2021-093 Ordinance - Laserfiche WebLink

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EXHIBIT A: CHAPTER 928 (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 stormwater practice, in acres. Runoff coefficients required by the Ohio Environmental Protection Agency (Ohio EPA) for use in determining the WQv can be determined using the list in Table 1 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 1: Runoff Coefficients Based on the TVDe 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 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) b. An additional volume equal to 20% of the WQv shall be incorporated into the stormwater practice for sediment storage. This volume shall be incorporated into the sections of stormwater practices 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. Stormwater quality management practices 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 2. e. Sites within watersheds of coldwater habitat streams shall include SCMs