2008 005 Ordinance - Laserfiche WebLink

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C=0.85813 - 0.7812 + 0.7741+0.04, where: i =fraction of the drainage area that is impervious 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 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 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) b. An additional volume equal to 20% of the WQv shall be incorporated into the storm water practice for sediment storage. c. Storm water 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. Table 2: Draw Down Times for Storm Water Management Practices Best Management Practice Drain Time of WQv Infiltration Facilities 24 - 48 hours Extended Conveyance Facilities (Vegetated Swales, Filter Strips) 24 hours ^ Extended Conveyance Detention Design ^ Flow Through Design Extended Detention Facilities ^ Extended Dry Detention Basins 48 hours ^ Wet Detention Basins ** 24 hours ^ Constructed Wetlands (above permanent pool) 24 hours ^ Media Filtration, Bioretention 40 hours * Size to pass a hydrograph with a volume equal to the WQv, a duration of 2 hours, and peak rainfall intensity of 1 inch/hour at a depth of no more than 3 inches. The use of this criterion is limited to sites where the total area disturbed is 5 acres or less. **Provide both a permanent pool and an extended detention volume above the permanent pool, each sized with at least 0.75*WQv. d. Each practice shall be designed to facilitate sediment removal, vegetation management, debris control, and other maintenance activities defined in the Inspection and Maintenance Agreement for the site. 3. Additional criteria applying to infiltration facilities.