A new county study on rising sea levels shows just how quickly Stinson Beach is being lost to the ocean—and outlines possible responses to a crisis compounded by the presence of septic systems in already flood-prone areas.

The latest vulnerability assessment by the county’s Stinson Beach Adaptation and Resilience Collaboration, or ARC, follows up on a 2016 vulnerability assessment of Marin’s coastline, which showed that Stinson Beach is the most vulnerable of Marin’s coastal communities. 

More than 250 buildings in the seaside village are at risk from a 100-year storm—one that has a 1 percent chance of happening each year—and over 120 buildings are at risk from a 100-year creek flood. A preponderance of the town’s septic systems are built on the Seadrift spit and are already under threat of water infiltration and severe damage. 

The study, authored by the firm Environmental Science Associates, used the latest sea-level rise projections from the California Ocean Protection Council that show that seas could rise 1.5 feet by 2050 and over 6.5 feet by 2100. The projections date to 2018. Isaac Pearlman, a senior planner for the county who is leading the ARC project, said the council will publish updated projections later this year.

With just 1.6 feet of sea-level rise, beaches will shrink by 30 percent, turning the arc of Stinson Beach into a series of pocket beaches. The saltmarsh will flood, resulting in a 40 percent reduction in wetland habitat and routine inundation of Highway 1 and a majority of the Calles, a neighborhood abutting the beach. 

With 6.5 feet of rise, up to a few feet of water will breach the spit. Nearly all the low-lying areas and Highway 1 will be exposed to regular tidal inundation and coastal erosion. The beach will disappear almost entirely and saltmarsh habitats will convert to mudflats and open waters. 

The report states that over the past century, Seadrift’s shoreline has eroded significantly more than the southeastern beach, at an average of nearly 1 foot a year. Katie Beacock, who owns Seadrift Realty and has worked closely with the county and an organization focused on coastal property rights and land use, said that although Seadrift residents will be highly impacted by climate change, few have expressed concerns for the future.

“I’ve been expecting phone calls but haven’t received any,” she said. “Septic tanks are the number-one concern most of us have but I have great faith in the state and county water control. This is not an issue exclusive to Marin, but up and down the coast.”

This winter’s most damaging storm in Stinson Beach arrived on Jan. 5 amid a series of atmospheric rivers and coinciding with king tides. Wave runup substantially eroded the dunes on Stinson Beach, breached the spit, flooded the Calles and led to very high and emergent groundwater throughout the spit. 

According to the new vulnerability study, heavy rain flooded Easkoot Creek and damaged the National Park Service parking lot and adjacent areas. Portions of Highway 1 along the lagoon flooded and sediment flowed down from the Mount Tamalpais watershed. 

James Jackson, a hydrologist for Environmental Science Associates, said his firm had not determined whether to categorize the storm as a 10-year storm or something rarer.  

Easkoot Creek is fed by three tributaries that begin on Mount Tam and run down into the Bolinas Lagoon. Sediment flows can quickly change its makeup, leading it to breach its banks in the rainy season. The county built a sedimentation basin in 2013 opposite the Parkside Café to reduce flooding, and it dredges the creek annually. 

Now, the park service is considering constructing a channel from the sediment basin southwest through the dune field to the ocean to further mitigate flooding. 

The county dredged Easkoot Creek last fall in preparation for the rainy season, then again on New Year’s Eve, said Julian Kaelon, a spokesman for the county’s Department of Public Works. But several roads still flooded and nearly 70 homes suffered water damage. Around 20 houses in the Calles experienced structural and septic damage, and an additional 10 or so houses in Seadrift suffered minor flood damage.

The Stinson Beach County Water District visually assessed septic systems after the January storms and observed minor septic damage at five properties and major damage on one property where a leach field was exposed and carried away by stormwaters. The district provides 718 water connections and monitors 721 privately owned septic systems. Seventy percent of those systems are in the Calles, Patios and Seadrift areas, all of which are threatened by seasonal high groundwater.

After the early January storms, groundwater surfaced in various streets, saturating older gravity-fed leach fields, the report states. Modern septic systems in town typically have raised leach field beds and pumps that prevent groundwater from flowing into the systems.

“Ultimately, wastewater treatment and disposal were significantly affected for weeks from the high groundwater elevations that were reached following the early January 2023 rains,” the report states. 

The report outlines adaptation strategies ranging from elevating homes in wave and targeted flood zones, building coastal armoring along the beach and floodwalls along the lagoon, restoring dunes, elevating or rerouting Highway 1 and transitioning from septic systems to a community wastewater treatment system. 

Kent Nelson, the general manager of the Stinson Beach County Water District, said the district is in the “very early stage of looking at the feasibility and rough costs [of such a system].” The district applied for a state climate-change adaptation grant that will fund the roughly $400,000 feasibility study. Mr. Nelson said that a community collection system and wastewater treatment facility would cost around $31 million.

The next phase of the ARC project will consist of stakeholder engagement through website comments, focus groups and beach pop-ups. Last Friday, Mr. Pearlman manned an education station with information on rising sea levels and virtual reality equipment at the Stinson Beach Library. One resident put on the headset and watched as the spit was covered in water and the lagoon decimated by flooding in the year 2100.

“So I guess our new M.O. is to pick everything up and go?” he asked. 

“It’s easy to think that way,” Mr. Pearlman said. “But with all of the information we have here, we can start to say ‘Okay, what’s our first move in adapting to this?’”

The county will develop and analyze adaptation strategies throughout this year and present a draft adaptation roadmap report in June of next year. 

Besides the intrinsic value of preserving the seaside community of Stinson Beach, hundreds of thousands of visitors from all over the Bay Area and beyond visit the beach. 

A new study from Point Blue Conservation Science showed that over 2.6 million people visited the beach between 2017 and 2021. The area is a refuge for those who don’t have immediate access to a recreational water source, and its use skyrocketed for disadvantaged communities during the pandemic.

To watch a simulation of sea-level rise at Stinson Beach, visit https://www.youtube.com/watch?v=StVbIMYkoEM&t=285s