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Cl-IAGFit~ <br />,--- EAR~,TEtk~ t? E [3 <br />PtS.-:TIVE6~S <br />May 2016 <br />(1) Through a site hydrologic study approved by the Village Engineer that <br />uses continuous hydrologic simulation; site-specific hydrologic <br />parameters, including impervious area, soil infiltration characteristics, <br />slope, and surface routing characteristics; proposed SCMs controlling the <br />amount and/or timing of runoff from the site; and local long-term hourly <br />records, or <br />(2) Using the following equation: <br />WQv <br />where terms r <br />WQv <br />C <br />P <br />A <br />= C*P*A/12 <br />ave the following meanings: <br />= water quality volume in acre-feet <br />=runoff coefficient appropriate for storms less than 1 in. <br />= 0.75 inch precipitation depth <br />= area draining into the stormwater practice, in acres. <br />Runoff coefficients required by the Ohio Environmental Protection <br />Agency (Ohio EPA) for use in determining the WQv can be determined <br />using the list in Table 1 or using the following equation to calculate the <br />runoff coefficient: <br />C=0.85813-0.7812+ 0.7741+0.04, where: <br />i =fraction of the drainage area that is impervious <br />Table 1: Runoff Coefficients Based on the Type of Land Use <br />Land Use Runoff Coefficient <br />Industrial & Commercial 0.8 <br />High Density Residential (>8 dwellings/acre) 0.5 <br />Medium Density Residential (4 to 8 dwellings/acre) 0.4 <br />Low Density Residential (<4 dwellings/acre) 0.3 <br />Open Space and Recreational Areas 0.2 <br />Where land use will be mixed, the runoff coefficient should be calculated using a weighted <br />average. For example, if 60% of the contributing drainage area to the stormwater treatment <br />structure is Low Density Residential, 30% is High Density Residential, and ] 0% is Open <br />S ace, the runoff coefficient is calculated as follows (0.6)(0.3)+(0.3)(0.5)+(0.1)(0.2) _ (0.35) <br />b. An additional volume equal to 20% of the WQv shall be incorporated into the <br />stormwater practice for sediment storage. This volume shall be incorporated into <br />the sections of stormwater practices where pollutants will accumulate. <br />Each individual SCM must be sized to treat the WQv associated with its entire <br />contributing drainage area. Exceptions to this may be granted by the Village <br />Engineer and/or the OEPA on a case-by-case basis. <br />d. Stormwater quality management practices shall be designed such that the drain <br />time is long enough to provide treatment and protect against downstream bank <br />erosion, but short enough to provide storage available for successive rainfall <br />21 <br />