Greening the grey with green infrastructure

San Francisco’s paved surfaces, including buildings, streets and parking lots, cover most of the city, preventing rainwater from infiltrating into the ground. The result is that stormwater runoff travels across these impervious surfaces picking up pollutants including oil, bacteria and debris before it flows into the city’s combined sewer system.

Usually, the combined stormwater and wastewater is transported to a wastewater treatment plant (WWTP) where it is treated to secondary treatment standards before being discharged into the bay or ocean. However, during heavy rainfall events the large influx of stormwater can exceed the capacity of the city’s WWTPs.

During these heavy rainfall events the sewer system treats the combined effluent to the equivalent of primary treatment standards before discharging it into the bay or ocean. In addition, the sudden influx of stormwater can lead to flooding in low-lying areas of the city.

Greening the grey in San Francisco

To reduce the quantity of stormwater entering the sewer system San Francisco is integrating green infrastructure into streetscape projects to reduce stormwater runoff as well as provide additional benefits including higher water quality, greening of neighborhoods and traffic calming. The green infrastructure installed in streets mimics the natural hydrologic processes of the landscape by increasing:

  • Retention: Capturing and holding water onsite through infiltration, rainwater harvesting or evapotranspiration from plants and soil
  • Detention: Collection and holding of runoff in temporary or vegetated systems before it is slowly released
  • Filtration: Removal of solid matter from water using porous materials including sand and soil. Filtration can also be used to remove contaminants (biofiltration)
  • Infiltration: Penetration of water into the soil from the ground surface
Greening the streets of San Francisco

Greening the streets of San Francisco

Mandating green infrastructure

To further reduce stormwater runoff, San Francisco requires all new and redevelopment projects that disturb 5,000 square feet or more of ground surface to manage stormwater that falls on their site with green infrastructure. As part of this requirement the city has developed The Stormwater Management Requirements and Design Guidelines that outlines the mandatory requirements for managing post-construction stormwater runoff and provides guidance on how to incorporate green infrastructure into site design. Activities approved by the Design Guidelines to create or replace impervious surfaces include rainwater harvesting, installing vegetated roofs, laying permeable pavement and developing bioretention swales, ponds and constructed wetlands. 

The take-out

To manage heavy storm events in a sustainable manner cities need to build green infrastructure on both public and private lands.

*Robert C. Brears is the author of Urban Water Security (Wiley). Urban Water Security argues that, with climate change and rapid urbanization, cities need to transition from supply-side to demand-side management to achieve urban water security.

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