Transportation Authority of Marin and Arup unveil new study addressing impacts of sea level rise on Marin County’s transportation network

News

The Arup-developed report outlines a county-wide approach for protecting critical infrastructure in one of California’s most climate-vulnerable areas

Cars driving on flooded highway in Marin County

Author

Kathryn Hohs

Americas Media Officer

Los Angeles

Author Contact Information

americaspress@arup.com

+1 310 578 4400

Last updated: 16th July 2025

Arup recently delivered an adaptation study for the Transportation Authority of Marin (TAM) to help safeguard Marin County’s transportation network against the impacts of coastal flooding and sea level rise. Accepted at the TAM Board of Commissioners meeting in late June, the study identifies the climate vulnerabilities of Marin’s roads, active transportation infrastructure, and transit network and sets forth long-term adaptation strategies.

Marin County, located across the Golden Gate Bridge from San Francisco, is one of the areas in California most vulnerable to climate change. Sections of the county’s main highway corridor, US-101, are built through wetlands that are prone to flooding. Frequent high tide events, like winter king tides, cause major interruptions for commuters and degrade essential roadways and pathways, and are expected to worsen with sea level rise. These impacts can obstruct evacuation routes and restrict access to community resources.

To address the growing risks, TAM commissioned Arup, in partnership with WRT and Pathways Climate Institute, to conduct a study looking at the vulnerability of its transportation assets and provide recommendations for how to protect them. The resulting study outlines any and all potential adaptation projects and policy initiatives relevant to Marin County’s conditions that will help protect critical transportation infrastructure for decades to come.

Aerial view of flooded intersection in Marin County

Frequent high tide events, like winter king tides, cause major interruptions for commuters © Josh Edelson Photography

A person riding a bicycle through a flooded path on the coast of Marin County

Arup's study identifies the climate vulnerabilities of Marin’s transportation assets, including active transportation infrastructure © Josh Edelson Photography

Anne Richman, TAM’s Executive Director, said, “This study helps TAM advance the work done in the County and lays out the paths toward protecting the transportation system, community, and critical assets. We are excited to use the work as the foundation for TAM’s Sea Level Rise Program into the future, while we coordinate and collaborate with partners across Marin County, the Bay Area, and the State.”

“This work is about the people and communities who depend on critical infrastructure every day,” said Meg Ackerson, a climate resilience engineer at Arup. “The study offers a blueprint for holistic adaptation that secures a resilient future for generations of Marin County residents.”

The study identified 19 focus areas across the county and then developed 7 deep dives of those focus areas looking more discreetly at what adaptation solutions may help protect vulnerable assets and communities. For each deep dive, Arup identified the potential climate impacts and proposed opportunities for adaptation projects. The comprehensive planning study identifies both hard infrastructure enhancements, like elevating roads, and nature-based strategies, such as marshland restoration.

Flooded intersection in Marin County on the coast of California

The study calls for a voluntary adaptation policy to ease adoption across jurisdictions in Marin County © Josh Edelson Photography

Central to this work is the recognition that sea level rise and flooding solutions span jurisdictions and agencies, requiring partnerships across departments and between cities and counties for effective implementation and long-lasting adaptation benefits. In alignment with California’s Senate Bill 272, the study aims to increase transparency and collaboration for cross-agency teaming for the common goal of sea level rise adaptation.

To ensure a comprehensive, well-coordinated plan of action, Arup engaged countywide stakeholder groups for input on the study. Arup and TAM presented the findings to local environmental and equity focus groups, including representatives from individual cities, county government officials, and other groups advocating for environmental and equity needs.

The study calls for a voluntary adaptation policy, which will ease the adoption of climate-focused enhancements to the transportation network in jurisdictions across the county. With such a policy, upgrades like elevating roads can be folded into maintenance projects already underway, and TAM’s Measure AA sales tax could be used to support adaptation efforts, to be determined by TAM. This approach will enable Marin County to act nimbly in adapting to the changing climate by funding and implementing vital projects across jurisdictions in a consistent and coordinated manner.

Now that the study has been accepted, TAM will begin implementing program recommendations in the next phase of their county-wide sea level rise adaptation program. TAM plans to share the study, including the voluntary adaptation policy, with local jurisdictions and explore the development of a technical assistance program and countywide design principles to support coordinated efforts to adapt to sea level rise.