Resilience Scanner

100-Year Storm Flood Risk Mapping Tool

Solution Overview

The San Francisco Public Utilities Commission (SFPUC) launched the 100-Year Storm Flood Risk Map in 2018 to inform property owners and tenants about stormwater flood risk so they can take actions to protect their property and belongings.[1] This informational tool identifies more than 4,000 properties that are vulnerable to storm runoff occurring during a 100-year storm—a term used to describe an extreme storm that has a 1 percent chance of happening during any given year.[5] The approach was developed to help property owners, developers, and City agencies understand and address the potential for flooding.[8]

Technical Components

The SFPUC created this mapping tool using computer modeling that simulates the citywide operation of the stormwater system during a 100-year storm to identify areas subject to flooding.[4][6][10] The sophisticated computer model simulates how rainfall flows through the City's sewer system and on its streets, with the model calibrated and validated using data from past storms to ensure its accuracy.[3] The modeling platform utilizes InfoWorks ICM software, which includes a two-dimensional (2D) module that generates a surface mesh using ground surface elevation data and can route overland flows by solving the surface flow transport equations for each mesh element in extreme storm events.[2] InfoWorks ICM also features a Real Time Control (RTC) module that allows programming different types of pump stations, gate and valve structures and their operational set points for different treatment pathways during varying storm conditions.[2] The modeling was based on historical storm data to simulate flooding scenarios.[1] The map shows the depth of potential flooding, ranging from 1 to 3 inches to over 2 feet.[3] The Flood Map depicts areas that experience flooding at least six inches deep during a 100-year storm.[1] The tool specifically shows parcels that are highly likely to experience deep and contiguous flooding during a 100-year storm, where deep and contiguous flooding means flooding that is at least 6 inches deep, spanning an area at least the size of half an average city block.[7] The Flood Map, updated in December 2024, now shows the flood elevations for these areas in San Francisco, by flood elevation zones.[7] Flood elevation refers to the height (rounded to the nearest foot) that stormwater flooding is likely to reach at a specific location during a 100-year storm, measured from the San Francisco City Datum, a consistent reference point for vertical measurements used by the design and development industry in San Francisco.[1]

Implementation Details

The preliminary flood risk maps presented by the SFPUC highlight areas prone to deep and contiguous flooding across multiple neighborhoods.[9] Areas with stormwater flooding risks include the Islais Creek area (Cayuga/Alemany), South of Market, Inner Mission, and Civic/Center Western Addition.[4] The SFPUC identified neighborhoods with the greatest flood risks and prioritized projects in those areas, with proposed upgrades to the flood-prone neighborhoods totaling $700 million.[5] The agency emphasized the importance of prioritizing projects based on flood risk, with a focus on high-risk areas marked in red on the maps, with the goal to eliminate these high-risk zones over time, although it was acknowledged that not all neighborhoods could receive immediate attention due to budget constraints and competing infrastructure needs.[9] The SFPUC is investing over $600 million to expand the capacity of the system in specific low-lying areas.[1] The agency is actively working on several capital projects, with over $700 million proposed for flood management initiatives, including ongoing construction in neighborhoods like Ingleside Terrace and Sunnyside.[9] To tackle these challenges, the SFPUC is planning to invest approximately $1.6 billion in capital projects over time to enhance the city's infrastructure to better manage stormwater and minimize flooding risks.[9] Following the map's launch, a 2019 ordinance requires sellers and landlords to disclose if a property lies in the Flood Map's risk zone.[1] San Francisco law requires sellers or landlords of property in San Francisco to disclose to buyers or tenants if the property is within the 100-year storm flood risk zone shown on the SFPUC's Flood Map.[7] Flood elevations are not required to be disclosed but are provided to help renters and owners take steps to protect their properties and belongings and minimize potential flood impacts.[6]

Benefits and Impacts

The 100-Year Storm Flood Risk Map serves as a valuable tool for residents, helping them understand their flood risk and take steps to protect their properties, contributing to a more flood-resilient city.[6] The mapping tool functions as a planning tool and does not affect flood insurance requirements.[8] The map shows flood risk from heavy rainfall that can temporarily overwhelm the capacity of the sewer system, also known as stormwater flooding.[3] The Flood Map shows flooding due to stormwater runoff, not other potential flooding sources, such as water coming in from San Francisco Bay or the Pacific Ocean.[1] This Draft Map shows flood risk from storm runoff only and does not consider flood risk in San Francisco from other causes such as inundation from the San Francisco Bay or Pacific Ocean.[4]

Climate Adaptation Relevance

San Francisco faces unique flooding challenges due to its topography and infrastructure configuration. The city has extremely steep hills, low valleys, and lots of flat low-lying areas with paved surfaces, which means rain has no place to go other than their combined sewer system before being discharged to the San Francisco Bay or the Pacific Ocean.[2] San Francisco is the only coastal city in California with a combined sewer system that collects and treats both wastewater and stormwater in the same network of pipes.[2] A 100-year storm is a very large storm that has a 1% chance of happening in any given year and would bring intense rainfall over a short period of time that could overwhelm the City's sewer system and cause flooding.[3] The SFPUC is using the latest climate science to understand how flood risk will change in the future and to plan for a more resilient San Francisco.[3]

Business Analysis

The SFPUC owns and operates close to 1,000 miles of sewer mains, 3 treatment facilities, 200 million gallons of storage, 26 pump stations and 36 combined sewer discharge outfalls.[2] The agency's capital investment strategy reflects a comprehensive approach to flood management, with proposed upgrades to flood-prone neighborhoods totaling $700 million and over $600 million specifically allocated to expand system capacity in low-lying areas.[5][1] The SFPUC is planning to invest approximately $1.6 billion in capital projects over time to enhance the city's infrastructure to better manage stormwater and minimize flooding risks.[9] The implementation includes ongoing construction in neighborhoods like Ingleside Terrace and Sunnyside as part of the over $700 million proposed for flood management initiatives.[9] The regulatory framework supporting this investment includes the 2019 ordinance requiring property disclosure, which creates market transparency around flood risk and incentivizes property-level adaptation measures.[1]

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