Sea level rise threatens Highway 37; leaders prepare billion dollar plan to stop it

Highway 37 serves as a key artery of Bay Area traffic from Marin County to Vallejo, but its low-lying place in former wetlands makes it susceptible to flooding and sea level rise over coming decades.

Leaders in transportation will need to address two issues at once to ensure the long-term sustainability of the key corridor: the creation of flood-resistant, sea-level impervious infrastructure and the environmental restoration of the wetlands.

“You can't do the environmental restoration and address sea level rise without doing the transportation project. And you can't do the transportation improvement projects without addressing sea level rise,” said Suzanne Smith, the executive director of the Sonoma County Transportation Authority.

More than 40,000 drivers use the roadway each day, serving commuters, agricultural products and the tourism industry of Sonoma Valley, in addition to serving as a vector for evacuation efforts. The highway has been shut down for flooding in the past, one example being in 2019 from a broken levee and rainwater flooded parts of the thoroughfare.

By 2030, Highway 37 will be subject to frequent flooding, said Eamon O’Byrne, executive director of the Sonoma Land Trust. And by 2040, parts of Highway 37 will be underwater from sea level rise.

“The hint that we should have had years ago, is when a sea lion tried to make it across 37,” said Madolyn Agrimonti, a Sonoma City Council member and a former representative of the city on the SCTA. “The highway patrol came and they had to get the sea lion off Highway 37, so that's a hint that the water’s rising.”

With so much on the line, Caltrans, the Metropolitan Transportation Committee, the Sonoma County Transportation among other county transportation authorities have planned two projects, which total more than $5 billion altogether, to protect the vital 21-mile transportation corridor.

The first is a $500 million dollar effort to widen the highway and ease traffic congestion. The second is an estimated $5-10 billion project to lift the highway and restore the wetlands that sit in the floodplain near San Pablo Bay, Smith said.

“What we've done over the past almost-decade now is develop the sort of underlying planning efforts to help establish what phases of work need to go forward and start on the environmental process on getting those projects delivered,” Smith said.

But recently, another group involved with planning efforts, the Sonoma Land Trust, has broken from the rest over the interim plan to widen the highway before addressing the long-term issues of restoring the wetlands and raising the highway.

“They completely agree that the long-term solution is the elevated causeway,” O’Byrne said. “The problem is that they've put forward a plan that's ‘an interim’ solution that, unfortunately, doesn't deal with climate change. It doesn't mention the word.”

Moving forward with the interim plan only addresses traffic congestion, O’Byrne said. And if the floodplain has not been restored to wetlands, sea level rise will imperil and flood the interim project by 2040 and “it will become a drowned muddy slew.”

Sonoma Land Trust is working on a multi-decade restoration effort of the wetlands. The organization tore down a levee in 2015 to bring waters from San Pablo Bay to flow naturally into the area for the first time in a century, O’Byrne said.

“If you just build the causeway, it might not be high enough. But if you restore the wetlands, that's an added level of buffering for for the causeway,” O’Byrne said. “So you protect the causeway, plus you create up to 20,000 acres of habitat for the plants and animals that call that place home.”

Highway 37’s two-lane roads are subject to daily bottlenecks, however, Smith said. Agrimonti said she and her husband bring “little opera binoculars” in the car to look ahead for traffic congestion.

Meanwhile, the long-term project is estimated to take two decades to complete, Smith said.

“When you've got a highly used roadway, and a very sensitive environment, it's a little bit fraught. And so trying to navigate good solutions that address both the traveling public's needs, and the environmental needs is really what we’re trying to do,” Smith said.

Without Highway 37, commuters would be forced to drive almost double the number of miles to go between Marin County and Vallejo, Smith said. Basically, there is no alternative road prepared to handle the same volume of traffic as the flood-prone roadway.

“We might wind up with an endlessly built up berm on which Highway 37 sits that is still vulnerable to serious climate effects,” O’Byrne said. “We've been kicking the can down the road on Highway 37 since it was built in the 1930s. And if past actions are any guide to the future, the paint and patch solution only leads to more paint and patch solutions down the road.”

Contact Chase Hunter at chase.hunter@sonomanews.com and follow @Chase_HunterB on Twitter.