The Storm Water Control Plan will also contain the following information depending on the size of the development sites as well as any additional information, required by the City of Middleburg Heights engineer.
(a) General Requirements: All Storm Water Control Plans must contain the following information pertaining to post-construction water quality control:
(1) A description of the post-construction BMP(s) that will be installed during the 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.
(2) Detail drawings must be provided for all post-construction BMP(s).
(b) Development Sites Smaller than Five Acres: A development site that will disturb one (1) or more, but less than five (5) acres of land and is not a part of a larger common plan of development or sale which will disturb five or more acres of land shall identify:
(1) A description of measures that will be installed during the construction process to control pollutants in storm water discharges that will occur after construction operations have been completed.
(2) Structural measures should be placed on upland soils to the degree attainable.
(3) The Storm Water Control Plan shall include an explanation of the technical basis used to select the practices to control pollution where flows exceed predevelopment levels.
(4) Velocity dissipation devices shall be placed at discharge locations and along the length of any outfall channel to provide non-erosive flow velocity from the structure to water course so that the natural physical and biological characteristics and functions are maintained and protected (e.g., no significant changes in the Hydrological regime of the receiving water).
(c) Development Sites 5 Acres or Larger: A development site that disturbs five (5) or more acres of land or will disturb less than five (5) acres, but is a part of a larger common plan of development or sale, which will disturb five (5) or more acres of land shall identify:
(1) Post Construction Water Quality Storage: The Post-Construction BMP(s) chosen must be able to detain storm water runoff for protection of the stream channels, stream erosion control, and improved water quality.
(2) Structural BMPs: Structural (designed) Post-Construction storm water treatment practices shall be incorporated into the permanent drainage system for the site.
(3) 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 0.75-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 the 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 storm water 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 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 and the City Engineer.
(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 Practice | 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 pool) | 24 hours |
Media Filtration, Bioretention | 40 hours |
*Provide both a permanent pool and water quality volume above the permanent pool, each sized at 0.75 * WQv
(7) The owner may request approval from the City of Middleburg Heights Engineer to use alternative structural Post-Construction BMPs if the alternative structural post-construction BMPs have been approved by Ohio EPA pursuant to the currently effective Ohio EPA NPDES Construction Activity General Permit or an individual NPDES Permit issued by Ohio EPA.
(8) 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 that 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.
(9) For redevelopment projects (i.e., developments on previously developed property), Post-Construction practices shall either ensure a 20 percent net reduction of the site impervious area, provide for treatment of at least 20 percent of the WQv, or a combination of the two.
(Ord. 2008-18. Passed 2-26-08.)