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

Flow-based control measure design standards apply to control measures whose primary mode ofpollutant removal depends on the rate of flow of runoff through the facility or device. Examplesof control measures in this category include swales, sand filters, diversion structures for off-linecontrol measures, and many proprietary products. Typically, flow-based design criteria calls forthe capture and infiltration or treatment of the flow runoff produced by rain events of a specifiedmagnitude. For the local area, the intensity of such a storm event is 0.20 inches/hour for theCities of Folsom and Roseville and 0.18 inches/hour for other cities in Sacramento County andunincorporated Sacramento County.This method satisfies the provisions of the Sacramento Areawide and City of Roseville NPDESMunicipal Stormwater Permits, which requires that flow-based measures be designed for at leastthe maximum (peak) flow rate of runoff produced by the 85 th percentile hourly precipitationintensity multiplied by a factor of two, referred to here as the flow-based 85 th percentile method.(CDM, 2003). This criterion is the same as the one prescribed by the 2003 California BMPHandbook. From Appendix D of that handbook, the 85 th percentile hourly precipitation intensityfor the Sacramento gage is approximately 0.09 inches/hour. Multiplying by two, the requiredintensity is at least 0.18 inches/hour. The factor of two specified for this method by the municipalstormwater permits appears to be provided as a factor of safety: therefore, caution should beexercised when applying additional factors of safety during the design process so that overdesign can be avoided. (CASQA, 2003).The flow-based BMP design criteria should be used in conjunction with the Rational Formula, asimplified, easy to apply formula that predicts flow rates based on rainfall intensity and drainagearea characteristics. The Rational Formula is as follows:whereWQF = flow in ft 3 /si = rain intensity in inches/hrA = drainage area in acresC = rational runoff coefficientWQF (cfs) = C i AThe Rational Formula is widely used for hydrologic calculations, but it does have a number oflimitations. For stormwater treatment control measure design, a key limitation is the ability of theRational Formula to predict runoff from undeveloped areas where runoff coefficients are highlyvariable with storm intensity and antecedent moisture conditions. This limitation is accentuatedwhen predicting runoff from frequent, small storms used in stormwater quality treatment designbecause many of the runoff coefficients in common use were developed for predicting runoff fordrainage design where larger, infrequent storms are of interest. Table 5-3 in the California BMPHandbook (May 2003) provides some general guidelines on use of the Rational Equation. Insummary, the Rational Formula, when used with commonly tabulated runoff coefficients inundeveloped drainage areas, will likely result in predictions higher than will be experiencedunder actual field conditions. However, given the simplicity of the equation, its use remainspractical and it is therefore the preferred method recommended by the local permitting agencies.The following steps describe the approach for application of the flow-based design criteria. Forsimplicity, the worksheets presented in Appendix D (see Step 3 of Appendix D-1 and D-2)already incorporate these steps.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007 Appendix E Page 2