Parts of some bayside communities in Marin are sinking at an accelerated pace, making them more vulnerable to threats from the rising sea, according to a federal study.
Areas of eastern San Rafael and Corte Madera are subsiding at a rate of more than 0.4 inches per year, largely because of sediment compaction, according to the report by the National Aeronautics and Space Administration.
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In the lowest areas, sea levels could rise more than 17 inches by 2050, more than double the estimated average of 7.4 inches for the region.
“In many parts of the world, like the reclaimed ground beneath San Francisco, the land is moving down faster than the sea itself is going up,” said Marin Govorcin, the lead author of the study and a scientist at NASA’s Jet Propulsion Laboratory in Southern California.
The study used satellite imagery and models based on tide gauge measurements.
Areas of other Bay Area communities, including Foster City and Bay Farm Island next to the Oakland airport, were found to be sinking at the same pace as San Rafael and Corte Madera. Areas of Point Reyes and the Russian River Valley were also found to be sinking, but at a slower rate, according to the study.
The subsidence is happening in landfilled areas made from sand and mud dredged from the sea. Landslides and erosion could also lead to subsidence, the study said.
Tectonic activity, or the movement of the Earth’s plates that activate volcanoes and earthquakes, could have an opposite effect, with rising land offsetting and the rising water, the study said.
As coastal communities develop adaptation strategies, they can also benefit from a better understanding of the land’s role, the research team said.
The NASA study follows a report from the California Ocean Protection Council predicting that sea levels across the state will rise an average of 9.6 inches by 2050.
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In December, the San Francisco Bay Area Conservation and Development Commission adopted a first-of-its-kind regional shoreline adaptation plan to help coastal communities protect their shores from the rising sea.
According the commission, the state regulatory agency that oversees the bay, Marin County is facing about $17 billion in expenses to build adaptation projects, such as living shorelines, marshes and potentially seawalls and levees, to protect itself from inundation.
The regional plan is a first step to meet mandates set by Senate Bill 272, signed in 2023 by Gov. Gavin Newsom. The bill requires local governments in the coastal zone or around San Francisco Bay to develop a sea-level rise plan that includes adaptation strategies and recommended projects by 2034.
The findings of the new NASA report align with the studies that San Rafael is using to strategize its defense, said Kate Hagemann, climate adaptation planner for the city.
“Parts of San Rafael are sinking faster than the sea is rising, and it’s happening at the same time,” Hagemann said. “When you have those two changes happening simultaneously, it means our flood risk is happening faster, and you have less time to plan, which is a complication for us.”
The city is immersed in a feasibility study to develop approaches to protect its most vulnerable areas, primarily the Canal neighborhood and low-lying areas near the San Rafael Creek, Hagemann said.
The city has partnered with the Multicultural Center of Marin and Canal Alliance nonprofits, as well as researchers at the University of California at Berkeley, to do the study.
Hagemann and the team plan to present an update on their work the San Rafael City Council when it meets Monday. The session is set for 6 p.m. at City Hall at 1400 Fifth Ave. Meeting information is at cityofsanrafael.org.
In Corte Madera, R.J. Suokko, director of public works, said the findings of the NASA study are consistent with the town’s assessment.
“Based on the assessments and plans prepared over the last few years, the town’s top priority is focusing efforts on fortifying and raising the flood berm that protects the Marina Village neighborhood and many of the businesses, homes and Highway 101 behind this area,” Suokko said. “Our recent analysis has shown that the flood berm is at risk of overtopping now, in the scenario that a significant storm surge and a king tide event occur simultaneously.”
The town has applied for a $900,000 grant from the Federal Emergency Management Agency’s flood mitigation assistance program to begin design and environmental review, Suokko said.
“We are anxiously awaiting authorization to proceed with the work from FEMA, which we’re told should happen this upcoming summer,” Suokko said.