Study to evaluate living shorelines as fix for rising waters

02/19/2020

By 2030, the projected sea level combined with a particularly nasty storm event could flood nearly everything west of Highway 1 in Stinson Beach: 590 parcels, 430 buildings and several miles of road. By the middle of the century, every high tide will bring flooding, and the roadways will likely need to be altered to maintain access to the low-lying town. 

The Marin County Community Development Agency, in a countywide sea-level rise adaptation report published two years ago, selected both Stinson Beach and the eastern shore of Tomales Bay as priorities for mitigation efforts. The projections for the impact on those two areas—based on a coastal storm modeling system created by the United States Geological Survey—are among many alarming revelations in a sea-level rise vulnerability assessment released in 2016. 

Last September, thanks to funding from the California Coastal Conservancy, the county awarded a consultant $190,000 to complete a feasibility study that examined whether a nature-based, living shoreline project along Stinson Beach could adequately protect nearby private homes and public resources; another $214,714 was awarded for a similar project on Tomales Bay. The studies were intended to evaluate alternatives to traditional armoring, such as sea walls.    

The research is still a year from completion, but this month a representative from Environmental Science Associates planned visits to the towns to describe preliminary findings. 

James Jackson, a coastal engineer who helped the county complete the vulnerability and adaptation analyses, presented to a packed room at the Stinson Beach Community Center last Wednesday night. He will speak about the Tomales Bay project on Tuesday, Feb. 25 from 7 to 8 p.m. at the Dance Palace. 

“This is an issue worth pursuing right now,” Mr. Jackson said. “I know that the community is no stranger to coastal hazards, and there have been extreme events over the last several decades. It’s really important to focus on the problem. Stinson Beach is a great asset, both to the community that lives here—some of you are here and enjoy the beach every day—as well as people like me and others around the greater Bay Area that come to Stinson Beach to play and recreate and enjoy the sun.”

Mr. Jackson and his team have analyzed the shoreline dynamics at the beach, including trends of erosion, wave exposure and beach width, and how these factors fluctuate over time and in storms. 

The team is now undertaking a comparative evaluation of possible alternatives, looking at their effectiveness at reducing flooding and erosion hazards as well as how long they will last, their cost, their environmental impacts and their coastal recreation benefits. The consultants are also looking into possible sources for materials, and at potential regulatory barriers to implementation.

To evaluate the shoreline dynamics, Mr. Jackson divided the beach into zones. Moving west to east, these were the tip end of Seadrift, the section of the beach in front of the Patios, the section of beach in front of the Calles, and the portion of the beach managed by the National Park Service, which has the most public access. 

Mr. Jackson said it appears that the sand in the Seadrift section—where in the ‘80s a rock revetment was installed after a series of severe storms—has been eroding by half a foot per year since the ‘90s. The rest of the beach has been relatively stable: sand and sediment washes away during storms and high tides, and then returns.

The width of the beach varies from section to section. On average, the beach can stretch up to 250 feet during the fall after the calmest weather months, with the exception of the Seadrift area, where the maximum width is more like 150 feet. During the El Niño years of 1998 and 2016, however, the beach shrunk by 100 to 150 feet in some places. “This beach has seen a lot of wave action,” Mr. Jackson noted.

He also emphasized that Stinson Beach is a good candidate for a living shoreline project. At some point it could become cost-prohibitive to maintain a natural barrier, he admitted, but the feasibility study would give the county some idea of when that might be.

After analyzing shoreline dynamics, Mr. Jackson and his team have turned their focus to evaluating alternatives. They have already identified four possible strategies, which were originally outlined by the county: large dunes, smaller dunes known as foredunes reinforced by vegetation, a cobble berm (imagine a thick layer of the rock from a riverbed piled into a mound), and a combination of dunes and a berm.  

A key aspect of the feasibility for the project selected down the line will be obtaining necessary materials. The Bolinas Lagoon, the Russian River, Bodega Harbor, and various dredging sites around the San Francisco Bay are possibilities. Dredging Easkoot Creek, which has caused severe flooding in Stinson, including in recent years, could solve two local flooding problems at once, Mr. Jackson said. 

Yet receiving permitting to take those materials could be an obstacle, he added, especially for sites in national marine sanctuaries and considering that other areas of the coast will also likely need resources in the near term.  

Living shoreline concepts have already been implemented elsewhere in the state. There’s a thick cobble berm piled at Linda Mar in Pacifica and at Surfer’s Point in Ventura County. San Francisco has experimented with using dump trucks to mound sand onto sections of Ocean Beach that border essential water system infrastructure, though the city is currently seeking permitting for a more permanent strategy.

Mr. Jackson didn’t hint at a preference among the four alternatives for Stinson Beach. Over the next year, his company will develop the concepts to get a better understanding of how long each would last and the cost in maintenance over time. 

The reception from the community was largely positive last Wednesday night, with many expressing gratitude for the county choosing to fund the study. 

One man criticized county representatives for not moving more quickly, however. “First, I’d like to thank Marin County for finally doing something really progressive, and trying to save what we already have, besides just fighting with the California Coastal Commission,” he said. “But a one-year study? How will we be able to see what the vegetation does? What putting down stone on the beach does? How will we see what these mitigating factors are going to accomplish if we can’t even test them?”

Jack Liebster, a planning manager with the Community Development Agency who has spearheaded Marin’s sea-level rise planning, stepped in to address the question. The point of the study, he said, is to see if a nature-based solution is even feasible. After that, the county will prepare a community plan for Stinson Beach—as well as for Marshall—that gives detailed recommendations for adaptation actions. Community hazard plans for other West Marin towns will follow.

Mr. Liebster said it was likely the county would fund a pilot program for Stinson Beach, the most vulnerable town; homeowners could also elect to fund their own or joint projects. 

The feasibility report will provide essential information about the county’s overall approach, Mr. Liebster emphasized. “Are these new-fangled ways of doing things viable, and how do they compare to the traditional ways?” he asked at the start of the meeting.

The county’s 2016 vulnerability report found that around 1,300 parcels; 1,000 buildings; 20 miles of roads; 1,800 acres of wetlands; and numerous other assets could be exposed to sea-level rise and storms by 2100, when seas could rise by roughly six feet. 

The adaptation report, brought before supervisors a year later, discussed protecting those assets through building elevation, floodproofing and nature-based strategies in the near to medium term. For the long term, solutions like elevating and armoring roads as well as developing new wastewater treatment systems were fleshed out. 

The evaluations in Tomales and in Stinson are just two of the “many balls in the air,” Mr. Liebster said. The county is also currently undertaking the environmental hazards portion of its update to the Local Coastal Program, which will outline its approach to new development in line with the California Coastal Commission’s standards.