The Marin County Flood Control & Water Conservation District has been working for the past several years on the San Anselmo Flood Risk Reduction Project (SAFRR), which is a significant effort to protect lives and property during severe storms. A key part of the project is the removal of a bridge known as Building Bridge 2 (BB2) in downtown San Anselmo. BB2 obstructs creek water flow, and its removal will lower the Base Flood Elevation (BFE) for approximately 500 homes and reduce the flooding risk in downtown San Anselmo. The District has worked diligently with its regional partners and affected property owners to make strategic flood control improvements.
In addition to reducing flood risk, BB2 is being removed as it’s been deemed structurally unsound and a threat to public safety. It will remain inaccessible to public access until it is removed.
Due to required mapping revisions from FEMA and the need to conduct additional community outreach, the bridge removal will not take place in 2024 as previously expected, but now is tentatively scheduled to be removed in 2025-2026.
General Project Questions
Q: What exactly is SAFRR and the BB2 project?
A: BB2 removal is a significant component of the SAFRR. The bridge’s foundations sit in the creek channel and obstruct water flow. This is a serious concern – especially during storm – and removal of BB2 will reduce the base flood elevation to approximately 500 parcels. These components lower creek flows and reduce flood risk by:
- Using a flood diversion and water storage basin where Sunnyside Nursery used to be located
- Increasing capacity by changing the shape of the creek channel
- Removing BB2 and its supports that obstruct water flow
In other words, the construction of the basin will divert water during peak hour storms and the removal of BB2 and modifications to creek banks will allow more water to stay in the channel instead of overtopping onto streets. This project is an important part of the District’s commitment to public safety.
Q: Will the removal of BB2 help downtown San Anselmo?
A: Computer modeling shows a reduction in BFE on San Anselmo Avenue and more water will remain in the channel which reduces flood risk. San Anselmo's creek upgrades and the removal of BB2 are important components to the Town's Creekside Park Project.
Q: What impact will the removal of BB2 have on bank stability/erosion?
A: The slight increases in maximum flow velocities and potential increases in scour and erosion that could arise from BB2 removal and the other project elements would occur only for brief periods in large and infrequent flood events, and in only a few locations. This impact would be less than significant.
Q: When will BB2 be removed?
A: The existing buildings on top of BB2 bridge were demolished in 2020, and BB2 will be removed following required mapping revisions from FEMA and the need to conduct additional community outreach. It could occur in 2025-2026.
Q: Why has this project taken so long to complete?
A: A key reason for the delay in removing BB2 is the requirement to make less severe any impacts associated with an increase in BFE caused by the bridge removal. Mitigations, if any, must occur prior to the demolition of BB2. Work during much of 2023 has been the modeling of stream flows to reflect the changes from the 2018 certified Environmental Impact Report (EIR) model, and the District spent late 2023/early 2024 meeting with property owners to discuss the specific impact on each parcel that shows an increase in BFE. In January 2024, the County and District determined that flood map revisions must occur, resulting in further delays.
Q: How will the removal of this bridge impact San Anselmo during storm season?
A: Removal of BB2 is an important component of SAFRR that reduces BFE for approximately 500 property owners. Removal of BB2 will reduce flood risk by helping to keep water within the channel during storms since it obstructs the natural flow within the channel. More water will remain in the creek instead of being diverted onto San Anselmo Avenue.
Q: How will this project affect the current plaza area and are there plans for a new community space?
A: BB2 is structurally compromised, obstructs the natural flow of water within the channel, and increases flood risk to upstream properties and in downtown San Anselmo. Removing BB2 reduces flood risk, and once BB2 is removed, the Town of San Anselmo will restore the area as a park. More information about the plaza project is available from the Town of San Anselmo.
Q: Why were the existing buildings on top of the bridge removed?
A: Removing them helped improve San Anselmo’s Community Rating System eligibility, resulting in potentially lower flood insurance rates for property owners in the floodplain. The benefits of removing the remaining bridge platform are immense and will greatly reduce the community’s flood risk.
Q: What is FEMA’s role in this project?
A: FEMA issues Flood Insurance Rate Maps for flood prone communities based on best available data. Historically, FEMA used a 1-Dimensional Hydraulic Model; the District has developed a more sophisticated 2-Dimensional Model that more accurately depicts the community’s flood risks. A key difference is that our modeling takes into account the impact of the complex topography of the flood plain outside the channel, which is a major factor missing from FEMA’S existing model. Local jurisdictions are obligated to share information with FEMA that better reflects actual risk than FEMA’s maps. In early 2024, the bridge removal was postponed in order to conduct a map revision process.
Property Related Questions
Q: What are FEMA’s requirements around flood mitigation measures and changes in BFE as they relate to individual properties?
A: Typically, FEMA defers to the local jurisdiction regarding the appropriate mitigation measures.
Q: How is the District informing affected property owners about project impacts?
A: District officials have met with individual property owners and a Community Open House was held in January 2024. Media stories, website content and social media posts have also been employed.
Q: Aren’t some properties downstream from BB2 going to be placed at higher risk of flooding when the bridge is removed?
A: All properties currently in FEMA Flood Insurance Rate Maps Special Flood Hazard Areas are at risk of flooding. Mitigation measures depend on where a specific property’s Base Flood Elevation (BFE) is after removal of BB2 in relation to Finished First Floor Elevations (FFE); the increase in BFE after removal determines what mitigation measures, if any, are required. The latest, most advanced model show that BB2 removal will reduce flood risk for 502 parcels, and that the impacts of the removal on downstream properties will be less than 3.9 inches of rise in the Base Flood Elevation (BFE) (100 year-storm Water Surface Elevation) with more than three quarters of the impacted parcels seeing less than 2 inches of rise in the BFE and more than half will see less than a 1 inch increase in the BFE. After completing surveys and obtaining the finished first floor elevations of impacted structures, we concluded that no structures will need to be elevated and only a very few will require adjustments to electrical or other equipment in basements/crawlspaces. The District will cover the cost of those required mitigations
Q: How is the District determining which properties will receive financial assistance to mitigate changes caused by the project?
A: When the Base Flood Elevation (BFE) increase occurs below FFE, there may be electrical and/or mechanical equipment under structures (such as crawlspaces) that might need to be raised. After meeting with the subset of property owners who meet that criteria, we will make arrangements with them to survey under the structure to determine whether there is indeed electrical and/or mechanical equipment that must be raised, and we will work closely with them through the process. The removal of BB2 will not require the District to elevate any homes or other structures.
Q: If mitigation is required for a specific home, how long will it take to make the changes and who will pay for them?
A: Depending on what needs to be done, such as moving electrical and/or mechanical equipment above BFE, work can often be done within a week. The District’s model show that no home or structure will need to be elevated. The District is financially responsible for the cost of any required mitigation as long as the property owner agrees with the proposed mitigation.
Q: What is the best way to find up-to-date project information?
A: The District is committed to keeping the public up-to-date, and full information is available on the project website.
Q: If I want to talk to someone about the impact on my property or neighborhood, who can I talk to?
A: Send email to FloodInquiry@marincounty.org or call 415-473-7101.