The landscape of Stinson Beach will change in the coming decades. The question is whether that change will be made deliberately by people or chaotically by the rising sea.
According to a recent analysis, Stinson’s sandy beach and the vegetated marsh along the lagoon will lose an estimated 30 to 40 percent of their surface area if sea levels rise by 15 feet between 2040 and 2050.
There is no one perfect solution, according to a new report from Marin County. The report provides an easy-to-read summary of more than a dozen ways to divert water away from the homes, roads and businesses that make up the community of about 500 residents.
The popular oceanfront destination attracts approximately 750,000 visitors annually. The tourist infrastructure is also at risk.
The solutions range from simple sand dunes to raising the city. Each option contains pros, cons and price tags. The key point: Responding to sea level rise will require a lot of coordinated community action.
The Marin County Community Development Agency has scheduled a virtual public webinar for June 17 at 5:30 p.m. The public is invited to discuss the findings and discuss next steps.
“Our report shows that addressing sea level rise in Stinson Beach will take a combination of community actions, and a lot of collaboration and partnership,” said Isaac Pearlman, senior planner at the Marin Community Development Agency.
The move to find solutions follows a 2023 provincial study to identify the extent to which buildings, habitats, infrastructure and community services are at risk from future flooding due to sea level rise. The analysis found that Stinson Beach’s homes and infrastructure can withstand a 100-year flood – a very intense storm that has a 1% chance of occurring each year or a 26% chance of occurring during a mortgage with a term of 30 years.
However, this measure does not take future sea level rise into account. Increased wave run-up, flooding of lagoons and creeks, rising groundwater and coastal erosion place the beach community at great risk.
The study concluded that if no action is taken, more intense storms and sea level rise will make flooding in Stinson Beach more frequent and destructive, and the impacts will last longer.
All homes in Stinson Beach rely on septic systems, so several options address the problem of these systems failing when groundwater swells. A community wastewater treatment plant could provide the collection, treatment, and distribution of treated wastewater. Horizontal dikes can be used to reduce flood risk for low-lying areas around fluvial and estuarine environments. They resemble traditional dikes, but have flatter, wider slopes on the flood side. They are covered with native plants and cover the elevation range from highlands to shallow waters.
Wind-driven waves break on the long slope and promote the healthy deposition of sediments. They also digest and clean wastewater when it is injected deep under the roadside.
“Ecotone or horizontal levees often replace or are built on top of mudflat habitat and/or elevated shoreline,” says Pearlman, referring to the environmental impacts of building a levee in an existing wetland such as Bolinas Lagoon. “The end result is restored and improved habitat, but ecotone levees are not built in a vacuum, so one type of habitat is converted into another.”
The report lists temporary solutions such as replenishing the beach, building seawalls or adding rock revetments to the beach to displace wave energy. Higher defensive approaches include building steel bulkheads with sump pumps, or waterproofing buildings with watertight doors and walls. Raising roads and moving buildings to higher ground are forms of disengagement, as are development interventions such as buyouts, purchasing a plot of land to remove it from a coastal hazard such as flooding or erosion.
