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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 storm water treatment <br />structure is Low Density Residential, 30% is High Density Residential, and 10% is Open <br />Space, 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 />storm water practice for sediment storage. This volume shall be <br />incorporated into the sections of storm water practices where <br />pollutants will accumulate. <br />c. Storm water quality management practices shall be designed such that the <br />drain time is long enough to provide treatment and protect against <br />downstream bank erosion, but short enough to provide storage available for <br />successive rainfall events as defined in Table 2. <br />Table 2: Draw Down Times for Storm Water Management Practices <br />Best Management Practice Drain Time of WQv <br />Infiltration Facilities* 24 - 48 hours <br />Extended Conveyance Facilities (Vegetated Swales, Filter Strips) <br />^ Vegetated Filter Strip with Berm 24 hours <br />^ Enhanced Water Quality Swale 24 hours <br />^ Flow Through Design ** <br />Extended Detention Facilities <br />^ Extended Dry Detention Basins*** 48 hours <br />^ Wet Detention Basins + 24 hours <br />^ Pocket Wetland^ 24 hours <br />^ Constructed Wetlands (above permanent pool) 24 hours <br />^ Bioretention* 40 hours <br />^ Sand and other Media Filtration 40 hours <br />