Fortifying the bay's shoreline with levees fronted by restored tidal marshes is a cheaper, more aesthetic and ecologically sensitive way to protect Marin and other bayside counties from sea level rise, according to a new report by a Bay Area environmental group.
The Bay Institute's report — the subject of a panel discussion earlier this month in San Francisco — proposes restoring tidal marshes with sediment from local flood control channels and irrigating the marshes with treated wastewater. The plan also calls for "horizontal levees" that are a hybrid of traditional earthen levees and restored marshes. The conclusion was based partly on research done in the lower Corte Madera Creek watershed.
Tidal marsh restoration in the bay has been a priority for environmental groups since the 1970s. More than 5,000 acres have been restored in the past two decades, with another 30,000 acres purchased and slated for restoration.
"Marshes do a great job of protecting land from storm surges," said Beth Huning, coordinator of the Marin-based San Francisco Bay Joint Venture, which brings together public and private agencies and others to restore wetlands and wildlife habitat. "They slow down the wave action and that will be important as sea levels rise."
The tall, dense vegetation of tidal marshes absorb a significant amount of the energy of surging ocean waves during storms.
"The concept is a good one. The physics of it are accurate," said Lisamarie Windham-Myers, a wetland ecologist at the U.S. Geological Survey. "It's been proven over and over that wetlands help reduce storm surges." Therefore, she said, levees don't have to be as tall.
The Bay Institute estimates that shorter levees fronted by tidal marshes would bring down the cost from more than $12 million to less than $7 million per mile, while providing the same level of flood protection. With 275 miles of bay shoreline to protect, total savings could eventually exceed more than $1 billion.
"We knew the cost would be reduced, but we were shocked at the actual savings," said Marc Holmes, the Bay Institute's marsh restoration program director.
Recent work in the Corte Madera watershed by the Bay Conservation and Development Commission — which regulates shoreline development — helped demonstrate to researchers how waves attenuate over marshes, sapping the waves' power as they hit land.
The Bay Institute report was released after U.S. Sens. Dianne Feinstein and Barbara Boxer reintroduced the San Francisco Bay Restoration Act, which seeks federal funding for wetland restoration and water improvement projects in the bay and its watershed.
"We hope we have now given them the ammunition to say, 'Look, this is not going to cost us money, it is going to save us money,'" Holmes said.
As carbon emissions cause the Earth to warm, polar ice melts and warmer ocean water expands. Sea levels in the bay have risen 8 inches in the past century, leading to occasional flooding of major regional roadways such as Highway 37 — which links Marin, Sonoma and Solano counties — and Highway 101 during winter storms.
Though rising sea levels are a concern, winter storms riding in on higher tides can cause the most havoc.
"In the next century, we're going to get more storms, fiercer storms," Holmes said. "Locations that were once outside of the danger zone are now inside, simply because storms are arriving on higher sea levels."
The goal of the Bay Institute study was to find a way to build a cost-effective network of levees that could lessen the flood threat caused by storm surges, while also providing benefits to the environment. The Bay Institute report imagines tidal marshes filled with silt to create a gentle upward slope from the bay shoreline to the top of a wedge-shaped earthen levee. Near the levee, tall, quick-growing plants with deep root systems would be irrigated with wastewater from nearby water treatment plants.
The sloping marsh can slow down storm surges, and the dense vegetation can absorb it like a sponge. Using marshes to buffer storm surges means earthen levees built on the landward side of the marshes can be built half as tall. As the vegetation grows taller and the root systems expand, the horizontal levee will be able to protect against the rise in sea levels expected in the coming decades, provided the restoration begins sooner rather than later.
"The report shows healthy wetlands are the best way and the cheapest way to help protect homes, highways and businesses," said Caroline Warner, public outreach coordinator for the San Francisco Bay Joint Venture.
The Bay Institute report also recommends that Congress allocate $1 billion to establish a program administered by the Environmental Protection Agency that would coordinate the various efforts to restore and protect the bay. Congress has established a handful of comparable programs for nationally important regions such as the Great Lakes and Chesapeake Bay.
"Obviously," Holmes said, "we could make that case in San Francisco Bay persuasively."