Marin sea-level rise study focuses on transportation network

Elevating a flood-prone section of Highway 101 through Marin City on a causeway or viaduct could protect it and the nearby Bay Trail from rising seas.

The proposed project is one of the potential strategies in a new report on protecting roads, paths and transit in seven areas in Marin County that are most vulnerable to sea-level rise.

An executive committee of the Transportation Authority of Marin voted unanimously this month to recommend the report to the agency’s board for approval. The final adaptation study and plan is expected to be presented at the board’s meeting on June 26.

Mikaela Hiatt, planner for the agency, said that when the study began two years ago, several other local, countywide and regional sea-level planning efforts already were underway. That includes sea-level studies such as Marin County’s BayWAVE and CSMART studies as well as Caltrans’ study on state highway vulnerability.

“We took this study as an opportunity to build off of those efforts,” Hiatt said. She said staff were directed to move past a vulnerability assessment “and to start to actually think about adaptation planning.”

The authority is Marin County’s traffic congestion management agency and administers sales tax and vehicle registration fee collection through Measures AA and Measure B. The Measure AA sales tax measure, renewed by voters in 2019, allots 1% of its revenue, or $250,000 annually, toward sea-level rise protection.

Hiatt said regional planners have estimated it will cost about $17 billion for Marin County to fully adapt to sea-level rise, making funding and prioritization challenging.

The study was conducted by the Arup Group, a multinational design and planning company based in London. The Transportation Authority of Marin allocated $550,000 for the study in 2023.

Meg Ackerson, the consulting planner, said the study initially identified 19 areas of concern.

The areas of concern are Bel Marin Keys; Belvedere and Tiburon; Bolinas; the Hamilton Wetlands; Inverness; Marin City; Manzanita and Tamalpais Junction; Mill Valley; downtown and western Novato; Santa Venetia; Sausalito; Stinson Beach; Highway 37; Highway 101; San Rafael; Kentfield; Corte Madera; Terra Linda; and Larkspur.

“Working with those, we filtered out a number of them to focus on seven areas for what we’re calling a deep dive,” Ackerson said. “We decided to focus on areas where TAM could move the needle a little bit, and progress on some of the sea-level rise adaptation in the area.”

The seven focus areas include Sausalito; Tam Junction and Marin City; Mill Valley; the Corte Madera Creek watershed; San Rafael’s Canal neighborhood; Santa Venetia; and Novato. The report provides detailed adaptation summaries that highlight the flood-prone roads and infrastructure and the best types of projects to mitigate risks.

The report also provides a timeline of when planning should begin for each project and when each should be constructed. The report also projects a “tipping point,” which identifies the date when flooding is expected to be a regular, if not daily, occurrence in each problem area.

In the Tam Junction and Marin City area for example, the report addresses flooding at Highway 101, the Bay Trail and the Bothin Marsh and Coyote Creek.

The report identifies potential strategies to prevent flooding in the area, including elevating roads; developing coarse grain beaches; and constructing levees and tide gates. Installing new pump stations and restoring marshlands are also possible projects.

Ackerson said planning needs to start now for most project development.

Staff said that in implementing the plan, Transportation Authority of Marin will serve as a lead or facilitator in some instances, and a partner or adviser in others.

Marin County Supervisor Stephanie Moulton-Peters, a TAM board member, told staff at the June 9 committee meeting that she had questions.

“Our transportation network is essential and TAM doesn’t own it, so I’m trying to understand the roles that TAM will play,” she said.

“There’s a lot about sea-level rise that is complicated and this is one of those pieces the whole sort of governance and who does what question,” said Anne Richman, executive director of the agency.

Richman said TAM will take the lead on transportation projects like roads and other infrastructure. However, projects that involve levees, tide gates or floodwalls are not transportation infrastructure work. In those cases, TAM will advise “about the effectiveness of the project or how it relates to the transportation network,” Richman said.

TAM will also be in a position to help identify funding opportunities.

Moulton-Peters said she would like some more examples presented when it returns to the full board on June 26.

“It’s a really great report,” Moulton-Peters said. “I thought your trigger points and just looking at planning was really helpful.”

Along with the study and implementation plan, staff developed an adaptation policy that will be proposed for adoption to officials in all of Marin’s cities and towns.

If the plan is approved, staff will begin working with cities and towns to fine-tune the policy for each respective jurisdiction, ensuring goals are aligned.

The board meeting is 6 p.m. June 26 in Room 330 at the Marin County Civic Center at 3501 Civic Center Drive in San Rafael. More information is at tam.ca.gov.