Methods for controlling increases in storm water runoff peaks and volumes may include, but are not limited to permanent retention ponds and lakes, dry detention basins, and subsurface detention tanks.
(a) Storm Water Basins.
(1) Pool geometry. The recommended length-to-width ratio for the pond is 3:1 (the length will be three (3) times the width).
(2) Riser in embankment. The riser shall be located within the embankment for purposes of maintenance access. Access to the riser will be by manholes.
(3) Water drains. Each retention basin shall have a drainpipe that can completely drain the pond. The drain shall have an elbow within the pond to prevent sediment deposits from plugging the drain.
(4) Adjustable gate valves. Both the storm water management and water quality basin drains shall have adjustable gate valves. Valves shall be located inside the riser at a point where they will remain dry and can be operated in a safe and convenient manner. During annual inspections the valves shall be fully opened and closed at least once, and the certifying official shall attest to this on the inspection form. To prevent vandalism, the handwheel shall be chained to a ringbolt or manhole step.
(5) Principal spillway. Each principal spillway shall be designed in accordance with the NRCS standards and specifications for the office serving the county. Each principal spillway shall have the capacity to pass the 100-year design storm flow. The inlet or riser size for the pipe drops shall be designed so that the flow through the structure goes from weir flow control to pipe flow control without going into orifice control in the riser. The crest elevation of the primary spillway shall be no less than one foot below the emergency spillway crest. Premium joint pipe is required and a removable trash rack shall be installed at each location. Anti-seep collars shall be provided for all pipe conduits through an embankment.
(6) Emergency spillway. An emergency spillway shall be provided on each storm water management basin. Emergency spillways shall convey flood flows safely past the embankment, and shall be designed in accordance with NRCS standards and specifications for the office serving the local county. Emergency spillways shall have a 100-year design storm capacity unless exempted in writing by the City Engineer.
(b) Embankments. Each dam embankment shall be designed in accordance with the standards and specifications in the current edition of the Ohio "Rainwater and Land Development" manual, the NRCS Field Office Technical Guide for Cuyahoga County, the Ohio EPA standards, and Chapter 1059,"Ponds, Lakes, Embankments, Reservoirs or Other Impoundments of Water" whichever are more stringent. Anti-seep collars shall be provided for all pipe conduits through an embankment.
(1) Safety features.
A. The primary spillway opening shall not permit access to the public and other non-maintenance personnel.
B. The perimeter of all water pool areas that are deeper than three (3) feet shall be surrounded by benches that meet the following:
1. A safety bench, with a maximum slope of 3%, which extends outward, on dry land, from the shoreline. This bench will be a minimum of 25 feet wide to provide for the safety of individuals and maintenance vehicles that are adjacent to the water pool. The safety bench may be landscaped, without the use of structures, to prevent access to the water pool.
2. Side slopes between the safety bench and the aquatic bench shall not be steeper than 3:1 (3 feet horizontal for every 1 foot vertical).
3. An aquatic bench that extends inward from the shoreline far enough to ensure public safety and has a maximum depth of 15 inches below the normal water surface elevations. The aquatic bench may be landscaped to prevent access to the deeper water pool.
4. Side slopes beyond the aquatic bench and below the permanent water level shall not be steeper than 2:1 (2 feet horizontal for every 1 foot vertical).
5. The contours of the pond will be designed and managed to eliminate drop-offs and other hazards.
6. Side slopes getting to the pond shall not exceed 3:1 and shall terminate on a safety bench.
(c) Runoff Rate. The peak runoff rate from the development area shall not be greater after development than it was before development. The applicant shall provide calculations demonstrating no increases in the runoff rates from the one (1), two (2), five (5), ten (10), twenty-five (25), fifty (50) and one hundred (100) year storms.
(d) Runoff Volume. Increases in the runoff volume shall be offset by further restricting runoff rates. Based on the increase in runoff volume, the applicant shall determine the critical storm for the development area. The runoff rate from the critical storm shall be restricted to the one (1) year pre-development storm runoff rate. The critical storm shall be calculated as follows:
(1) Determine the total volume of runoff from a one-year frequency, twenty- four hour storm, occurring on the development area before and after development.
(2) From the volumes in paragraph (1) determine the percent of increase in volume of runoff due to development (2) using this percentage, select the critical storm from this table:
Table 4: Critical Storm Selection
The Percentage Increase In Volume Of Runoff Is: | ||
Equal To Or Greater Than | And Less Than | The 24-Hour “Critical Storm” For Discharge Will Be |
0 | 10 | 1 Year |
10 | 20 | 2 Years |
20 | 50 | 5 Years |
50 | 100 | 10 Years |
100 | 250 | 25 Years |
250 | 500 | 50 Years |
500 | ---- | 100 Years |
(e) Water Quality. To meet the post-construction requirements of this Chapter, the Storm Water Management and Erosion and Sediment Control Plan must contain a description of the Post-Construction Best Management Practices (BMPs) that will be installed during construction for the site and the rationale for their selection. The rationale must address the anticipated impacts on the channel and floodplain morphology, hydrology, and water quality.
(1) Structural BMPs. Structural (designed) Post-Construction storm water treatment practices shall be incorporated into the permanent drainage system for the site.
A. Properly Sized BMPs. The BMP(s) chosen must be sized to treat the water quality volume (WQv) and ensure compliance with Ohio's Water Quality Standards in OAC Chapter 3745-1. The WQv shall be equivalent to the volume of runoff from a 0.90-inch rainfall and shall be determined according to the following equation:
WQv = Rv *P* A/12 (Equation 1)
where:
WQv = water quality volume in acre-feet
Rv = the volumetric runoff coefficient calculated using Equation 2
P = 0.90 inch precipitation depth
A = area draining into the BMP in acres.
Rv = 0.05 + 0.9i (Equation 2)
where i = fraction of post-construction impervious surface
B. An additional volume equal to 20 percent of the WQv shall be incorporated into the BMP for sediment storage and/or reduced infiltration capacity. The BMPs will be designed according to the methodology included in the Ohio Rainwater and Land Development manual, ODOT Post-Construction storm water standards, or other manual that is acceptable to Ohio EPA.
C. The BMPs listed in Tables 5a and 5b below shall be considered standard BMPs approved for general use. However, communities with a regulated MS4 may limit the use of some of these BMPs. BMPs shall be designed such that the drain time is long enough to provide treatment, but short enough to provide storage available for successive rainfall events and avoid the creation of nuisance conditions. The outlet structure for the post-construction BMP must not discharge more than the first half of the WQv in less than one-third of the drain time. The WQv is the volume of storm water runoff that must be detained by a post-construction practice specified by the most recent edition of the Rainwater and Land Development manual.
D. Post-construction practices shall be sized to treat 100% of the WQv associated with their contributing drainage area. If there is an existing post-construction BMP that treats runoff from the disturbed area and the BMP meets the post-construction requirements of this permit, no additional post-construction BMP will be required. 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.
Table 5a Extended Detention Post-Construction Practices
With Minimum Drain Times
Extended Detention Practices | Minimum Drain Time of WQv |
Wet Extended Detention Basin 1,2 | 24 hours |
Constructed Extended Detention Wetland 1,2 | 24 hours |
Dry Extended Detention Basin 1,3 | 48 hours |
Permeable Pavement - Extended Detention 1 | 24 hours |
Underground Storage - Extended Detention 1,4 | 24 hours |
Sand & Other Media Filtration - Extended Detention 1,5 | 24 hours |
Notes:
1. The outlet structure shall not discharge more than the first half of the Wqv in less than one-third of the drain time.
2. Provide a permanent pool with a minimum volume equal to the WQv and an extended detention volume above the permanent pool equal to 1.0 x WQv.
3. Dry basins must include a forebay and a micropool each sized at a minimum of 0.1 x WQv and a protected outlet, or include acceptable pretreatment and a protected outlet.
4. Underground storage must have pretreatment for removal of suspended sediments included in the design and documented in the SWP3. This pretreatment shall concentrate sediment in a location where it can be readily removed. For non-infiltrating, underground extended detention systems, pretreatment shall be 50% effective at capturing total suspended solids according to the testing protocol established in the Alternative Post-Construction BMP Testing Protocol.
5. The WQv ponding area shall completely empty between 24 and 72 hours.
Table 5b Extended Detention Post-Construction Practices
With Minimum Drain Times
Infiltration Practices | Maximum Drain Time of WQv |
Bioretention Area/Cell 1,3 | 24 hours |
Infiltration Basin 2 | 24 hours |
Infiltration Trench 3 | 48 hours |
Permeable Pavement - Infiltration 3 | 48 hours |
Underground Storage - Infiltration 3,4 | 48 hours |
Notes:
1. Bioretention soil media shall have a permeability of approximately 1 - 4 in/hr. Meeting the soil media specifications in the Rainwater and Land Development manual is considered compliant with this requirement. Bioretention cells must have underdrains unless in-situ conditions allow for the WQv (surface ponding) plus the bioretention soil (to a depth of 24 inches) to drain completely within 48 hours.
2. Infiltrating practices with the WQv stored aboveground (bioretention, infiltration basin) shall fully drain the WQv within 24 hours to minimize nuisance effects of standing water and to promote vigorous communities of appropriate vegetation.
3. Subsurface practices designed to fully infiltrate the WQv (infiltration trench, permeable pavement with infiltration, underground storage with infiltration) shall empty within 48 hours to recover storage for subsequent storm events.
4. Underground storage systems with infiltration must have adequate pretreatment of suspended sediments included in the design and documented in the SWP3 in order to minimize clogging of the infiltrating surface. Pretreatment shall concentrate sediment in a location where it can be readily removed. Examples include media filters situated upstream of the storage or other suitable alternative approved by Ohio EPA. For infiltrating underground systems, pretreatment shall be 80% effective at capturing total suspended solids according to the testing protocol established in the Alternative Post-Construction BMP Testing Protocol.
E. The owner may request approval from the City Engineer to utilize innovative or experimental post-construction storm water management technologies.
1. Alternative post-construction BMPs shall previously have been tested to confirm storm water treatment efficacy equivalent to those BMPs listed in Tables 5a and 5b using the protocol described in this section. BMP testing may include laboratory testing, field testing, or both.
2. Permittees shall request approval from Ohio EPA to use alternative post-construction BMPs on a case-by-case basis. To use an alternative post-construction BMP, the permittee must demonstrate that use of a BMP listed in Tables 5a and 5b is not feasible and the proposed alternative post-construction BMP meets the minimum treatment criteria as described in this section. The permittee shall submit an application to Ohio EPA for any proposed alternative post-construction BMP. Where the development project is located within a regulated municipal separate storm sewer system (MS4) community, the use of an alternative practice requires pre-approval by the MS4 before submittal of the Ohio EPA permit application. Ohio EPA requires that approvals for alternative post-construction BMPs are finalized before permittees submit an NOi for permit coverage.
3. In addition to meeting sediment removal criteria, the discharge rate from the proposed alternative practice shall be reduced to prevent stream bed erosion and protect the physical and biological stream integrity unless there will be negligible hydrological impact to the receiving surface water of the state. Discharge rate is considered to have a negligible impact if the permittee can demonstrate that one of the following three conditions exist:
a. The entire WQv is recharged to groundwater;
b. The larger common plan of development or sale will create less than one acre of impervious surface;
c. The storm water drainage system of the development discharges directly into a large river with drainage area equal to 100 square miles or larger upstream of the development site or to a lake where the development area is less than 5 percent of the watershed area, unless a TMDL has identified water quality problems into the receiving surface waters of the state.
4. 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.
5. Alternative Post-Construction BMP Testing Protocol. 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 6, and total suspended sediment (TSS) concentrations within 10% of 200 mg/L (180 mg/L - 220 mg/L TSS). For an alternative BMP to be acceptable, the test results must demonstrate that the minimum treatment rate is 80% TSS removal at the design flow rate for the tested BMP.
Table 6 Particle Size Distribution for Testing Alternative
Post-Construction BMPs
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 |
a. 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. For an alternative BMP to be acceptable, the test results must demonstrate the minimum treatment rate is 80% TSS removal for influent concentrations equal to or greater than 100 mg/L TSS. If the influent concentration to the proposed alternative BMP is less than 100 mg/L TSS in the field, then the BMP must achieve an average effluent concentration less than or equal to 20 mg/L TSS;
b. Testing of alternative post-construction BMPs shall be performed or overseen by a qualified independent, third-party testing organization;
c. 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;
d. 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 80% treatment efficacy;
e. Testing shall indicate the recommended maintenance frequency and maintenance protocol to ensure ongoing performance of the alternative post-construction BMP.
6. The alternative post-construction BMP testing protocol described in this section is similar to testing requirements specified by the New Jersey Department of Environmental Protection (NJDEP) for storm water Manufactured Treatment Devices (MTD) and therefore testing results certified by NJDEP shall be accepted by Ohio EPA. For examples of BMPs that have been tested using New Jersey Department of Environmental Protection's procedures, see the website: www.njstormwater.org.
7. Another nationally recognized storm water product testing procedure is the Technology Assessment Protocol - Ecology (TAPE) administered by the State of Washington, Department of Ecology. The TAPE testing procedure describes testing to achieve 80% TSS removal using a sediment mix with a particle size distribution with approximately 75% of the mass of the aggregate with particle diameters less than 45 microns. Overall, this particle size distribution is finer than the distribution in Table 5. Therefore, if TAPE testing results are available for a proposed alternative post-construction BMP, those results shall be accepted by Ohio EPA. The State of Washington, Department of Ecology website is https ://ecology. wa.gov/.
8. 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 90 percent 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 3) 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 (Equation 3)
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)
9. 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, and rain gardens. The Director may also consider non-structural post-construction approaches.
F. Construction activities shall be exempt from this condition if it can be demonstrated that the WQv is provided within an existing structural Post-Construction BMP which is part of a larger common plan of development or sale or if structural Post-Construction BMPs are addressed in a regional or local storm water management plan. A municipally operated regional storm water BMP can be used as a post-construction BMP provided that the BMP can detain the WQv from the entire drainage area and release it over a 24 hour period upon written permission from the Community Engineer.
G. For a previously developed area, one or a combination of the following two conditions shall be met:
1. A 20 percent net reduction of the site's volumetric runoff coefficient through impervious area reduction with soil restoration or replacing impervious roof area with green roof area (for these purposes green roofs shall be considered pervious surface) or
2. Treatment of 20 percent of the WQv for the previously developed area using a practice meeting Table 5a/5b criteria.
H. Where there is a combination of redeveloped areas and new development, a weighted approached shall be used with the following equation:
WQv = P *A* [(Rvt0.2) + (Rv2 - Rv1)] / 12 (Equation 4)
where
P = 0.90 inches
A = area draining into the BMP in acres
Rv1 = volumetric runoff coefficient for existing conditions (current site impervious area)
Rv2 = volumetric runoff coefficient for proposed conditions (post-construction site impervious area)
I. 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.
J. 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:
1. Impervious surface disconnection
2. Rainwater harvesting
3. Bioretention
4. Infiltration basin
5. Infiltration trench
6. Permeable pavement with infiltration
7. Underground storage with infiltration
8. Grass swale
9. Sheet flow to filter strip
10. Sheet flow to conservation area
(2) Water quality basin. If a Water Quality Basin is needed and cannot be incorporated into an existing or planned Detention or Retention Basin then a separate Water Quality Basin will need to be planned, designed, constructed and maintained into perpetuity.
(f) The requirements shall be satisfied at each location where runoff leaves the development area. The runoff rates and volumes shall be considered for both the conditions before and after development at these locations.
(g) To ensure the continued functioning of storm water control structures, the following information shall be identified to the satisfaction of the City Engineer:
(1) The person or entity responsible for continued maintenance of the storm water control structure shall be identified to the satisfaction of the City Engineer.
(2) Maintenance requirements and schedules;
(3) Permanent access easements required for those conducting maintenance to perform inspection and maintenance of storm water control structures and storm water conveyance systems. The minimum size of an easement shall be approved by the City Engineer. Within such easements, no person shall plant trees, shrubbery, plantings, or construct buildings, fences, or walls, that obstruct free flow of storm water or obstruct the movement of construction equipment.
(4) An owner or owners of two or more existing or proposed lots may apply for approval of a SWM plan serving more than one lot. Such plan shall comply with applicable provisions of this chapter and all other relevant laws and ordinances. No subdivision of lands or building permit shall be approved under any multiple lot plan until owners of all affected lots have
entered into a mutual agreement or formed an association for joint maintenance of the SWM facility and until all easements needed to effectuate the plan have been granted. The mutual agreement or association and the easements must be signed, acknowledged, delivered and recorded in the manner for documents relating to real estate titles.
(h) Detention or retention basin exemption for redevelopment or for expansion of existing facilities:
(1) For any development regulated by this chapter, the construction of a detention or retention basin may not be required for the development if the post-development peak discharge for a 100 year frequency 24 hour storm causes no increase to the existing peak discharge using the TR-55 method of calculation or other method approved by the City Engineer.
(Ord. 2022-051. Passed 3-21-22.)