Frequently Asked Questions (FAQ)
Overview
The Marin County Flood Control & Water Conservation District (District) has been working since 2016 on the San Anselmo Flood Risk Reduction Project (SAFRR), a significant and complex effort to protect lives and property by decreasing flood risk during severe storms. A key part of the project is removal of a bridge, known as Building Bridge 2 (BB2), in downtown San Anselmo. BB2 obstructs creek water flow, and its removal will reduce historic flooding risk for hundreds of properties in downtown San Anselmo and the Town of Ross. In addition to posing a flood hazard, BB2 is structurally unsound and is a risk to public safety. BB2 removal is part of the District's ongoing efforts with regional partners and property owners to implement strategic flood control improvements.
Given the project’s complexity, which includes FEMA mapping requirements and the need for close coordination with local jurisdictions and stakeholders, bridge removal is tentatively set for 2026.
For definitions of key terms and acronyms referenced throughout this FAQ, please refer to the project glossary.
General Project Questions
Q: What is SAFRR and how is the BB2 project part of it?
A: SAFRR has three major elements to protect lives and property during severe storms:
- The flow diversion and storage (FDS) basin where Sunnyside Nursery used to be located (constructed in 2022);
- The removal of buildings (completed in 2020) and the Building Bridge 2 structure in downtown San Anselmo, along with creek bank improvements; and
- Flood mitigation of building structures downstream of BB2 that are affected by its removal.
The removal of BB2 is an important part of SAFRR. The bridge obstructs creek water flow, and its removal will reduce historic flooding risk for hundreds of properties in downtown San Anselmo and the Town of Ross. In addition to posing a flood hazard, the bridge is structurally unsound and a risk to public safety.
Q: Will the removal of BB2 enhance public safety in downtown San Anselmo?
A: Yes. District modeling shows a decrease in flood damage to homes and businesses in downtown San Anselmo. The bridge removal will allow more stormwater runoff to remain in the creek; without the bridge removal, this stormwater runoff would overflow into the neighborhood. The bridge removal also complements the Town of San Anselmo’s Reimagined Creekside Park Project, which will be implemented after project completion.
Q: What impact will BB2 removal have on bank stability/erosion?
A: Impacts on long-term bank stability and erosion would not be significant, as evaluated in the Environmental Impact Report. Removing the bridge will result in more water remaining in the creek during extreme storms (e.g., 25-year, 100-year storm events) rather than spilling out onto streets. With more water in the creek, there will be a slight increase in the amount of water moving through the creek. Increases in erosion would occur only at a few locations and for brief periods during large and infrequent flood events. In-stream stabilization improvements have been designed to control excess creek erosion where the bridge will be removed.
Q: When will BB2 be removed?
A: Removal of the bridge is planned for the summer of 2026. The buildings on top of BB2 were demolished in 2020.
Q: What progress has been made on the SAFRR project?
A: The District has made significant progress on the SAFRR project since 2016. This includes: the removal of buildings from the bridge (completed in 2020); the construction of the Sunnyside FDS Basin (completed in 2023); as well as modeling, design, environmental permitting, and habitat mitigation for both BB2 and the FDS Basin. BB2 removal is a complex process involving close, ongoing collaboration among federal, state and local partners. A critical next step for the project is FEMA approval to move forward with the bridge removal.
Q: How will the bridge removal impact San Anselmo during storm season?
A: BB2 removal will reduce flood damage to hundreds of homes and businesses during extreme storms. As it stands, the bridge obstructs the creek’s natural flow. Removing this obstruction will decrease flood risk by helping keep water within the channel during these storms.
Q: How will this project affect the current plaza area, and are there plans for a new community space?
A: The plaza is not currently accessible to the public, as BB2 is structurally unsound. Once the bridge is removed, the Town of San Anselmo plans to construct a reimagined public park in the area, which will provide a new community gathering space. More information about the plaza project is available from the Town of San Anselmo.
Q: Why were the buildings on top of the bridge removed?
A: Building removal was a necessary first step to remove the bridge, which is structurally unsound and poses a risk to public safety.
Q: What is FEMA’s role in this project?
A: FEMA plays a crucial role in the SAFRR Project by providing regulatory oversight. FEMA manages Flood Insurance Rate Maps, which guide floodplain management and insurance requirements. The bridge removal project requires a Conditional Letter of Map Revision (CLOMR) from FEMA, which is a formal review to determine whether the project meets National Flood Insurance Program (NFIP) standards for modifying floodplains or floodways. This process provides FEMA an opportunity to review analyses before a project that would change a floodplain is implemented.
Q: How much will this project cost?
A: The bridge removal’s current estimated cost is approximately $11 million, mostly funded by grants, general fund dollars and fees. It is important to note that BB2 removal is estimated to save the region over $23 million over the designed life of the project (50 years) by decreasing flood risk to hundreds of homes and businesses along the creek. The District will continue to evaluate costs and savings as the project progresses.
Q: How does the Town of San Anselmo’s decision to leave the Marin County Flood Control and Water Conservation District impact this project?
A: Passage of Measure F in San Anselmo removes the Town of San Anselmo from Zone 9 of the flood district; however, property owners will continue to pay a flood fee until 2027, when it is scheduled to sunset. The measure is not anticipated to affect the project timeline or budget.
Q: What are 10-, 25- and 100-year storm scenarios?
A: The District employs models to simulate various storm scenarios. SAFRR project models comprise hydrology (analyzing rainfall into runoff) and hydraulic (analyzing runoff into water depth and speed) simulations for storm events that occur, on average, once every 10-, 25- and 100-years. These scenarios look at how the creek would handle large amounts of rainfall and associated flooding. Another way to look at this is these events have a 10%, 4%, and 1% chance of occurring in any given year, respectively. While the 10-year storm scenario is less severe than the 100-year storm, it occurs 10 times more often.
Q: What are 1-D and 2-D models? Why is the District using two different hydraulic models?
A: A one-dimensional, or 1-D, hydraulic model simulates water flow depth and velocity (speed) along a linear creek path (upstream to downstream). This model has been used traditionally by engineers as an industry standard.
A two-dimensional (2-D) hydraulic model simulates flow depth and velocity over an area, taking into account flow in multiple directions. This model is state-of-the-art for simulating overland flow on a floodplain and has been increasingly used by engineers in recent years.
The District is using a combined 1-D/2-D model to evaluate environmental impacts and benefits of the SAFRR project as part of the Environmental Impact Report in alignment with the California Environmental Quality Act (CEQA). The 1-D component is being used to model creek hydraulics, and the 2-D component is used to model creek overflow on the floodplain. The District uses the 1-D/2-D model as a planning tool to evaluate improvements to reduce flood risk in the Ross Valley watershed and has utilized the dynamic model for the SAFRR project.
The hydraulic model used to develop the current FEMA Flood Insurance Rate Map (FIRM) for San Anselmo Creek is a 1-D model. FEMA prefers the use of the same 1-D model to evaluate changes to mapped floodways and floodplains. This provides more of an apples-to-apples comparison to past modeling and mapping results. Using a 1-D model is simpler and is recognized as standard practice for FEMA’s review of the CLOMR application.
Q: What is the best way to find up-to-date information about the bridge removal?
A: Up-to-date, complete information is available on the BB2 webpage.
Property Related Questions
Q: What are FEMA’s requirements around flood mitigation measures and changes in Base Flood Elevation as they relate to individual properties?
A: As part of the District’s Conditional Letter of Map Revision (CLOMR) application, FEMA requires an assessment of the following:
- Rise: Which properties experience a modeled rise in the Base Flood Elevation (i.e., 100-year storm event);
- Impact: The impact, if any, of this rise on building structures; and
- Mitigation: The appropriate mitigation, if any, to alleviate that impact.
Further, the CLOMR application requires hydraulic modeling files; proposed changes to the floodway and floodplain map; surveyed building elevations, e.g., First Finished Floor (FFF), Lowest Adjacent Grade (LAG), in relation to the modeled rise in Base Flood Elevation; and other supporting documentation.
Q: How is the District informing involved property owners about project impacts?
A: The District engages with property owners through online and in-person communications to address comments and questions and to enhance project transparency. If there is a modeled rise in Base Flood Elevation to a property due to bridge removal, the District will inform the property owner(s) before submitting its CLOMR application to FEMA. These notifications are anticipated to happen in the second quarter of 2025. After FEMA deems the CLOMR application submittal complete, the District will notify applicable property owners of impacts, as defined by FEMA, to building structures and necessary mitigation. This is currently anticipated by the end of 2025. Please note that timelines may change due to the complex nature of the project, including coordination with multiple local, state and federal entities. For more information about how the District is communicating with property owners, see the related news post.
Q: Are some properties downstream from BB2 going to be placed at higher risk of flooding when the bridge is removed?
A: No. It is important to understand that the modeled rise in the creek water surface is localized, limited and only occurs during large and rare storm events (e.g., 100-year). The modeled water surface rise would not occur for relatively moderate storm events (e.g., 10-year). These smaller storms are the biggest contributors to flood risk of homes and businesses since they occur much more often than larger and rarer storm events (e.g., 100-year).
Modeled rise of the creek water surface does not necessarily equate to direct structural impact. For the 100-year event, much of the flooding of structures along the creek is caused by overflow from the street side, not from the creek. The modeled water surface on the street side, which is higher than that on the creek side, drops in all modeled flood events with the bridge removed.
The benefits of removing the bridge far outweigh the impacts. Additionally, any modeled rise in flooding, which results in new impacts to structures, would be mitigated by the District, per FEMA and EIR requirements.
As included in the EIR, several other flow-constraining bridges in San Anselmo, Fairfax, and Ross will likely be removed and replaced by the respective towns. These projects, which are separate from the SAFRR project, would allow for greater flow volumes to pass downstream into Corte Madera Creek (formed at the confluence of San Anselmo Creek and Ross Creek), where the channel has more capacity. One of these bridges is the Winship Bridge, and its replacement would remove barriers to flow and reduce channel constriction along the reach of San Anselmo Creek upstream to Barber Avenue, where new inundation would occur from the removal of BB2 during the 25-year event. According to the District’s 1-D/2-D model, if the Winship Bridge is removed, then the Project would not cause an increase in flood inundation levels to homes related to BB2 removal.
Q: What are the criteria for determining whether properties require mitigation? Have there been discussions or decisions about providing mitigation for these properties?
A: The criteria for determining whether properties require flood mitigation are based on FEMA regulations and guidance (44 CFR 60.3 and 44 CFR 65.12), as well as the SAFRR project EIR (Mitigation Measure 4.9-4).
Yes, the District has had discussions about mitigation internally and with FEMA. It is important to note that a rise in the modeled water surface on a property does not directly translate to an impact to a building structure. Also, the impact to a structure does not directly translate to a need for mitigation. The District does not anticipate that mitigation will involve the elevation of building structures. However, it is anticipated that some mitigation will be necessary for a limited number of properties. For basements, crawl spaces and garages, this may include elevating utilities or other equipment off the floor and flood venting so that flood waters have a path to drain out of non-living spaces. For pier-supported structures in the creek, this may include structural reinforcement of the pier.
Q: Where can I find a list of properties affected by this project?
A: The District is gathering field survey data and performing the technical analysis necessary to identify the following associated with bridge removal:
- Properties with a modeled rise in Base Flood Elevation;
- Affected building structures; and
- Appropriate mitigation.
A comprehensive list of affected properties will be compiled once surveying and analysis are complete. The list will be included in the CLOMR application and available to the public after the CLOMR is finalized.
Q: If I want to talk to someone about the impact on my property or neighborhood, who can I reach out to?
A: Please send an email to FloodInquiry@marincounty.gov.
As of April 2025, District staff are hosting drop-in office hours in San Anselmo or Ross to give residents an additional opportunity to connect directly with the project team. Specific dates and locations can be found in the related news post.
In addition to the responses to the above frequently asked questions, we have also received additional inquiries that are more technical and specific in nature. Responses to these questions can be found in the Technical/specific Q&A.