(a) Stormwater Design General Information. No person shall develop any real property or connect or cause to be connected any building or other structure, either directly or indirectly, with a drain for the removal of surface, roof, ground or other water to be discharged into a ditch, swale, waterway, stream or an existing storm drainage system for such real property, without complying with the performance standards and paying the charges set forth in this chapter.
(b) 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 watercourse restoration project and comply with Chapter 1051 - Establishment of Riparian Zones.
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 or another design manual acceptable for use by the City and Ohio EPA.
SCMs that meet the criteria in this regulation, and additional criteria required by the City 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 1246
, 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 pre-development runoff patterns, volumes, and peaks from each lot.
(4) Stormwater facilities in water resources. SCMs and related activities shall not be constructed in water resources 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 1050.07
, and the activity is in compliance with Chapter 1052
- Erosion and Sediment Control and Chapter 1051
- Establishment of Riparian Zones, all as determined by the City Engineer.
(5) Stormwater facilities in the floodplain. Stormwater facilities constructed, manufactured or otherwise, that provide treatment of the water quality volume (see Table 3 of this section), detention, retention, and/or infiltration, and all related activities, shall not be constructed in any special flood hazard area, as defined in Chapter 1464
- Flood Damage Prevention.
(6) Stormwater ponds and surface conveyance channels. All stormwater pond and surface conveyance designs must provide a minimum of one foot freeboard above the projected peak stage within the facility during the 100-year, 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.
(7) Exemption. The site where soil-disturbing activities are conducted shall be exempt from the requirements of this section if it can be shown to the satisfaction of the City Engineer that the site is part of a larger common plan of development where the stormwater management requirements for the site are provided by one or more 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 Engineer.
(8) Maintenance. All SCMs shall be maintained in accordance with inspection and maintenance agreements approved by the City Engineer as detailed in Section 1050.08
.
(9) Agreements with sub-dividers or developers. A sub-divider or developer shall be required to construct an on-site SCM for the purposes of water quality and water retention approved by the City Engineer. The combination of stormwater quality and quantity requirements for two or more developments may be placed into one detention basin to be located at a strategic site given that a separate agreement with all parties is developed. The City shall enter into an agreement with the sub-divider or developer, to be approved by Council, containing the following conditions:
A. The sub-divider of a major subdivision shall require the formation of a homeowners' association, which shall assume responsibility for all maintenance, upkeep, repair, replacement and management of the SCM. In other developments, the sub-divider or developer shall make provisions acceptable to the City for maintenance of the SCM area as stated in Section 1050.10. Blocks and/or easements shall be granted to the City for access to and maintenance of the stormwater management area.
B. If more than one development is to use a single SCM, a separate association of all members using that SCM shall be formed. This association will be held responsible for all future maintenance and repairs of the SCM as stated in this chapter.
C. The sub-divider or developer shall be exempt from the application of this section only if authorized by the City Engineer.
D. Where a sub-divider or developer is exempt from the provisions of this section, that sub-divider or developer shall comply with and pay fees in accordance with this chapter, governing subdivisions and development prior to the effective date of this section.
E. All SCMs will be placed within blocks and/or easements to allow City access (See Section 1050.11).
(10) 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 un-concentrated 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.
(11) Preservation of wetland hydrology. Concentrated stormwater runoff from SCMs to wetlands shall be converted to diffuse flow before the runoff enters wetland(s) in order to protect the natural hydrology, hydro period, and wetland flora. The flow shall be released such that no erosion occurs down slope. Practices such as level spreaders, vegetative buffers, infiltration basins, conservation of forest covers, and the preservation of intermittent streams, depressions, and drainage corridors may be used to maintain the wetland hydrology.
If the applicant proposes to discharge to natural wetlands, a hydrological analysis shall be performed to demonstrate that the proposed discharge matches the pre-development hydro periods and hydrodynamics.
(c) 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) Stream/storm sewer discharge. The stormwater facility (storm sewer main or natural watercourse) that will convey the discharge from the site shall be analyzed to determine if it is capable of conveying the additional storm sewer discharge from the site of a 100-year/24-hour storm. If the designated outlet is not capable of conveying the discharge from the site during the 100-year/24-hour storm, then additional storage must be placed onsite to store the additional volume for a period of thirty-six hours.
(2) Surface water protection. The City 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 1050.07
, and the activity is in compliance with Chapter 1052
- Erosion and Sediment Control and Chapter 1051
- Establishment of Riparian Zones all as determined by the City 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. Relocation of a major ditch needs to comply with Section 1050.151
with the approval of the City Engineer. Roadside ditch enclosure must be in compliance with Section 1050.151
. Relocation of minor ditches must show that all affected drainage flowing into that ditch has been accommodated as approved by the City Engineer and complies with Section 1050.153
.
(3) 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 City 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.
(4) 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. Flow shall be directed into an open channel, storm sewer, or other SCM from areas too long and/or too large to maintain sheet flow, all as determined by the City Engineer.
(5) Open channels. Unless otherwise allowed by the City Engineer, drainage tributary to SCMs shall be provided by an open channel with landscaped banks and designed to carry the 10-year, 24-hour stormwater runoff from upstream contributory areas.
(6) 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 City Engineer. The following criteria shall be used to design storm sewer systems when necessary:
A. Storm sewer design flow shall be based on the Rational Method. 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 or eighteen inches below proposed basement elevations if structures are not serviced by a sump pump, whichever is more restrictive during a 25-year, 24-hour storm. The system shall be designed to meet these requirements when conveying the flows from the contributing area within the proposed development and existing flows from offsite areas that are upstream from the development. These calculations will be reviewed and approved by the City Engineer prior to design acceptance. Rainfall data shall be obtained from the latest volume of the NOAA Rainfall ATLAS 14 or per Table 1. Runoff coefficients shall be per Table 2.
Table 1 - Rainfall per Storm Frequency | |
24-Hour Storm (year) | Rainfall (in.) |
Table 1 - Rainfall per Storm Frequency | |
24-Hour Storm (year) | Rainfall (in.) |
2 (50% storm) | 2.44 |
5 (20% storm) | 3.06 |
10 (10% storm) | 3.55 |
25 (4% storm) | 4.35 |
50 (2% storm) | 5.08 |
100 (1% storm) | 5.92 |
Table 2. Rational Method Runoff Coefficients (C) for City of Avon
| ||||
Runoff Coefficients for Hydrologic Soil Groups | ||||
Table 2. Rational Method Runoff Coefficients (C) for City of Avon
| ||||
Runoff Coefficients for Hydrologic Soil Groups | ||||
Cover Description | A | B | C | D |
Cultivated agricultural land | 0.17 | 0.3 | 0.43 | 0.50 |
Pasture or range land continuous grazing | 0.08 | 0.16 | 0.36 | 0.47 |
Meadow protected from grazing | 0.06 | 0.13 | 0.30 | 0.43 |
Woods | 0.05 | 0.10 | 0.29 | 0.41 |
Woods/grass combination (orchard, tree farm, etc.) | 0.07 | 0.14 | 0.33 | 0.45 |
Lawns, parks, golf courses, cemeteries, etc. | 0.08 | 0.16 | 0.36 | 0.47 |
Paved streets, parking lots, roofs, driveways, etc. | 0.96 | 0.96 | 0.96 | 0.96 |
Gravel areas | 0.40 | 0.59 | 0.69 | 0.74 |
Residential Areas | ||||
Average lot size & average % impervious area | ||||
1/8 acre or less 65 | 0.41 | 0.59 | 0.72 | 0.77 |
1/4 acre 38 | 0.16 | 0.37 | 0.54 | 0.64 |
1/3 acre 30 | 0.12 | 0.32 | 0.50 | 0.61 |
½ acre 25 | 0.09 | 0.29 | 0.47 | 0.59 |
1 acre 20 | 0.06 | 0.26 | 0.45 | 0.57 |
2 acres 12 | 0.05 | 0.23 | 0.41 | 0.50 |
Dirt or graded areas | 0.41 | 0.61 | 0.74 | 0.83 |
B. Rainfall intensity will be calculated using the equation i = a/(t+b)c Where:
i = Rainfall intensity (in./hour)
t = Time of concentration (minutes)
Refer to Ohio Department of Transportation's Location & Design Manual, Volume 2 (or latest edition) Drainage Design, Figure 1101-2 for Rainfall Intensity Constants (a, b & c).
C. The maximum slope allowable shall be a slope that produces no less than 2.5-fps and no more than a 10-fps velocity within the pipe barrel under design flow conditions.
D. In areas where foundation (footer) drains are connected to the storm sewer by gravity connections, the hydraulic grade line for the 25-year, 24-hour storm, shall be eighteen inches below the planned basement elevations. The planned basement elevations shall be clearly shown in the profile view of the storm sewer system. In areas where foundation (footer) drains are connected to the storm sewer via a pumping system, the hydraulic grade line for the 25-year, 24-hour storm shall be one foot below the highest elevation to which the foundation (footer) drain outflow is pumped prior to the outflow flowing by gravity to the receiving storm sewer. Identity sump pump invert at the wall in the plans.
E. The minimum inside diameter of pipe to be used in public storm sewer systems is twelve inches.
F. All stormwater conveyance 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. The following sources may be used to determine tailwater elevations:
1. Previous studies on file with the City.
2. FEMA Flood Insurance Rate Maps.
3. Calculations prepared by a registered professional engineer.
G. Catch basin design spread calculations shall be submitted to the City Engineer for review to determine catch basin spacing and sizing. At a minimum, there shall be at least one total clear lane during a 25-year, 24-hour storm.
H. 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. Exception: Any structure capturing runoff directly from a roadway shall have a minimum twelve inch sump to help reduce sediment from reaching the storm sewer pipe system.
I. Headwalls shall be required at all storm sewer inlets or outlets to and from open channels or lakes.
J. Outlets discharging into an open-water conveyance structure shall have an invert at a minimum of three inches above the average water depth during the snow-melt season.
K. The flood elevation for a 100-year, 24-hour storm must be a minimum of ten feet away horizontally from the perimeter of any homes within the new subdivision, nor cause any home flooding to adjacent neighboring properties, and shall be at least two feet below the finished grade elevation of any livable structure.
L. All storm sewer outlets from a subdivision must flow either into a public storm sewer, stream of the State, or a major ditch unless authorized by the City Engineer.
M. The maximum distance for sheet flow shall be 300 feet before entering a stormwater control measure. Except, that the maximum overland drainage area tributary to the storm structure shall be no greater than 1.5 acres.
(7) 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 for the minimum 100-year, 24-hour storm or as indicated by the City Engineer. The maximum allowed headwater for such a storm shall be twelve inches below pavement crown elevation. Water crossings carrying receiving waters located near upstream City borders shall convey no more flow than currently designed to carry unless directed by the City Engineer.
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, unless piers are situated outside of the ordinary high water mark stream channel.
C. If a culvert or other closed bottom crossing is used, twenty-five percent of the cross-sectional area or a minimum of one foot of box culverts and pipe arches must be embedded below the channel bed, with this cross-sectional area not calculated in the capacity of the crossing. The conduit or conveyance must to be sized to carry the 100-year storm under these conditions.
D. The minimum inside diameter of pipes to be used for crossings shall be twelve inches.
E. The maximum slope allowable shall be a slope that produces no more than 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 five 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 one-foot 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.
(8) 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 two feet 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.
(9) 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 per Section 1464.25
.
(10) 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.
(d) Stormwater Quality Control.
(1) Direct runoff to a SCM. The site shall be designed to direct runoff to one or more of the following SCMs. These practices are listed in Table 3 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 City Engineer may require a soil engineering report to be prepared for the site to demonstrate that any proposed infiltration facilities meet these performance standards.
C. For sites disturbing less than five acres, but greater than one acre and not part of a common plan of development or sale, where five or more acres are disturbed, the City Engineer may approve other SCMs if the applicant demonstrates to the City Engineer's satisfaction that these SCMs meet the objectives of this regulation.
D. For sites disturbing five or more acres, or less than five acres but part of a larger common plan of development or sale which will disturb five or more acres, the City Engineer may approve other SCMs if the applicant demonstrates to the City Engineer's satisfaction that these SCMs meet the objectives of this regulation as stated in division (d)(6) of this section, and has prior written approval from the Ohio EPA.
E. For sites with less than one acre of disturbed land and not part of a common plan of development, stormwater quality measures are not required to be implemented. The stormwater quantity controls in subsection (e) hereof shall apply.
F. For the construction of new roads and roadway improvement projects by public entities (i.e. the state, counties, townships, cities, or villages), the City Engineer may approve SCMs not included in Table 4 of this regulation, but must show compliance with the current version of the Ohio Departments of Transportations "Location and Design Manual, Volume Two Drainage Design".
(2) Criteria applying to all SCMs. Practices chosen must be sized to treat the water quality volume (WQv) and to ensure compliance with Ohio Water Quality Standards (OAC Chapter 3745-1). Water quality volume (WQv) orifices smaller than 3-inches in diameter shall incorporate design features that minimize the risk of clogging (e.g., reverse-flow pipe, downturned inlet elbow, perforated tee assembly buried in stone, etc.).
A. The WQv shall be equal to the volume of runoff from a 0.75 inch rainfall event and shall be determined according to one of the following methods:
1. Through a site hydrologic study approved by the City Engineer that uses continuous hydrologic simulation; site-specific hydrologic parameters, including impervious area, soil infiltration characteristics, slope, and surface routing characteristics; proposed SCMs controlling the amount and/or timing of runoff from the site; and local long-term hourly records, or
2. Using the following equation: WQv = C*P*A/12
where terms have the following meanings:
WQv = water quality volume in acre-feet
C = runoff coefficient appropriate for storms less than 1 in.
P = 0.75 inch precipitation depth
A = area draining into the, in acres.
Runoff coefficients required by the Ohio Environmental Protection Agency (Ohio EPA) for use in determining the water quality volume can be determined using the list in Table 3 or using the following equation to calculate the runoff coefficient:
C = 0.858i3-0.78i2+ 0.774i+0.04, where:
i = fraction of the drainage area that is impervious
Table 3: Runoff Coefficients Based on the Type of Land Use | |
Land Use | Runoff Coefficient |
Table 3: Runoff Coefficients Based on the Type of Land Use | |
Land Use | Runoff Coefficient |
Industrial & Commercial | 0.8 |
High Density Residential (>8 dwellings/acre) | 0.5 |
Medium Density Residential (4 to 8 dwellings/acre) | 0.4 |
Low Density Residential (<4 dwellings/acre) | 0.3 |
Open Space and Recreational Areas | 0.2 |
Where land use will be mixed, the runoff coefficient should be calculated using a weighted average. For example, if 60% of the contributing drainage area to the stormwater treatment structure is low density residential, 30% is high density residential, and ten percent is open space, the runoff coefficient is calculated as follows (0.6)(0.3)+(0.3)(0.5)+(0.1)(0.2) = (0.35) | |
B. An additional volume equal to twenty percent of the WQv shall be incorporated into the SCM for sediment storage. This volume shall be incorporated into the sections of SCMs where pollutants will accumulate.
C. Each individual SCM must be sized to treat the WQv associated with its entire contributing drainage area. Exceptions to this may be granted by the City Engineer and/or the OEPA on a case-by-case basis.
D. SCMs for quality shall be designed such that the drain time is long enough to provide treatment and protect against downstream bank erosion, but short enough to provide storage available for successive rainfall events as defined in Table 4.
E. All wet or dry extended stormwater detention facilities shall have a forebay and micropool as per the Rainwater and Land Development Manual.
Table 4: Draw Down Times for SCMs
| |
Stormwater Control Measure | Drain Time of WQv |
Infiltration Basin or Trench* | 24 - 48 hours |
Permeable Pavement - Extended Detention Permeable Pavement - Infiltration* | 24 hours 48 hours |
Extended Detention Facilities Dry Extended Detention Basins** Wet Extended Detention Basins + Pocket Wetland^ Constructed Wetlands (above permanent pool) # Bioretention Area/Cell*** Sand and other Media Filtration | 48 hours 48 hours 24 hours 24 hours 24 hours 24 hours |
*Practices designed to folly infiltrate the WQv shall empty within 48 hours to provide storage for subsequent storm events. ** The use of a forebay and micro pool is required on all dry extended detention basins. Each is to be sized at a minimum 10% of the WQv. +Provide both a permanent pool and an extended detention volume above the permanent pool, each sized with at least 0.75*WQv. ^Pocket wetland must have a wet pool equal to the WQv, with 25% of the WQv in a pool and 75% in marshes. The EDv above the permanent pool must be equal to the WQv. #Extended detention shall be provided for the full WQv above the permanent water pool. ***The surface ponding area shall completely empty within 24 hours so that there is no standing water. Shorter drawdown times are acceptable as long as design criteria in Rainwater and Land Development have been met. This includes grassed linear bioretention, which was previously titled enhanced water quality swale. | |
F. Each SCM shall be designed to facilitate sediment removal, vegetation management, debris control, and other maintenance activities defined in the inspection and maintenance agreement for the site.
G. The seasonal high groundwater elevation is the normal water line or normal elevation of saturation.
(3) Additional criteria applying to infiltration facilities.
A. Infiltration facilities shall only be allowed if the soils of the facility have an infiltration rate between 0.5 in./hr. and 4.0 in./hr. (or deemed acceptable by the City Engineer), if the seasonal high water table is at least three feet below the final grade elevation, and any underlying bedrock is at least six feet below the final grade elevation. Infiltration trenches should be designed to meet all criteria in the current edition of the Ohio Rainwater and Land Development Manual.
B. All runoff directed into an infiltration facility 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.
C. During construction, all runoff from disturbed areas of the site shall be diverted away from the proposed infiltration facility site. No construction equipment shall be allowed within the infiltration facility site to avoid soil compaction.
(4) Additional criteria applying to extended conveyance facilities.
A. Facilities designed according to the extended conveyance detention design drain time shall:
1. Not be located in areas where the depth to bedrock and/or seasonal high water table is less than three feet below the final grade elevation.
2. Only be allowed where the underlying soil consists of hydrologic soil group (HSG) A or B, unless the underlying soil is replaced by at least a 2.5 foot deep layer of soil amendment with a permeability equivalent to a HSG A or B soil and an under drain system is provided.
B. Concentrated runoff shall be converted to sheet flow, or a diffuse flow using a plunge pool, flow diffuser or level spreader, before entering an extended conveyance facility.
(5) 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 in less than one third of the drain time. If elevations allow, a valve shall be provided to drain any permanent pool volume for removal of accumulated sediments. 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.
2. The perimeter of all permanent pool areas deeper than four feet shall be surrounded by an aquatic bench that extends at least eight feet and no more than fifteen feet outward from the normal water edge. The eight feet wide portion of the aquatic bench closest to the shoreline shall have an average depth of six inches below the permanent pool to promote the growth of aquatic vegetation. The remainder of the aquatic bench shall be no more than fifteen 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 hardy native species of plants 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 each be equal to at least ten percent of the water quality volume (WQv).
(6) Owner: The owner may request approval from the City Engineer for the use of alternative structural post-construction SCMs if the owner shows to the satisfaction of the City Engineer that these SCMs are equivalent in pollutant removal and runoff flow/volume reduction effectiveness to those listed in Table 3. If the site disturbs more than five acres, or less than five acres but is part of a larger common plan of development or sale which will disturb five or more acres, prior approval from the Ohio EPA and City as necessary. To demonstrate the equivalency, the owner must show:
A. The alternative SCM has a minimum total suspended solid (TSS) removal efficiency of eighty percent, using the Level II Technology Acceptance Reciprocity Partnership (TARP) testing protocol.
B. The water quality volume discharge rate from the selected SCM is reduced to prevent stream bed erosion, 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 of the following conditions:
1. The entire water quality volume is recharged to groundwater.
2. The development will create less than one 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 100 percent 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 of fourth order or greater or to a lake, and where the development area is less than five percent of the water area upstream of the development site, unless a TMDL has identified water quality problems in the receiving surface water of the State.
(e) 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, or have a maximum allowable discharge of 0.25 cubic foot per second per acre of developed land.
(2) Developers or sub-dividers shall include in their preliminary plans a local watershed study to determine the impact from the development or subdivision caused by stormwater onto the lands adjoining or downstream from the area to be developed, to assure that said lands shall not be adversely affected by the proposed development or subdivision.
(3) 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 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.
(4) 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 City 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 numbers, time of concentration. Time of concentration shall also show the flow path and separation in flow type.
4. Rainfall data shall be obtained from the latest volume of the NOAA Rainfall ATLAS 14 or per Table 1.
5. Temporal Distribution - Use the SCS Type II rainfall distribution for all design events. Include lot coverage assumptions used for full build out of the proposed condition.
6. Curve numbers for the pre-development condition must reflect the average type of land use over the past ten years and not only the current land use. Curve numbers shall conform to the National Engineering Handbook Table 9-1.
a. Post-development curve numbers - All areas that are altered by construction practices shall use post-construction hydraulic soil groups from Rainwater and Land Development.
b. 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.
7. The volume reduction provided by permeable pavement, bioretention, or other 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 City Engineer.
8. 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.
B. From the volume determined in subsection (e)(4)A. hereof, determine the percent increase in volume of runoff due to development. Using the percentage, select the 24-hour critical storm from Table 4.
Table 5: 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: | |
Table 5: 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. | ||
C. For sites with less than one acre of disturbed land and not part of a common plan of development:
1. If the post-developed flow for the critical storm is less than 110 percent of the pre-developed critical storm flow, no stormwater quantity measures need to be implemented.
2. If the post-developed flow for the critical storm is equal to or greater than 110 percent of the pre-developed critical storm, stormwater quantity measures need to be implemented so that the post-developed critical storm flow is less than the pre-developed critical storm flow.
(f) Stormwater Management on Redevelopment Projects: Comprehensive Stormwater Management Plans for redevelopment projects shall reduce existing site impervious areas by at least twenty percent. A one-for-one credit towards the twenty percent net reduction of impervious area can be obtained through the use of pervious pavement and/or green roofs.
(1) Where site conditions prevent the reduction of impervious area, SCMs shall be implemented to provide to treat at least twenty percent of the WQv.
(2) When a combination of impervious area reduction and stormwater quality control facilities are used, ensure a twenty percent net reduction of the site impervious area, provide for treatment of at least twenty percent of the WQv, or a combination of the two.
(3) Where projects are a combination of new development and redevelopment, the total water quality volume required to be treated shall be calculated by a weighted average based on acreage, with the new development at 100 percent water quality volume and redevelopment at twenty percent.
(4) Where conditions prevent impervious area reduction or on-site stormwater management for redevelopment projects, practical alternatives as detailed in Section 1050.10 may be approved by the City Engineer.
(Ord. 133-16. Passed 12-27-16.)