Town of San Anselmo, Friends of Corte Madera Creek Watershed, Ross Valley Sanitary District
San Anselmo Creek, Corte Madera Creek
Public or private land?
When funding is obtained
San Anselmo Creek at Sanders Avenue, San Anselmo, California
The project would replace a bridge, relocate an active sewer, and install a roughened channel to remove the highest priority barrier to steelhead passage in the Corte Madera Creek Watershed and accommodate the 100-year flow in the channel.
Ross Taylor & Associates assessed and prioritized barriers to fish passage in the Corte Madera Creek watershed in 2006. A culvert containing an Alaskan steeppass fish ladder at the Saunders Avenue rrossing of San Anselmo Creek, a tributary to Corte Madera Creek, is the highest priority for treatment. An active 24-inch sewer rests on the bed of the creek immediately upstream of the box culvert. Each of these features individually would cause San Anselmo Creek to overtop the bank of the creek by about 6 inches during the 100-year event. To address both the flooding and fish passage problems requires a new bridge and rerouting the sewer so it can be buried in the stream bed.
Drinking Water Supply
Water Quality Improvement
Habitat Protection and Restoration
Water Quality: When flooding occurs, water quality is harmed by the materials carried into the creek by the out-of-bank flows: excessive sediment, gardening supplies and chemicals, automotive care products, and other materials typically found in garages and garden sheds.
Habitat Protection and Restoration: The project would remove a barrier to passage for steelhead trout and other fishes in San Anselmo Creek. Invasive non-natives would be removed from the bank within the project area on the high school campus and native plants would be installed.
Flood Protection: Replacing the bridge and relocating the sewer would remove up to 19 homes plus a portion of the Sir Francis Drake High School Campus from the 100-year flood plain of San Anselmo Creek.
It removes several parcels from the 100-year flood plain.
The project is a multi-benefit project.
Part 2 - Detail
A full-span bridge will be constructed, a 24-inch sewer and the lateral connecting Drake High School to the sewer will be relocated, and a roughened channel will be installed under the new bridge. The roughened channel is necessary to avoid undercutting retaining walls that have been built by homeowners all along the right bank in the project area and on the left bank downstream of the bridge. The new configuration of the sewer and the bridge will provide capacity for the 100-year flow in San Anselmo Creek and meet NOAA Fisheries and California Department of Fish and Game criteria for passage of all age classes of steelhead trout (Oncorhynchus mykiss) that are known to occupy the watershed.
Ross Valley Watershed Program
Michael Love & Associates and Stetson Engineers. 2009.
Saunders Roughened Channel and Sewer Conceptual Design.
Fish Passage: In the 1980s an Alaskan Steeppass fish ladder was installed and a low-flow channel constructed to provide upstream passage for adult anadromous steelhead. However, the Steeppass and low-flow channel are poorly suited for providing adult steelhead passage at typical migration flows. At migration flows the hydraulic capacity of the Steeppass is overwhelmed, and there is inadequate attraction flow for fish to find the outlet. At lower flows there is inadequate depth in the low-flow channel for adult steelhead to swim through. The fish passage facility also lacks a pool at the transition from the low-flow channel into the Steeppass, making entry difficult. Additionally, an Alaskan Steeppass does not provide passage for juvenile salmonids and is highly susceptible to plugging by debris.
Replacing the fish ladder and associated concrete structures with a roughened channel will provide passage for all age classes of steelhead and coho salmon (Corte Madera Creek is critical habitat for both species, although coho salmon were extirpated from the watershed by the mid-1980s).
Flooding: An active 24-inch sewer that forms a low dam on the bed of the creek causes water to leave the creek channel during the 100-year event; when that happens, the bridge is adequate to convey the flow that remains in the channel. Hydraulic modeling shows that if the sewer were removed from the bed of the creek, then the bridge itself would cause flooding. When the water leaves the channel, it floods a portion of the campus of Sir Francis Drake High School, 139 Saunders Avenue, and 18 properties on Agatha Court. To avoid flooding during the 100-year event, the sewer must be relocated and the bridge replaced.
Flood risk will be unabated in the vicinity of the Saunders Avenue Bridge, leaving many properties in the 100-year flood plain.
Migrating steelhead will be unable to pass the barrier at Saunders Avenue substantially all the time.
i. Water Supply (conservation, recycled water, groundwater recharge, surface storage, etc.)
ii. Water Quality
The degradation of water quality caused by flooding will e reduced. This includes excessive erosion of upland areas that flood, addition of household and garden chemicals to the creek when garages, homes, and garden sheds flood, and sewage overflows that typically occur when flooding occurs and overwhelms sewers.
iii. Flood and Stormwater Management
The 100-year flow will be retained in the channel if the project is implemented. This would remove 20 properties from the 100-year flood plain.
iv. Resource Stewardship (watershed management, habitat protection and restoration, recreation, open space, etc.)
The project would remove a barrier to passage for steelhead trout and other fishes in San Anselmo Creek. Invasive non-natives would be removed from the bank within the project area on the high school campus and native plants would be installed.
Increases Water Supply Reliability
Advances/ Expands Conjunctive Management of Multiple Water Supply Sources
Increases Water Use and/or Reuse Efficiency
Provides Additional Water Supply
Promotes Water Quality Protection
Reduces Water Demand
Advances/Expands Water Recycling
Promotes Urban Runoff Reuse
Addresses Sea Level Rise
Addresses other Anticipated Climate Change Impact (e.g. through water management system modifications)
Improves Flood Control (e.g. through wetlands restoration, management, protection)