The threat of sea-level rise looms especially near for Stinson Beach, where ocean waters have risen eight inches in the past century. Climate experts predict that by 2100, seas could rise 70 inches, putting over 700 homes in the popular beach town at risk of constant flooding.
Jeff Loomans, the Stinson Beach representative on the county’s Coastal Communities Working Group, said that Calle del Arroyo, the main road leading to Seadrift and other low-lying neighborhoods, is under immediate threat.
“An engineering study determined a couple years ago that it is now, at high tide during the year and routinely at king tides, an underwater road below sea level itself,” Mr. Loomans said.
Staff at the county’s Community Development Agency have worked for close to a decade to understand how rising seas will impact Stinson Beach’s vulnerable coastline. Last year, the agency received a $396,000 grant from the California Ocean Protection Council to fund the Stinson Beach Adaptation and Resilience Collaboration, or ARC, project. County officials took efforts a step further in July, approving a $547,000 contract with Environmental Science Associates to provide adaptation planning and engineering support for ARC.
The ARC project builds off Collaboration: Sea-level Marin Adaptation Response Team, or C-SMART, an effort led by the C.D.A. to understand the threat of sea-level rise. ARC is designed to address the adaptation recommendations made by C-SMART through the implementation of specific projects. It will analyze near-term solutions like educational outreach and emergency preparedness, medium-term solutions such as maintaining protections for existing structures, and long-term options like elevating structures.
ARC prioritizes nature-based solutions, such as the construction of dunes that the Stinson Beach Nature-Based Adaptation Feasibility Study predicted would be an effective method of dispersing storm waves and blocking winter tides in the short or medium terms. Compared to more traditional options like sea walls, nature-based strategies like manmade dunes provide an ecological, aesthetic and recreational benefit to the beach while protecting the development behind it.
Yet the Stinson Beach Nature-Based Adaptation Feasibility Study also found that additional adaptation actions—such as building seawalls and elevating structures—would be needed once seas rise more than 3.3 feet above current levels. ARC is using the feasibility study to determine if the pursuit of nature-based solutions is plausible.
Jack Liebster, a planning manager with the C.D.A., said past studies conducted under C-SMART were designed to understand the problem of sea-level rise, whereas ARC will address solutions. And although past feasibility studies have been crucial, he said they needed to be updated to address the latest projections.
The county estimates that the market value of assets exposed to storm and tidal impacts will be around $1.5 billion, based on predictions from a model created by the United States Geological Survey called the Coastal Storm Modeling System, or CoSMoS. The system makes detailed predictions of storm-induced coastal flooding, erosion and cliff failure, but the model is being retooled and is expected to provide updated predictions later this year.
ARC is also addressing new considerations that have arisen since the creation of C-SMART, such as the impact of sea-level rise on Stinson Beach’s septic systems.
“One of the things that was not really recognized several years ago was groundwater impacts,” Mr. Liebster said. “As sea level rises, it pushes groundwater to the surface and that means it’s going to have a debilitating effect on existing septic tanks, and all of Stinson Beach is on septic tanks.”
The Stinson Beach County Water District monitors 721 septic systems, and Ed Schmidt, the district’s outgoing general manager, said that rising seas will impact them. The systems disperse effluent into several feet of unsaturated native soils, where biological and chemical nutrient breakdown occurs. Flooding and higher groundwater elevations change the aerobic environment, which reduces the treatment of effluent and allows it to travel farther from its source.
Guidelines for protecting surface waters and groundwater from contamination are defined in state septic system code, and Mr. Schmidt said he worries that, at some point, Stinson won’t meet the requirements. Rising groundwater levels are anticipated to trigger higher pollutant counts as water is contaminated with effluent.
Mr. Liebster said the water district is studying the development of a centralized wastewater treatment system, which he said would be a key adaptation strategy. Raising buildings and increasing road elevations are other strategies that will be considered.
“One of the main objectives is to save the beach,” Mr. Liebster said. “Sea-level rise and coastal processes that are affected are going to combine to erode it more rapidly than in the past, and at the same time, higher water levels are going to drown it.”
The project also emphasizes community engagement. “We’re inviting people who have different concerns to participate in focus groups so that they have an opportunity to really articulate what their concerns are,” Mr. Liebster said.
Mike Matthews, the president of the Stinson Beach Village Association, said between 100 and 200 people attended half a dozen meetings or focus groups the county has already hosted. That’s notable, considering Stinson’s population is less than 600, he said.
Stinson Beach resident Annie Rand has been concerned about sea-level rise in Stinson for decades. She lived in the Calles during the storms of 1982 and ‘83, which devastated multiple houses. She remembers picking up pieces of her neighbor’s home along the flooded street.
“We had about 18 inches of sand in our yard, the fences got knocked down and there was septic water downstairs,” Ms. Rand said. “It was horrible.”
Afraid that climate-induced storms would persist, she moved her family up the hill a few years later.
Mr. Matthews acknowledges that while addressing the threat of sea-level rise is a big feat, he’s confident in the community’s resiliency.
“It is hard for any person to internalize a 100-year threat and decide what to do,” Mr. Matthews said. But, he added, “People are endlessly creative and, collectively, good ideas pop up. The orientation will be to find solutions that respect nature, minimize impact and preserve the essence of the town.”