Last Thursday’s storm revealed flaws in a flood repair project undertaken this year by the Golden Gate National Recreation Area, when two feet of water pooled in the northern section of the parking lot at Stinson Beach, spilling over berms and into homes on neighboring Calle del Pinos.
The parking lot sits in a low-lying flood plain between the beach and Easkoot Creek. A wall of dunes at least six feet high erected between the parking lot and the beach as part of the Stinson Beach Flood Repair Project, an $180,000 endeavor, blocked floodwaters from reaching the ocean.
In the aftermath of the storm, crews from the National Park Service breached a section of dune to let water pass to the ocean. By this week, as more storms rolled into West Marin, most of the waters in the parking lot had receded, but the sheer force of water spilling down to the beach in subsequent rains had widened the breach in the dune to nearly 10 feet. A section of the parking lot itself was also damaged.
“Water did not drain adequately out to the west side, so it stayed in the parking lot,” said Howard Levitt, the communications director for G.G.N.R.A. “We set up a system that was designed to contain flooding as best we could and to protect the community from wave action. But the water did not drain the way it was intended.”
As part of the flood project, which was spurred by a large storm system in February that caused flooding and damaged the parking lot, dunes between the lot and the beach were fortified to defend against erosion and tidal action. But until they were intentionally breached with heavy equipment last week, those dunes left no channel for water to flow out to the beach.
The project manager, Steve Ortega, was on leave this week and could not comment.
“The dune does not prevent flooding,” said Rollin Bruce, who owns a house on Calle del Pinos. “It’s actually holding water in. A pipe would have taken care of that problem. Or just some kind of water outlet—a pipe or concrete waterway.”
According to Mr. Levitt, the dunes were expected to be low enough for the water to flow over them. “The idea was for water to flow in a sheet over the dune,” he said. “But it didn’t quite function as it had been expected.” He added, “This storm was pretty intense. The amount of water was extreme.”
Other flood-control projects at Stinson Beach fared better.
In 2013, the Marin County Flood Control and Water Conservation District built a 150-foot long, 35-foot wide basin designed to collect sediment that Easkoot Creek brings down from Mount Tamalpais during heavy rains. Like G.G.N.R.A.’s dune restoration, the county’s sediment basin was tested during last week’s storm.
When storms hit Stinson, the volume of water flowing in the creek behind the Parkside Café naturally increases. The basin was put in place to collect sediment that otherwise would pile up in the bottom of the creek.
“It was lucky we had built it before the February storm,” said Chris Choo, a senior watershed planner for the Department of Public Works. “That trap is the first location where the sediment deposits.”
The basin cost the county $67,000 to build and another $9,000 to clean after the February storm. So far, according to the county, the basin has proved effective.
“It’s been working as designed,” said Tracy Clay, the principal civil engineer for the county’s Flood Control and Water Conservation District. “We removed sediment prior to the storm and kept the water flowing through the creek.”
Permit restrictions meant to protect salmon and steelhead habitat prevent the county from removing sediment during dry months; however, in anticipation of last week’s storm, the county acquired an emergency permit to dredge the sediment basin a day before the storm hit. As a result, the basin kept the creek bed deeper and gave the creek more breathing room to keep waters from jumping the banks.
“The work we had done to remove sediment was done because it was basically an emergency flood,” Ms. Choo said. “We knew the creek would flood if it was full already.”
Thursday’s storm was a litmus test to plan for future flood control projects at Stinson Beach, where a special election will be held in March on a parcel tax whose revenues would be used to fund a selection of projects that have not yet been chosen.