Stormwater Quality Design Manual (May 2007) - City of Sacramento ...

4. Design the OutletThe outlet should be sized to release the 75 percent of the wet weather pool volume in aminimum of 24 hours and release the remaining volume over an additional 16 to 24 hours. Forthis example the outlet is assumed to be a stand pipe with the top of the pipe at the wetweather pool elevation and a single orifice placed just above the permanent pool elevation.The orifice can be sized using the orifice equation and a simple spreadsheet calculation suchas the one shown on Exhibit 1. As shown on Exhibit 1, an orifice with an area of 23.8 squareinches will produce the desired drain time. The size of the stand pipe is based on the capacityrequired to pass the appropriate design storms.5. Flood Control DesignWater quality basins can often be used to provide flood control benefits. The storage volumeabove the water quality pool can act to reduce peak flood flows downstream of the basin. Therequired storage volume and storm outlet size are typically determined by routing floodhydrographs through the detention basin using a computer program such as SacCalc, HEC-1,HEC-RAS, or other equivalent program. For this example, it is assumed that flood routingwas performed and 6.8 acre-feet of storage is required to attenuate flood flows to the desireddownstream level. To meet this storage requirement, the basin is widened to create a floodcontrol bench just above the maximum water quality pool as shown in the following crosssection.Bank Elev. 110’Flood Control Pool(6.8 ac-ft)Wet Weather Pool(2.5 ac-ft)Permanent Pool (3.2 ac-ft)Elev. 109’Elev. 106.5’Elev. 104’100’200’