San Francisco Stormwater Design Guidelines
San Francisco’s Stormwater Infrastructure While the creation of these Guidelines is driven primarily by regulatory requirements for the City’s separate sewer areas, the majority of San Francisco (90%) is served by a combined sewer system (see Figure 6). The stormwater management goals for areas served by separate storm sewers are different from those for areas served by the combined sewer system. Despite this, many of the fundamental design concepts for stormwater management apply to both areas, and as such, the Guidelines can be used as a tool in both the separate and combined sewer areas of San Francisco. Using landscape-based stormwater infrastructure will enhance and diversify the functions of both the separate and combined systems. Approximately 10% of the City is served by a separate storm sewer system or is lacking stormwater infrastructure; in most of these areas stormwater flows directly to receiving waters without treatment. In the separate storm sewer areas, the primary reason for implementing post-construction controls is to improve stormwater quality before it reaches a receiving water body. These controls are aimed at removing specific pollutants of concern and treating what is known as the “first flush”. The first flush is the dirtiest runoff, usually generated during the beginning of a rain event; it mobilizes the majority of the pollutants and debris that have accumulated on impervious surfaces since the last rain. A combined sewer system conveys wastewater and stormwater in the same set of pipes. The combined flows receive treatment at wastewater treatment plants before being discharged to the Bay and Ocean. Conventional separate storm sewer systems provide no stormwater treatment, while combined sewer systems treat most urban runoff to secondary standards, including the first flush and most additional stormwater runoff. However, when the capacity of the system is exceeded by large storm events, localized flooding and combined sewer discharges (CSDs) can occur. In the event of a CSD, the system discharges a mixture of partially treated sanitary effluent and stormwater to receiving water bodies. While these discharges are dilute (typically consisting of roughly six percent sewage and 94 percent stormwater), they can cause public health concerns and lead to beach or Bay access closures. The primary reason for implementing LID measures in a combined sewer system is to reduce and delay the volumes and peak flows of stormwater reaching the sewer system. Volume reductions and peak flow desynchronization can help reduce the number of CSDs, reduce flooding, and protect water quality. Post-construction controls in the combined system can also improve the capacity and efficiency of the City’s treatment facilities.
Figure 6. Combined sewer systems (top) serve 90% of San Francisco. Separate sewer systems (bottom) serve 10%. Image: modified from King County Wastewater Management Division San Francisco Context
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