Ideas for monumental scale interventions have started cropping up on the desks and drawing boards of Bay Area planners faced with the prospect of sea level rise. “We’re hearing about tidal barriers at the Golden Gate, and bigger and bigger levees, but then what happens to our marshes and mudflats?” says engineer Bob Battalio of the consulting firm ESA PWA. The answer, according to a new model developed by Battalio and colleague Jeremy Lowe — with the help of coastal ecologist Peter Baye, the Ora Loma Sanitary District’s Jason Warner, and East Bay Dischargers’ Mike Connor — is to integrate flood control with habitat restoration in a way that could bring back a lost ecosystem and streamline wastewater treatment, at a significant cost saving over traditional approaches. A recent report by The Bay Institute calls this concept the “horizontal levee.”
The term was borrowed from the Netherlands, but that’s the only resemblance to Dutch flood control practice. “They use large levees with asphalt and turf grass,” Battalio explains. “We emphasize natural shoreline formations and ecological function.” What’s being proposed is a gently sloping rise from the salt marsh along the Bay up into a wet marshy meadow in front of the levee (see diagram). Depending on lots of variables, the wet area on the landward transitional edge can be a “freshwater meadow,” “back marsh,” “brackish marsh,” or for purists, “estuarine terrestrial ecotone.”
The tidal marsh slows wave action and prevents levee overtopping in a flood or storm surge, while the upper marshy area gains elevation. Plant species adapted to fresh or brackish water grow faster and put down more roots than salt marsh species. By bulking up faster, and tolerating thin sediment deposits, the transitional marsh can keep pace with sea level rise longer.
“The concept is pretty simple: it’s a big, extra-wide, wet levee, runny (seeping) and supporting wetland vegetation on the slope instead of artificially dry upland vegetation.” says Peter Baye.
What’s missing for the Bay scenario is enough sediment and a source of fresh water to the transitional marsh, which was once fed by natural runoff. And that’s where the wastewater from Ora Loma, and material dredged out of flood control channels, comes in.
“The really new thing here is the idea of using treated wastewater to facilitate a brackish transition marsh farther upslope,” says Battalio. Treated wastewater may contain nutrients, which act like fertilizer. “Slope marshes with high nutrients are like wet prairies — very productive, depositing stable belowground biomass in soil that sequesters carbon and nutrients,” says Baye.
The proof of the concept may come at the Hayward shoreline. As Lowe explains it, a city council member persuaded the Hayward Area Shoreline Planning Area to commission a study of the shoreline’s future from ESA PWA. That led to the design of a demonstration project and The Bay Institute report. Lowe says Ora Loma Sanitary is interested because of the water quality benefits and potential reduction in operations costs. According to The Bay Institute’s Marc Holmes, the S.F. Bay Regional Water Quality Control Board also seems favorably inclined. He’d like to expand the scope to include other parts of the South Bay. But what may make the demo work in Hayward is a combination of forward-looking leadership and an effective local governance structure. Holmes hopes other cities will buy in, building a legislative campaign.
As for cost, Holmes says he was surprised by the difference between the horizontal levee and a more traditional 13-foot-high mound of dirt: “We did not anticipate the scale of the savings.” The study suggests that a traditional levee without a marsh would cost more than $12 million per mile over 50 years. With a 200-foot-wide salt marsh on the Bay, and/or a brackish marsh on the upland side of the levee, the price almost halves (see graph).
To Lowe and Battalio, using natural topography and native vegetation to knock down waves is not a radical departure. Others have floated the idea in the recent past. “Using the natural environment as much as possible is the kernel of our practice,” says Battalio. “We need to recognize the value of mudflats and marshes, and consider where we draw the line. Otherwise we might wind up spending more money and being less effective.”