WHILE REGIONAL AGENCIES WEIGH a proposal to raise a flood‑prone stretch of U.S. Highway 101 between the Manzanita park-and-ride and Donahue Street in Marin County, a local scholar has explored an alternate vision: putting the freeway in a tunnel beneath a new linear park.
The two views highlight the stakes for Marin City, a low‑lying community that is already likened by experts to a bathtub with an inadequate drain as sea levels rise.
There is currently a preliminary plan to elevate Highway 101 from Manzanita to Donahue at a rough estimate of $1.2 billion, with an extra $33 million for stormwater pumps and a drain pipe to carry rainwater beneath a shopping center that sits on slightly higher ground between Marin City’s entrance road and Richardson Bay.
It is part of Plan Bay Area 2050+, a long-range regional urban plan for the nine-county Bay Area authored by two regional governing bodies — the Metropolitan Transportation Commission and the Association of Bay Area Governments. The plan is required by state and federal governments as a guide for meeting future transportation, housing and infrastructure goals.
On Wednesday, the MTC will consider adopting the new plan. It is the first such plan to include a resilience project list, an inventory of infrastructure projects that will prepare the region for about 4.9 feet of sea level rise over the coming decades. The total project budget for the Bay Area is approximately $96 billion, with about $12 billion for sea level rise adaptation projects in Marin County, including the Highway 101 elevation shoreline stretch.
Michael Germeraad, associate planner with ABAG, said many of the projects on the resiliency list are conceptual in nature.
“It’s meant to give the region a tool to understand the level of funding that’s needed to meet this challenge,” Germeraad said. “Our approach is allowing locals or asset managers to lead an adaptation planning … as opposed to us coming in at the regional level and suggesting a specific approach.”
Germeraad said there is ongoing community outreach with local jurisdictions along the Marin City stretch of shoreline, with Caltrans as an asset manager for Highway 101. The current concept is to raise the highway to protect it from sea level rise.
The alternate vision
Kristina Hill, associate professor of landscape architecture and environmental planning at the University of California, Berkeley’s College of Environmental Design, has advised Marin City on a temporary solution for its chronic flooding.
They acquired portable pumps that move water around the shopping center to the Bay. That solution passed an important test when record king tides hit the region in January. Marin City was one of the only shoreline cities in the county that saw no flooding.
“If they elevate [Highway 101] on an earthen berm, it could make Marin City’s flooding worse, unless there are several large culverts with tide gates,” said Hill, adding that if they made it an elevated bridge or causeway, it would let the rain out, but it would also let the tides flood in.
“A causeway would need to be torn out and raised again in the future,” she said. “So, it’s a lot of money for a short-term solution. The only permanent fix that allows Marin City to adapt long-term is the tunnel. It lets tides in, yes, but it also lets rain out and won’t need to be raised in 50 to 100 years, when all hell will have broken loose and we won’t have money to raise everything.”
Hill’s cut‑and‑cover tunnel concept, developed in a graduate design studio, proposes lowering Highway 101 into a covered trench through Marin City. A project sketch shows the buried highway capped with a linear park, with Donahue Street carried over the tunnel toward Sausalito’s Bridgeway and bike routes connecting Marin City to nearby communities.
Under the proposal, the tunnel would contain car exhaust and protect the freeway from flooding while allowing Marin City’s central pond to be reconnected to the Bay through a surface channel, restoring saltwater habitat and natural tidal exchange. Connection to nature and reduced traffic-related air pollution are two items on the wish list of a collective of residents called Marin City Climate Resilience and Health Justice. A tunnel could also establish an elevated foundation for potential new housing.
“We wouldn’t claim it is anything more than a concept, but it raises important issues,” Hill said. “I’d like to see simple cut and cover tunnels considered more often by MTC and Caltrans.”
“A tunnel is technically a potential solution,” said Caltrans spokesperson Matt O’Donnell. “It’s hard to tell from the drawing, but the expansive nature of the work required for this tunnel project seems likely to be in billions of dollars. Such a proposal could be cost-prohibitive if other viable solutions identified were more cost-effective.”
O’Donnell said there is a large need and limited money for projects, although Marin County is certainly high on the urgency list.
“If we spend billions of dollars on a single project when less expensive projects will address the issue in the medium-term, then the money we use there can’t be spent on other urgent needs around the Bay,” he said.
Plan Bay Area 2050+ also forecasts a rise in climate risks should more people work from home. Housing pressures and transportation patterns will reshape the Bay Area through mid‑century, underscoring the need to protect key roadways that serve as evacuation and commute routes.
As agencies move toward final approval of Plan Bay Area 2050+ this week, the question for Marin City is whether its future will be defined by a higher highway overhead or by a buried one that reshapes its shoreline.