Every few years during the rainy season, certain neighborhoods of San Francisco flood.

If it rains too hard or for too long, the sewer pipes fill, pushing water (and raw sewage) out of floor drains and sewer grates.

Residents and businesses near 17th and Folsom streets in the Mission spend every winter anxiously preparing for floods. They line the sidewalks with city-issued plastic barriers and improvise their own designs. The damage is so costly, at that intersection and elsewhere, that sometimes businesses shutter.

Why do San Francisco’s sewers do this?

Illustration showing a car near a flooded street, a water burst, and a diagram explaining how San Francisco’s sewage flowed downhill to the bay in the city’s early infrastructure.
A worker in a raincoat opens a street drain as water flows in; nearby, another worker connects a pipe as water sprays out. Text explains how sewage systems changed cities.
A map shows San Francisco’s lost marshes, rivers, and current flood risk zones, highlighting changes from landfill and the impact of earthquakes and flooding.
A diagram shows how city creeks and natural watersheds reduce flooding, while drained creeks and stormwater pipes in urban, uphill areas cause water to flood lower neighborhoods.
A map of San Francisco showing three major sewage treatment areas: North Point, Oceanside, and Southeast, with labeled facilities and major sewer lines.
A woman reacts with concern as floodwater enters her home, covering floors and rising up the stairs. Text explains the dangers and consequences of sewage flooding homes.
Illustration showing sewage management in San Francisco with concrete transport/storage boxes, pumps, and overflow pipe leading into the bay, explained through diagrams and captions.
A diagram shows how stormwater and sewage mix in sewer boxes during heavy rainfall, causing untreated sewage to flow out with stormwater instead of settling.
Cartoon showing sewage flowing from a pipe into a waterway. Text highlights issues with cleaning underground sewage boxes in San Francisco.
Illustration showing sandbag distribution, residents managing floodwater, and information on city flood prevention efforts and funding in San Francisco during rainstorms.
Storm gates with sandbags block a garage door; workers in hard hats work in a street with pipes; captions discuss flood prevention and infrastructure upgrades.
A hand holds a newspaper with the headline "Green Infrastructure" and an illustration of a street with rainwater. Text discusses solutions to urban flooding in San Francisco.
A flowchart shows how SFMTA, SFPUC, and SF Public Works must collaborate for infrastructure projects, with an example highlighting challenges in adding speed bumps to a city street.
A person collects trash in a flooded urban area, while two people sit at a "Flood Fund" booth under heavy rain. Background text discusses city sewage and flooding issues.

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Ronna Raz is an illustrator and intern with Mission Local.

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6 Comments

  1. Super impressed by this piece, thank you Ronna! Great job creating a technically accurate simplification of a complicated and historic wastewater system that most folks don’t think about in their daily routine.

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  2. Lovely article and art!!! I feel like I know way more about our sewers now. Would love to see more informative comics like this one!

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  4. Amazing article and graphics! Thank you for explaining our historic/aging sewer system and its limitations. Its a great argument for permeable, green spaces.

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  5. Westside Water Resources is working to disconnect our roof rainwater from the sewer system to allow it to naturally filter back into the ground. This project, the Westside Basin Aquifer Recharge, will 1) restore the San Francisco peninsula’s Westside Basin Aquifer, 2) demonstrate the feasibility and environmental benefits of restoring the groundwater basins through distributed aquifer recharge, plus 3) relieve a stormwater/sewer system that the SFPUC has admitted cannot handle the volume of the projected rainfall.

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  6. Very good. As an aside it amazes me how many people choose to rent or (worse) buy homes in that area of 17th Street knowing, as they surely do or should do, that they have a fair to good chance of finding themselves under two feet of water every winter.

    I guess those homes are cheaper, perhaps?

    The further up a hill you are, the less risk you face. My excess rainfall passes through underground pipes I installed into the yard of the guy beneath me on my hill. No doubt he passes it on to the guy downhill from him. And so on all the way down to Folsom and 17th, where there is no more downhill.

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