2/23/13
As global warming escalates, San Francisco Bay's existing flood protection system will be no match for rising sea levels. But according to a new report by a Bay Area environmental group, fortifying the bay's shoreline with levees fronted by restored tidal marshes will be a cheaper, more aesthetic and ecologically sensitive alternative to traditional levees.
The Bay Institute's report 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.
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 act as the lungs of the bay," said John Bourgeois, manager of the South Bay Salt Pond Restoration Project. "They can clean and filter the water that comes down our tributaries before it hits the bay."
The tall, dense vegetation of tidal marshes can also 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 a billion dollars.
"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.
Funds to build and maintain levees have come over the years in piecemeal chunks from the federal government and local floodplain control agencies. The result has been a patchwork quilt of aging earthen levees, designed to protect against present-day sea levels.
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 South Bay Salt Pond Restoration Project is considering a similar "horizontal levee" for its Alviso flood protection plan, which will be released later this year.
