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February/March 2016

How Restoring Wetlands Will Prepare Us for Sea Level Rise

Last October, an excavator breached an old levee at Sears Point to initiate restoration of farmland back to tidal marsh. Photo courtesy of Sonoma Land Trust/Corby Hines.

By Robin Meadows

After California’s worst drought in 500 years, we’re finally enjoying a rainy winter thanks to one of the strongest El Niños on record. Droughts interspersed with drenchings are nothing new for us — these extremes are part of our normal weather cycle — and periodic wet years are nothing we can’t handle. But that’s about to change. In coming decades, sea level rise will amplify the storm surges and ultra-high “king” tides that send waves crashing over levees.

Making matters worse, sea level rise will also weaken the Bay Area’s resilience to floods. Tidal marshes edging the bay take the oomph out of waves and soak up water like sponges. However, according to a 2015 State Coastal Conservancy-led report, we stand to lose most of this natural flood protection to rising seas. The cost of an extreme storm to the Bay Area is estimated at $10 billion.

“Many of our salt marshes will be drowning,” said San Francisco Estuary Institute scientist Jeremy Lowe. “When they’re under water too long, the plants will start dying off and then we’ll have mudflats.” And while mudflats also help control floods, they are not nearly as effective.

Marshes won’t be the only things drowning. We’ve built cities and roads all the way down to the bay, and as it goes up, they will start to go under. “We need to start thinking about how to live with the bay as it moves,” Lowe said. Sea level rise is projected at roughly one to five feet by the year 2100, and our cities and roads can’t be easily reengineered to keep up with it. But our marshes can.

Tidal Marshes Then and Now

We have time, but we must start now — it takes decades to restore a tidal marsh. Fortunately, we’ve been restoring marshes here for about 40 years, so we’re good at it. Altogether, the bay needs 100,000 acres of tidal marshes to do the job. This is just over half their historical area circa 1800, before we started diking and draining them for agriculture, salt ponds, and other uses. Marshes around the bay were down to 40,000 acres in 1998, and since then about that many more have been restored or are in the works, leaving about 20,000 to go. Other reasons to restore these wetlands include that they help purify water, and provide habitat for at-risk species such as the California clapper rail and the salt marsh harvest mouse.

Most of the original marsh was in low-lying lands fringing the North, Suisun, and South bays, and that’s where most of the restoration is too. Of course the Central Bay also had wetlands, but they were smaller due to steep, rocky shorelines. Even so, the East Bay can still be a key player in adapting to sea level rise.

Giving Wetlands Room to Move

As the water creeps higher, marshes will need to shift inland. In the East Bay, a $2 million experimental levee — part of a $9 million project at the Oro Loma wastewater treatment plant in San Lorenzo — is testing a new way of giving wetlands room to move. The site used to have a wall-like levee right along the bay, which would have blocked wetlands from moving inland. Now, the levee is a gently sloping wedge that stretches up from the bay; this will let marshes migrate up the slope as sea level rises. “We hope to expand this upland restoration to the whole shoreline of the East Bay,” said Lowe, who directed the project.

The sloping levee mimics the gradual transition from wetlands to uplands and should, like natural marshes, slow waves from storms and king tides. “It’s a cool idea,” said UC Berkeley environmental engineer David Sedlak. Grasses, sedges, and other native plants will stabilize the levee, keeping the soil in place and building it up. To give the plants a head start in their manmade wetland, they will be irrigated with effluent from the wastewater treatment plant.

Native plants could also purify the effluent of nitrate, a nutrient that can cause harmful algae blooms. Most of the nitrate in the bay is from urine. “It comes from us,” Sedlak said, adding that our wastewater treatment plants do not remove nutrients, and that retrofitting them to do so could cost more than a billion dollars.

The Latest in Marsh Restoration

More traditional marsh restoration also needs to prepare for sea level rise, and a new project in Sonoma County’s Sears Point incorporates a sloping levee as well as what we’ve learned from previous restorations. The Sonoma Land Trust is restoring nearly 1,000 acres of diked agricultural land on the bay side of Highway 37. In addition to keeping water off the highway, the new sloping levee’s uplands will give wildlife a place to go during king tides.

Sears Point was diked and pumped dry 140 years ago, and exposure to air made the soil decompose and subside. It will be rebuilt naturally over the next 20 to 30 years, as tides bring in sediment. “We’re relying entirely on the bay and tides to bring in six feet of soil,” said project manager Julian Meisler.

Sediment won’t settle out when water is choppy, though. “The site is nearly three miles long and that’s enough for the wind to make waves,” he said. “We need calm conditions.” The old way to break up waves was finger-like peninsulas extending from the shore into the restored marsh, but these also let predators like coyotes trot in. Instead, the Sears Point project is dotted with more than 500 island-like mounds to break up the waves.

Letting the tide rebuild the marsh will cut costs but, at nearly $18 million to buy and reengineer the land, the project is still expensive. The same holds elsewhere around the bay. To help raise funds for the remaining wetland restoration we need, the San Francisco Bay Restoration Authority has placed a region-wide, $12 parcel tax on the June 7, 2016 ballot. The tax would raise $500 million over 20 years, enough to build 20 miles of new levees and restore an estimated 15,000 acres of wetlands.

Robin Meadows covers water for the Monitor.

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