Low Impact Development: San Francisco's Green Approach to Stormwater Management

This paper presents the methods, findings, and policy implications of a comprehensive evaluation of low impact development (LID) techniques for the City of San Francisco. LID is an innovative approach to stormwater management that relies on decentralized, small-scale stormwater facilities and site-design techniques to reduce wet weather flows and to remove stormwater pollutants. The LID analysis presented in this paper investigated the extent to which select LID techniques could be implemented in San Francisco, and the potential for those techniques to reduce citywide runoff volumes and peak flow rates. Using a GIS spatial analysis, the project team identified opportunities for retrofitting existing rooftops with ecoroofs and rainwater harvesting systems. The team also found opportunities for street tree planting, and for retrofitting streets, sidewalks, and parking lots with lined bioretention and permeable pavement facilities. The potential for infiltration systems was found to be severely limited due to high bedrock and groundwater, soil contamination, and unstable soils throughout much of San Francisco. Modeling of LID implementation scenarios showed quantifiable reductions in both the volume of stormwater entering the combined sewer system and the peak flows during wet weather events. These findings are being used by the San Francisco Public Utilities Commission to help shape their LID programs and demonstration projects.