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

Appendix EDesign Requirements for Stormwater Quality Treatment Control Measures(Volume and Flow-Based Measures)The Sacramento Areawide and City of Roseville NPDES Municipal Stormwater Permits containprovisions that require the local municipal agencies to establish and enforce stormwater qualitytreatment standards for many new and redevelopment projects. This appendix presents theminimum standards for sizing the treatment control measures.General MethodologyFollow these steps:• Refer to Table 3-2 in this Design Manual to determine if your project requires treatmentcontrol measures.• Once you have made that determination, use the fact sheets in Chapter 6 to identify thetype of control measures most appropriate for your project site and whether or not thosemeasures are volume or flow-based.• (Optional) Use Chapter 5 and the runoff reduction worksheets in Appendix D to selectrunoff reduction measures for your project which will reduce the runoff discharged; thismay result in reduced treatment needs (and associated costs) for your project.• Use the rest of the information in this appendix to size the treatment control facilities foryour project.The local Sacramento agencies have developed a presumptive approach, whereby, if projectapplicants follow the methodology presented herein, it is presumed that the project is reducingstormwater pollution in runoff to the “maximum extent practicable” NPDES municipal permitstandard. A key principle here is that treatment control measures are most efficient andeconomical when they target small, frequent storm events that over time produce more totalrunoff than the larger, infrequent storms conventionally targeted for design of flood controlfacilities. Further, studies in other areas of the country have shown that much of the pollutantload is contained in the “first flush” of rainfall during a storm event, typically the first 0.5-inches.Targeting design storms larger than this may result in some improvements in pollutant removaleffectiveness, but at considerable cost.It is important to note that arbitrarily targeting large, infrequent storm events can actually reducethe pollutant removal capabilities of some treatment control measures. This occurs when outletstructures, detention times, and drain down times are designed to accommodate unusually largevolumes and high flows. When over-designed in this way, the more frequent, small storms thatproduce the most annual runoff and a large part of the pollutant load pass quickly through theover-sized facility and therefore receive inadequate treatment. (CASQA, 2003).Sizing Flow-Based Treatment Control MeasuresUse this method for sizing flow-based control measures (e.g., vegetated swales) in bothSacramento County and the City of Roseville.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007 Appendix E Page 1