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

Using the Effective Area Managed in Calculating Treatment RequirementAfter each Runoff Reduction measure has been addressed on the subforms, forms D-2a and D-2b, the A cis totaled. This managed area is subtracted from the Area Requiring Treatment (A T) found in Step 1. TheAdjusted Area Requiring Treatment (A AT ) is also used to find an Adjusted Impervious Fraction (I A ).After the A T , A AT , and I A have been calculated, the water quality flow and/or volume must be calculatedfor sizing treatment controls. Whether the designer needs to calculate flow-based treatment or volumebasedtreatment depends on the type of treatment planned.Treatment flow (WQF) is found using the standard flow equation,WQF = C i Awhere C is the rational runoff coefficient for A AT (assumed to be 0.95), i is the rainfall intensity, and A isthe adjusted area requiring treatment (A AT ). This value is to be used when determining sizing criteria forstructural treatment controls.Treatment Volume (WQV) is found using either the CASQA method (Roseville):V = A x SV / 12where A = the total area of the drainage shed,SV = SV = the Unit Basin Storage Volume; use C A adjusted for credits earned (seeAdjusted Runoff Coefficient, below).Or the ASCE-WEF method (Sacramento):WQV (ac-ft) = P 0 * A / 12where A = the total area of the drainage shed,P 0 = maximized detention volume using the ASCE-WEF method.Please refer to chapter 6 and Appendix E for the selection of the treatment measures and designrequirements. Then use form D-1f, Treatment – Volume Based (CASQA) for volume-based treatmentcontrols within the City of Roseville, or form D-1g, Treatment – Volume Based, for treatment controls inareas outside of the City of Roseville.Adjusted Runoff CoefficientThe Adjusted Impervious Fraction is converted to an Adjusted Runoff Coefficient, C A , using the empiricalregression equation presented in the California Stormwater BMP Handbook (CASQA, 2003).C = 0.858 I 3 – 0.78 I 2 + 0.774 I + 0.04where,C = runoff coefficientI = impervious fractionThe results of this equation for values of “I” between 0 and 1 are listed in Table D-2d of Appendix D-2.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007 Appendix D-4 Page 11