Glossary

Terms are alphabetical and numbers at at the end. Jump to the term "SAFRR."

BB2:

Building Bridge 2 is a concrete platform that stands in San Anselmo Creek in downtown San Anselmo. The bridge obstructs creek water flow, and its removal will reduce historic flood 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 threat to public safety. 

CEQA:

The California Environmental Quality Act is a state law enacted in 1970 that requires public agencies in California to identify and disclose the environmental impacts of their projects — like removing BB2 — and to avoid or mitigate those impacts if required. CEQA is the law that guided the District’s development of the EIR.  

CLOMR:

A conditional letter of map revision is a formal review by FEMA to determine whether a project meets National Flood Insurance Program standards for modifying floodplains or floodways. This process provides FEMA an opportunity to review analyses before implementation of a project that would change a floodplain. The bridge removal project requires a CLOMR to proceed. 

EIR:

An Environmental Impact Report is a detailed written evaluation of how a proposed project could affect the surrounding environment. In 2018, the Board of Supervisors certified the EIR for the SAFRR project. The EIR and its various addendums can be found on the documents page of the project website.  

FEMA:

The Federal Emergency Management Agency is a federal agency responsible for managing and overseeing national disaster preparedness, response and recovery, including the administration of the National Flood Insurance Program (NFIP). FEMA plays a crucial role in the SAFRR Project by providing regulatory oversight and manages Flood Insurance Rate Maps (FIRM), which guide floodplain management and insurance requirements.  

Hydraulic Models:

Engineers use these models to analyze how runoff translates into water depth and the speed that water travels.  

Hydrology Models:

Engineers use these models to analyze how rainfall turns into runoff.  

SAFRR:

The San Anselmo Flood Risk Reduction Project is a significant and complex effort to protect lives and property by decreasing flood risk during severe storms. SAFRR has three major elements:   

1. The flow diversion and storage (FDS) basin (constructed in 2022), where Sunnyside Nursery used to be located;  
2. The removal of buildings (completed in 2020) and the Building Bridge 2 (BB2) structure in downtown San Anselmo, along with creek bank improvements; and   
3. Flood mitigation of building structures downstream of BB2 affected by the bridge removal.  

1-D Model:

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. The hydraulic model used to develop the current FEMA Flood Insurance Rate Map 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.  

2-D Model:

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. 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.   

10-, 25- and 100-Year Storm Scenarios:

These scenarios are used to model how the creek would handle large amounts of rainfall and associated flooding. SAFRR project models simulate storms that occur, on average, once every 10-, 25- and 100-years. 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.