common plan of development or sale will create less than one (1) acre of impervious surface.
3. The development project is a redevelopment project with an ultra-urban setting, such as a downtown area, or on a site where one hundred percent (100%) of the project area is already impervious surface and the stormwater discharge is directed into an existing storm sewer system.
4. The stormwater drainage system of the development discharges directly into a large river with a drainage area equal to one hundred (100) square miles or larger upstream of the development site or to a lake where the development area is less than five percent (5%) of the watershed area upstream of the development site, unless a TMDL has identified water quality problems in the receiving surface waters of the state.
If the conditions above that minimize the potential for hydrological impact to the receiving surface water of the state do not exist, then the alternative post-construction BMP must prevent stream erosion by reducing the flow rate from the WQv. In such cases, discharge of the WQv must be controlled. A second storm water BMP that provides extended detention of the WQv may be needed to meet the post-construction criteria.
C. Alternative BMPs that utilize treatment processes such as filtering or centrifugal separation, rather than a detention and settling volume, must be designed to ensure treatment of ninety percent (90%) of the average annual runoff volume. For the design of these BMPs, the water quality flow rate (WQF) considered equivalent to the water quality volume (WQv) shall be determined utilizing the Rational Method (Equation 4) with an intensity (i) appropriate for the water quality precipitation event. This intensity shall be calculated using the table given in Appendix C.
WQF = C * i * A
Where:
WQF = water quality flow rate in cubic feet per second (cfs)
C = rational method runoff coefficient
i = intensity (in/hr)
A = area draining to the BMP (acres)
(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 one (1)-year, twenty-four (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 one hundred (100)-year, twenty-four (24)-hour storm shall have peak runoff discharge rates no greater than the peak runoff rates from equivalent size storms under pre-development 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 Village Engineer, the total volume (acre-feet) of runoff from a one (1)-year, twenty-four (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).
5. Temporal distribution - Use the SCS Type II rainfall distribution for all design events with a recurrence interval greater than one (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 ten (10) years and not only the current land use.
a. Pre-development curve numbers - For all areas, use listed values from TR-55 NRCS USDA.
b. 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.
a. Maximum sheet flow length is one hundred (100) feet.
b. 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 or a hydrologic model acceptable to the Village 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 division (d)(3)A. of this section, determine the percent increase in volume of runoff due to development. Using the percentage, select the twenty-four (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: | |
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 |
(e) Stormwater Management on Previously Developed Areas.
(1) Comprehensive stormwater management plans for previously developed areas must comply with the provisions of Ohio EPA General NPDES Permit # OHC000005.
(2) Where conditions prevent impervious area reduction or on-site stormwater management for redevelopment projects, practical alternatives as detailed in Section 1183.10 may be approved by the Village Engineer.
(Ord. 2019-13. Passed 6-18-19.)