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APPENDIX D <br />Appendix D is used for sizing infiltration facilities (TC 1) and extended detention basins (TC 5). The <br />sizing criteria was cletennined 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 D.l). 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 CW7/e was developed using U1e following parameters: <br />A c!rawdown time from the entire storage volume of 40 hours. Tilis 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 />Initial abstraction/depression storage on impervious surface of 0.06 inch. <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 />~ 2. <br />""3. <br />'--v <br />4. <br />Identify the appropriate zone for your location from Figure D.1 and Tabf D.1 and then <br />select the appropriate detention storage figure for your zone. \) ?1'.R-1 J <br />Determine for the catchment area the percentage of impervious area directly connected to the <br />storm drain system. Directly connected imp:rvious 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. )awns). ( ~ ') <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". On~~ cap~e goal has been established.dra~ a horizontal_ line from thez.:z~ , . A\ <br />capture goal to the appropnate DCIA curve, then a verucal !me to the urut storage volume. ( /J!(J; Jr;} <br />i,l ~V--~ \ @., OlA\) <br />Multiply the unit volume_ tiTJ}_es the total acreage of the catchment basin and convert to acre- <br />feet or cubic feet. <br />Municipal Handbook D · l 1ll13'92 · 0 R A F T