The Bay Is Rising, and Marin’s Most Traveled Roads Are in Its Way

Highways. Trails. Roads. Ferry terminals. Bike paths. Train lines. Much of Marin County’s spiderweb of coastal and bayside transit options is at risk of inundation from rising sea levels in the coming decades, according to a recent study identifying the climate vulnerabilities of its built infrastructure.

The county, surrounded by water on three sides, is already plagued by flooding during high tides several times a year. Now it has a greater understanding of its future sea level rise risk due to the effects of human-caused climate change, thanks to the study unveiled last week by the Transportation Authority of Marin, or TAM, in collaboration with environmental consulting firm Arup.

Researchers identified 19 areas along the Marin County shoreline that are prone to flooding, sea level rise and groundwater rise, noting that “tipping points” at which flooding becomes permanent are just decades out in some locations. In others, temporary flooding could spike in the years to come.

“The transportation system is connecting all of these places, and if it fails in one community, then it fails to work for the entire county and the region,” said Mikaela Hiatt, a senior transportation planner with TAM.

On the bayside of Marin, the researchers found a long stretch of spots that face an outsize flood risk: Sausalito, Tiburon, Belvedere, Marin City, Manzanita, Mill Valley, Corte Madera, Larkspur, Kentfield, San Rafael, Santa Venetia, Novato, Bel Marin Keys, Hamilton Wetlands, Bel Marin Keys, Highway 37 and Highway 101. On the Pacific coast, they found far fewer: Stinson Beach, Bolinas and Inverness.

It’s no surprise that most of the areas set to flood in the coming decades nearly perfectly align with the county’s densest population centers, where advocates in places like Marin City and San Rafael have long called for solutions for existing floodwaters.

“The populations at risk are in some of the most beautiful parts of the county right by the water,” said Meg Ackerson, senior climate resilience engineer with Arup. She noted that county leaders and developers built much of the bayside infrastructure on fill or in low-lying areas that were once marshland.

“It’s like the water wants to reclaim what was once there,” Ackerson said.

The transportation agency’s study doesn’t set any must-dos for the vulnerable areas because the agency doesn’t own roads, highways, ferry terminals, rail lines or trails but acts as a technical advisor. Instead, the study’s authors list hazards and a suite of solutions to curb flooding, including an outline of ongoing work by other agencies.

For example, Marin City, located just west of Highway 101 and Richardson Bay, has long been plagued by floodwaters. When it rains heavily and tides are high, floodwaters can block the only road in and out of the neighborhood, spill onto Highway 101 and flood part of a shopping center.

There, researchers found a “tipping point” by midcentury where “we start to see permanent inundation,” Hiatt said.

Strategies that could protect the area include building a sea wall, raising streets, elevating the shoreline, creating a second evacuation route, breaching creek channels, and deepening the capacity of the area’s stormwater pond, according to the researchers. Caltrans is also conducting a study to address the issue.

Another area that already floods on “sunny days” during high tides is the Mill Valley area, “where there’s a kind of well-known photo with the kayaker next to the bicyclist,” said Anne Richman, TAM’s executive director.

The tipping point for Mill Valley is generally around 2050, “though temporary flooding and closures are anticipated sooner,” the authors wrote.

The agency identified at least two existing ongoing flood mitigation plans for the area, developed by county and city officials. Solutions could include elevating roads and trails, and Richman said local authorities could also explore the use of a horizontal levee, which is a wide, gradually sloping land buffer meant to soak up floodwaters and stop them from moving inland.

San Rafael is a third hotspot for future flooding, with significant effects on transportation corridors by 2050 due to its proximity to San Rafael Bay and San Rafael Creek, which splits a large portion of the city’s densely populated area.

Future floodwaters could cut off access to downtown, inundate the low-lying Canal neighborhood, and complicate access to Highway 101, Highway 580, and the Richmond-San Rafael Bridge. Solutions here could include creating horizontal levees, elevating transportation routes and building out ponds on public property to capture excess stormwater.

“The 580-101 connection is critical to how people move in and out of the county,” Hiatt said. “These solutions could protect the thousands of people who live in the Canal neighborhood and who use the transportation system.”

Throughout the study, agency officials prioritized nature-based solutions to keep them at the forefront of the planning process as coastal cities work toward a state mandate to develop assessments of their vulnerability to sea level rise.

While the study doesn’t force cities to adapt to rising tides, Richman hopes the research serves as a launching point as communities begin to take the topic seriously.

“We’re in kind of an interesting position because we don’t actually own any of the roadways or pathways or bus stops,” Richman said. “Ultimately, it’s going to take all of these agencies pulling together to make these projects happen over time.”