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STORMWATER MANAGEMENT MANUAL <br />extended detention basins, retention ponds, wetlands, and other practices <br />depended on a design water quality capture volume. See UDFCD Urban Storm <br />Drainage Criteria Manual ( USDCM) Volume 3 Best Management Practices for <br />calculation details. <br />1604.2 Water Quality Capture Volume <br />All structural BMPs recommended in this Manual are based on the 80th <br />percentile storm event. Specific guidance for finding the needed WQCV for each <br />BMPs types' design section can be found in the USDCM Volume 3 chapter on <br />Structural BMP. This WQCV varies with the type of BMPs used and is based on <br />the time it takes to fully drain the brim -full WQCV, which varies from 6 -hour to 40- <br />hours, depending on the BMPs. WQCV can be determined using the following <br />relationships: <br />WQCV = K {a(0.91 *1 - 1.19*1 + 0.78 *1)} <br />Where: <br />WQCV is in watershed inches <br />K = Adjustment to equation for Mesa County area = ds/0.43 <br />a = Adjustment for BMPs drain time <br />BMP Drain Value of <br />Time (hrs) "a" <br />6 0.7 <br />12 0.8 <br />24 0.9 <br />40 1.0 <br />I = watershed impervious as a decimal <br />ds = Depth of average runoff producing storm <br />The average runoff producing storm value, d = 0.28 inches, was obtained from <br />Driscoll, et al. 1989. Analysis of Storm Event Characteristics for Selected <br />Rainfall Gages Throughout the United States. The value 0.43 is for the Denver <br />Metropolitan area, for which the original WQCV equation was derived ( UDFCD <br />1999). Therefore, K = 0.28/0.43 = 0.65, which indicates that the Mesa County <br />area requires approximately 35% less WQCV than along the Front Range. <br />Determine the required storage volume in acre -feet as follows <br />Required storage = [WQCV /12]* Area <br />Where: <br />Required storage = Required storage volume, in acre -feet <br />Area = Tributary watershed area, in acres <br />Required storage volume shall be based on 120% of calculated WQCV to allow <br />for sediment accumulation. <br />The local jurisdictions recognize that some sites cannot be graded in a <br />reasonable manner so that the entire site drains to the detention basin. If at least <br />95% of the site drains to the detention pond, then local jurisdiction may accept <br />DECEMBER 31, 2007 SECTION 1600 - POST - CONSTRUCTION ��- 1606 <br />STORMWATER MANAGEMENT <br />