County planners call for sea-level barriers on bay
Projects would protect shoreline near Inverness, Marshall
Marin County is proposing to test two sealevel rise defense projects on Tomales Bay to prevent the flooding of a vital evacuation route for more than 1,000 residents and wildlife habitats.
The two sites include Martinelli Park — best known by residents, visitors and photographers as the site of the S.S. Point Reyes shipwreck — near Inverness as well as the Audubon Canyon Ranch Cypress Grove wildlife preserve near Marshall. Given the sensitive habitat and popularity with the public along the 13-mile bay, the county plans to create natural barriers known as living shorelines as opposed to building seawalls.
Federal sea-level rise projections show much of the Tomales Bay shoreline faces regular flooding in the coming decades. While these projects are only in the early conceptual phases, county staff said they could eventually help to inform future defense projects along the entire bay.
“There are areas of the shoreline that already flood,” said county planner Leslie Lacko. “There are populations, neighborhoods where if it floods on Sir Francis Drake Boulevard there is no way out for those people until the flooding subsides.”
“Living shorelines are one possible alternative to hard shoreline armoring,” Lacko said. “Given that Tomales Bay sits in a national marine sanctuary makes it very difficult to put in hard
“There are populations, neighborhoods where if it floods on Sir Francis Drake Boulevard there is no way out for those people until the flooding subsides.”
— Leslie Lacko, county planner
shoreline armoring, it’s important to have some extra tools in the box.”
In the last two years, county staff and the Environmental Science Associates consulting firm mapped out the major features of the Tomales Bay shoreline and modeled how future sea-level rise and erosion would impact them. The two candidate project locations were chosen to test natural defenses against sea-level rise of about 1.6 feet projected by the U.S. Geological Survey to occur between 2038 to 2045.
The 7-acre Martinelli Park was selected in large part because of its proximity to a low-lying section of Sir Francis Drake Boulevard. The road is the only escape route for residents living in communities on the western shores of the bay such as Inverness.
The site includes marshland to the north where the wreckage of the S.S. Point Reyes sits next to the outlet where the First Valley Creek flows into the bay. The southern end includes a parking lot, two stores, marshland and a walking path that acts as an “Achilles heel” for future flooding on Sir Francis Drake Boulevard, said Environmental Science Associates engineer Dane Behrens.
The southern side of the marsh is not as protected, which puts it at greater risk from overtopping by high tides and waves that could flood on to Sir Francis Drake Boulevard. At 1.6 feet of sea-level rise, the study projected the area could have 56 yearly flooding events.
The S.S. Point Reyes, which washed up on the site in the 1970s or 1980s, would remain in place. The wreckage has worked to trap sediment which, over the decades, formed a beach and cordgrass stands on the northern end of the park that have protected the nearby marsh, Behrens said.
County staffers plan to replicate the defenses on the southern end of the marsh by creating a small beach that gradually slopes up to what would likely be a rebuilt walking path. The result would be to reduce wave
energy and prevent further erosion of the marsh.
“The goal of that would be to knock down some of the waves but also allow more trapping of sediment in this marsh area so that it’s more resilient to sealevel rise, so it has more of a chance of being there in the future and providing that coastal buffer to protect the shoreline,” Behrens said.
The walking path is supported by a vertical wooden berm and has a low-lying section at its southern end. The project would raise the path and replace the berm with a gradual slope into the marsh.
“By raising the path and tying that into higher locations, we would be lowering the risk of coastal overtopping and raising the threshold when flooding would happen,” Behrens said.
The county would also lower the berms along First Valley Creek to widen it and allow sediment in the water to be distributed more evenly. The changes could impact access to the shipwreck, but the study states this could be addressed by creating a creek crossing.
Cost estimates for this project range from $600,000 to $1.4 million. However, Behren said the project costs would ultimately be lower compared to keeping the area as it is and making repairs from the more frequent flooding.
The Cypress Grove preserve on the eastern shore
of the bay has already experienced flooding, most notably in 2006 when several Audubon Canyon Ranch facilities and homes were flooded during a storm. The preserve was donated to Audubon Canyon Ranch for its research efforts in 1988 and is used to study seabirds and shorebirds.
The county is proposing to restore a beach that once existed along the southern end of the property. To do so, crews would place wooden logs known as drift sills along the eastern shoreline as well as deposit sand at different locations. These logs could also be used for the benefit of recruiting native oysters to improve the local habitat.
Early estimates show the project could cost about $1.3 million to $2.7 million, depending on how much sand needs to be brought in, according to the study. In comparison, raising the existing Cypress Grove buildings is estimated to cost $1.5 million to $3.3 million while relocating them would cost $1.3 million to $2.7 million.
Nils Warnock, conservation science director at Audubon Canyon Ranch, lives at the Cypress Grove site with other researchers and helped advise the creation sea-level rise defense study. He said the research nonprofit had been planning for an eventual retreat and relocation from the site in the coming years, but would prefer if possible to keep it where it is.
“You can’t work in your office when it’s got a lot of water in it,” Warnock said. “It’s a historic site, it’s a beautiful site and it’s a functional site. We would always love to keep it here where it is and the state it’s in.”
One complication is that the property is next to waters protected under the Greater Farallones National Marine Sanctuary and includes sensitive eelgrass beds used by a variety of wildlife. Any impacts to eelgrass beds from the project would more than likely require the county to replant eelgrass in other locations of the bay.
“It’s a little bit of a balancing act,” Behrens said.
To address this, county staff are envisioning working with state and federal agencies to create a firstof-its-kind plan detailing the location of existing eelgrass beds and sites where new eelgrass beds could be established.
“Getting a program like that place in Tomales Bay will be really critical to doing more projects like this,” Lacko said.
The projects are still several years out. The county will need to obtain the funding to design and study the environmental impacts of the projects as well as get buy-in from the various private landowners.
More information about the projects is online at marincounty.org/main/ marin-sea-level-rise.