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

5. Enter the capture curve selected in Step 3 on the vertical axis at the “Capture, % Runoff”value identified in Step 4. Move horizontally to the right across capture curve until thecurve corresponding to the drainage area’s composite runoff coefficient “C” determinedin Step 2 is intercepted. Interpolation between curves may be necessary. Move verticallydown from this point until the horizontal axis is intercepted. Read the “Unit BasinStorage Volume” along the horizontal axis. If a local requirement for capture of runoff isnot specified, enter the vertical axis at the “knee of the curve” for the curve representingcomposite runoff coefficient “C.” The “knee of the curve” is typically in the range of 75to 85% capture.6. Calculate the required capture volume of the control measure by multiplying the drainageshed from Step 1 by the “Unit Basin Storage Volume” from Step 5 to give the designvolume. Due to the mixed units that result (e.g., ac-in., ac-ft) it is recommended that theresulting volume be converted to cubic feet for use during design.ReferencesCalifornia Stormwater Quality Association (CASQA), 2003. California Stormwater BMPHandbook for New Development and Redevelopment, www.cabmphandbooks.com.Development Standards Plan, Dec. 2003. Appendix F: Technical Memorandum- Review ofDesign Criteria for Stormwater Quality Treatment Facilities for the Sacramento StormwaterManagement Program.Placer Regional Stormwater Coordination Group (PRSCG). May 2005. Guidance Document forVolume and Flow-Based Sizing of Permanent Post-Construction Best Management Practices forStormwater Quality Protection.US Department of Transportation, Federal Highway Administration. November 1996. UrbanDrainage Design Manual, Hydraulic Engineering Circular No. 22, FHWA-SA-96-078.Water Environment Federation and American Society of Civil Engineers (WEF and ASCE).1998. Urban Runoff Quality Management. WEF Manual of Practice No. 23 and ASCE Manualand Report on Engineering Practice No. 87.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007 Appendix E Page 6