Post-Construction Standards
(a) Where permanent stormwater control facilities, such as retention basins, and/or other permanent BMPs are necessary to minimize the discharge of sediment into water resources and to minimize erosion of the water resources, the Owners/Operators shall include post- construction BMPs in their permit applications in compliance with the following requirements:
(1) Descriptions of BMPs which are appropriate for each construction operation and the rationale for their selection. The rationale must address the anticipated impacts on the channel and floodplain morphology, hydrology, and water quality;
(2) BMPs shall meet the standards and specifications in the current edition of the Ohio Rainwater and Land Development manual;
(3) BMPs shall preserve the existing natural conditions to the maximum extent practicable;
(4) Structural measures shall be placed on upland soils to the degree attainable; and
(5) BMPs may include the following structural and/or non-structural measures:
A. Permanent stormwater detention ponds that provide extended detention of the water volume;
B. Flow attenuation by use of open vegetated swales and natural depressions;
C. Onsite infiltration of runoff;
D. Sequential systems that combine several practices;
E. Permanent conservation easements, preferably with the easement being held by a third party with no vested interest in ever seeing the property developed;
F. Re-creating floodplains;
G. Allowing roof water from buildings to run across lawn areas to remove pollutants;
H. Onsite sewage disposals system replacement or conversion to sanitary sewers;
I. Low Impact Development Design; and/or
J. Aquatic benches in retention basins and ponds.
(b) Plans for Development Sites Smaller than Five Acres: This subsection applies to any projects for which subsection (e) hereof requires SWM/SC plans and for which post- construction BMPs are required, except for projects with earth-disturbing activities on five or more acres of land and except for projects that are part of a larger common plan of development or sale that will have earth-disturbing activities on five or more acres of land. The plan for any project subject to this subsection shall contain the following information:
(1) Stormwater Issues: A statement as to how the decreased stormwater quality that will be caused by the planned development project will be handled;
(2) Description of Measures: A description of the BMPs that will be installed during the construction process to control pollutants in stormwater discharges that will occur after construction operations have been completed;
(3) Map: A map of the entire site showing the overall development;
(4) Riparian and/or Wetland Setback: All riparian and wetland setback areas shall be identified on the plan. They shall also be marked in the field prior to the start of construction; and
(5) Technical Basis: The plan shall describe the rationale for selecting the BMPs used to control pollution and to maintain and protect water quality.
(c) Development Sites Five Acres or Larger: The SWM/SC plan for any development site required to adopt post-construction BMPs that will have earth-disturbing activities on five or more acres of land, or that will have such activities on less than five acres but is a part of a larger common plan of development or sale that will have such activities on five or more acres of land, shall contain the following information:
(1) Stormwater Detention: The post-construction BMP(s) chosen must be able to detain stormwater runoff for protection of the stream channels, stream erosion control, and improved water quality;
(2) Structural BMPs: Structural (designed) post-construction stormwater treatment practices shall be incorporated into the permanent drainage system for the site. Non-structural BMPs may also be used where appropriate;
(3) Properly Sized BMPs: The BMP(s) chosen must be sized to treat the water 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 0.75-inch rainfall and shall be determined according to one of the two following methods:
A. Through a site hydrologic study approved by the local municipal permitting authority that uses continuous hydrologic simulation and local long-term hourly precipitation records; or
B. Using the following equation:
WQv = C * P * A / 12 where: WQv = water quality volume in acre-feet C = runoff coefficient appropriate for storms less than 1 inch (see Table 1) P = 0.75 inch precipitation depth A = area draining into the BMP in acres.
Table 1: 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 |
(4) Where land use will be mixed, the runoff coefficient shall 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 10% is Open Space, the runoff coefficient is calculated as follows (0.6)(0.3) + (0.3)(0.5) + (0.1)(0.2) = 0.35;
(5) 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 shall be designed according to the methodology included in the Ohio Rainwater and Land Development manual, or the ODOT Post-Construction stormwater standards;
(6) 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 as described in Table 2 below:
Table 2: Target Draw Down (Drain) Times for Structural Post-Construction Treatment Control Practices
Best Management Practices | Drain Time of WQv |
Infiltration | 24 - 48 hours |
Vegetated Swale and Filter Strip | 24 hours |
Extended Detention Basin (Dry Basins) | 48 hours |
Retention Basins (Wet Basins)* | 24 hours |
Constructed Wetlands (above Permanent Pools) | 24 hours |
Media Filtration, Bioretention | 40 hours |
*Provide both a permanent pool and an extended detention volume above the permanent pool, each sized at 0.75 * WQv.
(7) The Owner/Operator may request approval from the Stormwater Manager to use alternative structural post-construction BMPs if the Owner/Operator can demonstrate that the alternative BMPs are equivalent in effectiveness to those listed in Table 2 above. The use of alternative or vendor supplied post-construction BMPs shall be limited to redevelopment projects where the Owner/Operator demonstrates that the traditional BMPs in Table 2 are technically and economically infeasible;
(8) A development is not required to install a structural BMP where the Owner/Operator demonstrates the WQv that would be provided by that BMP is instead being provided within an existing structural post- construction BMP as part of a larger common plan of development or sale or if the structural post-construction BMPs for this development are addressed in a regional or local stormwater management plan;
(9) For redevelopment projects (i.e., developments on previously developed property), post-construction practices shall ensure a 20 percent net reduction of the site’s impervious area, provide for treatment of at least 20 percent of the WQv, or achieve a combination of the two;
(10) Site description:
A. The prior land uses of the site;
B. The nature and type of construction activity (e.g., low density residential, shopping mall, highway, etc.);
C. Total area of the site and the total area of the site that is expected to be affected by earth-disturbing activities, as that term is defined in the definitions section of these regulations;
D. Amount of the impervious area and percent imperviousness created by the construction activity; and
E. Name and/or location of the immediate receiving stream or surface water(s), the first subsequent named receiving water, and the major river watersheds in which it is located;
(11) A vicinity sketch locating:
A. The development area;
B. The larger common plan of development or sale; and
C. All pertinent surrounding natural features within 200 feet of the development site including, but not limited to:
1. Water resources such as wetlands, springs, lakes, ponds, rivers and streams (including intermittent streams with a defined bed and bank);
2. Conservation easements; and
3. Other sensitive natural resources and areas receiving runoff from the development;
(12) The existing and proposed topography shown in the appropriate contour intervals;
(13) The location and description of existing and proposed drainage patterns and facilities, including any allied drainage facilities beyond the development area and the larger common plan of development or sale;
(14) Existing and proposed watershed boundary lines, direction of flow, and watershed acreage; and
(15) The location of any existing or planned riparian and/or wetland setback areas on the property.
(Ord. 07-40. Passed 7-16-07.)