Low impact development: San Francisco’s green approach to stormwater management

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Courtesy of Water Environment Federation (WEF)

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 nweather events. These findings are being used by the San Francisco Public Utilities Commission to help shape their LID programs and demonstration projects.

INTRODUCTION
San Francisco is one of only two cities in California with a combined sewer system. The system is the oldest on the west coast, with the majority of the sewers over 70 years old. While it meets current regulatory requirements, the aging infrastructure is occasionally overwhelmed by wet weather flows, resulting in street flooding and combined sewer overflows (CSOs). The San Francisco Public Utilities Commission (SFPUC) is currently developing a Sewer System Master Plan (SSMP), which will provide a 30-year roadmap for sewer system improvements. The SSMP aims to address many challenges, such as aging infrastructure, odors, and CSOs, while improving the sustainability of the sewer system and the quality of life for San Francisco residents.  As part of the SSMP, the SFPUC requested an analysis of low impact development (LID) to reduce wet weather flows into the combined sewer system.

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