Marin County to advance San Anselmo bridge removal project

Required flood mitigation will not prevent a plan to remove an aging bridge in downtown San Anselmo over the summer, Marin County officials said.

The removal of the structure, called “Building Bridge 2,” is a crucial part of a project that will reduce flooding risk for an estimated 400 Ross Valley homes while increasing it for a much smaller number of residences.

The other major component of the $26 million project is a 33-acre detention basin at the former Sunnyside Nursery growing grounds, just west of Fairfax, which was completed in 2022 but has not been put to use.

“We’ll be going out to bid for construction in February,” said Rosemarie Gaglione, the county’s director of public works. “We can’t get into the creek until June 15, but there’s mobilization and work-up above the bridge that can be done before that.”

The work needs to be completed before December 2024, the expiration of an $8.7 million state Department of Water Resources grant that is providing part of the funding. The project also will use $16.5 million in revenue generated by a stormwater drainage fee that is paid by Ross Valley residents.

The project has been on hold while public works officials assessed the number of parcels that will have an increase in flood risk because of the work.

An environmental impact report completed in 2018 estimated that 20 parcels would see added inundated area or an increase in flood water depth because of the project during 25- and 100-year storms.

Gaglione said that any additional flooding of homes caused by the project must be mitigated before demolition can proceed. She said the estimated number of parcels expected to see a rise in water level has increased since the EIR was completed.

That is because at the time the EIR was written, it was expected that Winship Bridge in Ross would be replaced before Building Bridge 2 was removed. The 94-year-old Winship Bridge impedes flow in the creek because of a center pier, which supports the bridge, and tight waterways.

“What we have are 49 parcels downstream that will see a slight rise in water levels on their parcel,” Gaglione said.

She said, however, that the increased water level for 29 of those parcels won’t affect structures.

“If you have a rise in the water elevation but it just gets the dirt wet and it doesn’t come anywhere near the house, there really is nothing to mitigate,” Gaglione said.

Gaglione said homes on five parcels will have some increase in water beneath the first floor or in the basement. She said some electrical and mechanical equipment under these homes might have to be relocated, but her department believes no homes or structures will need to be elevated.

Gaglione said additional homes will have higher water levels during flood events due to the project, but since these homes were already flooding before, the flood district isn’t required to provide mitigation.

The Federal Emergency Management Agency prohibits flood projects that result in any rise in base flood elevations. Gaglione, however, said that rule doesn’t apply to this project because FEMA defines a project as installing new structures.

“In this case,” she said, “we’re removing an obstruction.”

John Crane, who is concerned that his home just downstream of Winship Bridge in Ross will be affected, said, “I think their interpretation of FEMA is convenient but not accurate per the regulation.”

The public comment period on an addendum to the project’s 2018 environmental impact report closed on Dec. 15, but no date has been set for the Flood Zone 9 advisory board to consider approving the document.

The addendum was necessary because of minor changes to the project: the replacement of a retaining wall along San Anselmo Avenue, which will facilitate the construction of San Anselmo’s Creekside Park project, and the addition of abutments for a new pedestrian bridge.

The addendum also analyzed the option of installing a temporary concrete baffle after removing Building Bridge 2. When the initial EIR was done, public works officials still weren’t sure how much mitigation the project would require.

In the event that extensive mitigation was needed, the baffle would have allowed the department to remove the bridge without a lengthy delay that would have imperiled use of the $8.7 million state grant.

Now, Gaglione said, “We don’t believe the baffle is necessary.”

Ross officials disagree.

In a comment on the addendum, Ross Town Manager Christa Johnson wrote, “The town continues to have growing concerns about the district’s ability to mitigate the potential increase in water surface elevation on private properties located along the federally regulated floodways in Ross.”

The 2018 EIR stated that the project would cause increased flooding in a limited area around the Winship Bridge in Ross, between Barber Avenue and the Sir Francis Drake Boulevard Bridge. Gaglione said that a number of the parcels expected to see higher water levels due to the removal of Building Bridge 2 are in Ross, but said she couldn’t provide an estimate of how many.

Johnson wrote that the baffle qualifies as a mitigation measure and therefore “must be a primary project component of the San Anselmo Flood Risk Reduction project until the Winship Bridge ‘foreseeable’ project and all necessary mitigation downstream of the project is completed.”

Gaglione said that when Winship Bridge is removed, the number of parcels seeing increased flooding due to removal of Building Bridge 2 is likely to increase. However, she said the flood district doesn’t have to factor in the Winship Bridge removal because there is no date certain when the California Department of Transportation project will be completed.

Osha Meserve, a lawyer hired by Flood Safe Marin, a group of Ross and San Anselmo residents, asserted in a comment on the addendum that the document underreports the number of parcels that will have increased water rise because of the project.

Meserve wrote that information she obtained through a Public Records Act request to the county indicates that “approximately 71 parcels would be directly impacted by the project, with approximately 35 buildings intersecting that new elevation.”

Meserve said it also appears that the addendum used a different “roughness coefficient” in its modeling, producing results that show fewer parcels seeing increased flooding. The roughness calculation is based on the amount of vegetation and other materials in the creek that slow the flow and force water over the creek bank.

“It looks like the county wanted to get to an answer of less flooding,” Meserve said.

Gaglione wrote in an email that she suspects that the map Meserve is referencing might be a draft created when the public works department still lacked important FEMA data. She said her department’s engineers are evaluating Flood Safe Marin’s critique regarding the roughness coefficient.

Supervisor Katie Rice said, “I think people misunderstood the issue of a rise of water on a parcel versus a rise in the water that is actually going to affect a residential structure.”