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C <br />APPENDIX D <br />Appendix D is used for sizing infiltration facilities (TC 1) and extended detention basins (TC 5). The <br />sizing criteria was determined using the Storage, Treatment, Overflow, Runoff Model (STORM) as <br />developed by the U.S. Corp of Engineers. STORM was applied to long-term, hourly rainfall data at <br />fifteen (15) rain gauges across the State. Based on this analysis nine gauges were selected as being <br />representative for sizing detention storage facilities (see Figure DA). Zones were drawn to reflect <br />these nine gauges taking into consideration rainfall data and topography. The sizing curves should <br />apply to most areas within each zone. However, the zone boundaries are approximate and certain <br />areas within each zone will be strongly influenced by topographic features. <br />Each sizing curve was developed using the following parameters: <br />A drawdown time from the entire storage volume of 40 hours. This drawdown time allows <br />quiescent conditions to establish in the basin, resulting in at least 80% sediment removal for <br />most soils in California. <br />• <br />Initial abstraction/depression storage on impervious surface of 0.06 inch. <br />• <br />Runoff coefficient of 0.9 for impervious surfaces and 0.15 for pervious surfaces. <br />Appendix D may be used in the following manner: <br />1. <br />Identify the appropriate zone for your location from Figure DA and Tabl DA and then <br />select the appropriate detention storage figure for your zone. �41",��l <br />2. <br />Determine for the catchment area the percentage of impervious area directly connected to the <br />storm drain system. Directly connected impervious area (DCIA) is defined as the area <br />covered by pavement, building, and other impervious surfaces which drain directly into the <br />storm drain without first flowing across pervious areas (e.g. ,lawns). <br />3. <br />Choose a capture goal, and read the required unit volume required for the basin. In selecting <br />the capture goal consider the requirements of the local municipality or a point at the "knee of <br />the curve". Once the capture goal has been established draw a horizontal line from theCIZ, <br />capture goal to the appropriate DCIA curve, then a vertical line to the unit storage volume./V� <br />4. <br />t t Sil' z <br />Multiply the unit volume_ times the total acreage of the catchment basin and convert to acre- <br />feet or cubic feet. <br />Municipal Handbook D • 1 I V13/92 - D R A <br />