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Why Treat Urban Runoff?

Before San Francisco developed into the city it is today, it consisted of a diverse range of habitats including oak woodlands, native grasslands, riparian areas, wetlands, and sand dunes. Rainwater infiltrated into the soil, replenishing aquifers, lakes, and creeks. Today, impervious surfaces such as buildings, streets, and parking lots cover much of the City, preventing infiltration. Instead of percolating into the soil, runoff travels over these impervious surfaces, mobilizes pollutants like oil and debris, and washes them into the sewer system or into receiving water bodies like the San Francisco Bay, Pacific Ocean, and our local lakes and creeks. During large storm events, this runoff can contribute to localized flooding and combined sewer discharges. But these adverse effects can be reversed by design strategies.

 graphic example of creek daylighting project
A proposed creek daylighting project, a BMP that manages stormwater and provides both habitat and recreational space.

What is Low Impact Design (LID)?

Low Impact Design (LID) is a stormwater management approach that aims to mimic pre-development hydrologic processes by increasing infiltration and delaying and treating runoff. LID’s comprehensive, watershed-based approach emphasizes highly distributed source control and multi-functional design over conventional pipes and gutters. Decentralized, small-scale practices improve adaptability to changing environmental and economic conditions. For example, vegetated roofs intercept runoff from building rooftops. Downspouts redirect runoff to landscaped areas, rain barrels, or cisterns where the water can be stored and used during the dry season. Vegetated swales clean runoff from the street. LID has the potential to minimize combined sewer discharges and localized flooding events in San Francisco by intercepting stormwater before it reaches the sewer. At the same time, LID delivers many other benefits: greener neighborhoods, potable water offsets, and wildlife habitat. LID is the cornerstone of San Francisco’s stormwater program.

What are Stormwater Best Management Practices?

Best Management Practices (BMPs) are measures to reduce the amount of pollutants carried by stormwater, and in some cases retain and treat it before releasing it into receiving waters, groundwater, or into the sewer. BMPs can be behavioral in nature, ranging from programs to educate the public about pollution prevention in their households and workplaces, to municipal street sweeping programs. They can also be structural, such as vegetated roofs, rain gardens, and permeable pavement. The questions addressed in this FAQ concern structural BMPs, also known as Low Impact Design (LID).

Do Best Management Practices Clean Stormwater?

Yes! Properly designed, built, and maintained BMPs can effectively capture, store, and filter urban runoff before releasing it into receiving water bodies and sewers, or recharging groundwater. In fact, use of BMPs is the principal way in which the US Environmental Protection Agency (EPA) requires Municipalities to manage and treat stormwater. Proper site selection, design, and engineering will vary by site.

 graphic example of a complete street
A well designed “Complete Street” provides for all types of mobility, while providing shade and habitat. It beautifi es our neighborhoods and also captures and treats urban runoff.

In a dense city like San Francisco, is there enough space for Best Management Practices?

Yes! There are numerous types of BMPs that fit into the urban environment. They can be nestled along sidewalks, double as traditional landscaping, or be placed on rooftops. The increase in vegetation on our city streets also provides wildlife habitat and improves our own quality of life.

Who’s responsible for maintenance of Best Management Practices?

For private projects, it is the owner’s responsibility to conduct maintenance and to provide proof of maintenance (to either the SFPUC or the Port of San Francisco, depending on its location). For public projects, maintenance is the responsibility of the agency client. Projects may include community participation as well, depending on the nature of the site.

Do Best Management Practices accumulate pollutants?

Numerous studies demonstrate that stormwater BMPs can effectively retain and degrade sediment, oil and grease, and keep gross pollutants such as trash and debris out of receiving water bodies. In both vegetated and pervious pavement systems, soil microbes break pollutants down into non-harmful components.

Does infiltrating stormwater contaminate groundwater?

Designs must provide a minimum distance of three feet from the groundwater level to effectively remove pollutants. In areas with poor infiltration, BMPs can be lined and designed to capture and treat pollutants before releasing the water to a receiving water body or sewer.

Why harvest rainwater in a climate when it only rains in the winter when you don’t need the water for irrigation?

Rainwater harvesting has multiple benefits and is a valuable practice in any season. It not only protects the combined sewer, and Bay and Pacific Ocean, it can offset the demand on municipal drinking water where it is not needed. The SFPUC encourages the use of rainwater for toilet flushing and irrigation. Both can help save a substantial amount of water while protecting San Francisco’s watersheds. Recent analysis shows that a 5-story building that utilized both water conserving toilets and rainwater harvesting could meet more than 80% of its annual water demand for toilet flushing. Over time, heavy metals and persistent pollutants remain within the first few inches of soil, but do not migrate further into the ground. A standard maintenance routine includes periodic removal and replacement of the first few inches of soil to ensure the effectiveness of the system.

Will BMPs provide a breeding ground for mosquitoes?

Stormwater BMPs must be properly designed to absorb rainwater within 72 hours to avoid any ponding and standing water, thereby safely interrupting the breeding cycle of mosquitoes.

 bioretention planter in Mint Plaza
A bioretention planter in the Mint Plaza fits elegantly into this mixed use space in downtown San Francisco.
Can we replace the piped infrastructure with LID?

LID is a long-term and incremental approach for managing stormwater, so San Francisco plans to continue to manage and invest in the existing drainage system while implementing LID wherever it is appropriate and useful. LID will serve as a complement to the combined sewer. The City can work to keep as much stormwater out of the system as possible and then rely on its excellent treatment capabilities for the remaining runoff. LID will also assist the City in accommodating anticipated increases in storm events and droughts associated with climate change.

Why require BMPs when the City treats stormwater using a combined sewer system?

More than 90% of San Francisco is served by combined sewers, where both stormwater and sanitary flows collect into the same set of pipes and is pumped to wastewater treatment plants. These flows are treated to secondary standards, meaning that most organic material and bacteria are removed. However, during large storm events, when the system has reached its design capacity, combined sewer discharges (CSDs) can occur. During a CSD, partially treated sewage mixed with stormwater can discharge into the Bay and Pacific Ocean. Stormwater BMPs desynchronize these flows by either using or infiltrating stormwater, or temporarily detaining it before slowly releasing it to the system. Stormwater BMPs also reduce the total volume of stormwater running into the sewer system, thereby reducing the amount of power and chemicals needed to treat it. In addition, the SFPUC’s National Pollutant Discharge Elimination System (NPDES) Permit contains provisions for requiring stormwater management within the combined sewer because of the treatment that BMPs provide.

Can you still manage stormwater where you cannot infiltrate?

Yes! In places in the City with poor infiltration, or sites containing a high pollutant load, there are numerous BMPs that can either treat the stormwater, or simply detain it, before returning it to the sewer system. Many BMPs such as flow-through planters can be lined and equipped with perforated under drains. Some BMPs merely reduce the total amount of water that runs off the City’s surfaces by increasing vegetated areas on roofs, sidewalks, and parking lots. Rain harvesting collects water from rooftops into barrels or cisterns that store the water for later use.

Can you direct me to some good resources for comparing costs of BMPs?

For details about individual BMPs, including costs and other considerations, please look for

  • Our BMP Fact Sheets located on the Urban Watershed Management Program’s website,
  • EPA also provides fact sheets, case studies and other resources at
  • EPA’s Stormwater Program webpage,
Last updated: 3/13/2014 9:07:44 AM