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As ocean levels rise, coastal communities face an ever-increasing risk of severe flooding. The existing infrastructure protecting many of these communities was not built to withstand the combined threat of rising seas and severe storms seen in this century.

While reinforcing existing flood barriers poses a costly challenge for at-risk communities, it also provides the opportunity to introduce innovative solutions that can provide both flood prevention and environmental benefits.

A group of researchers at UC Santa Cruz and the U.S. Geological Survey has evaluated one such flood mitigation solution, which can reinforce levees while creating environmentally beneficial coastal habitats. In a published on May 9 in Scientific Reports, the team evaluated the effectiveness of "horizontal levees"—traditional levees retrofitted with a sloping, wetland border—as a means of strengthening shorelines against the threat of rising sea levels.

"Ecosystems play a really strong role in reducing climate risk," said Rae Taylor-Burns, a postdoctoral researcher at the UC Santa Cruz Center for Coastal Climate Resilience (CCCR) and the study's lead author. "So conservation of intact ecosystems and restoration of degraded ecosystems can support community safety and community resilience to climate change."

The oncoming storms

Rising sea levels will make severe weather, which brings storm surge and intense waves, more impactful in the near future. By 2050, the coastal flooding from extreme storms we currently consider once-in-a-lifetime events could occur every other year due to sea level rise. Further, the flooding expected today from a once-in-a-lifetime event could occur daily by the end of the century.

Around $250 billion dollars of property and infrastructure in California is threatened by coastal flooding in the 21st century. The densely populated shores of the San Francisco Bay account for two-thirds of this projected risk from storms and rising sea levels. However, the bay's communities rely almost entirely on an aging system of levees for their protection; levees that were not built to withstand the extreme conditions driven by climate change.

Aquatic habitats—such as coral reefs, mangroves and wetlands—have been shown to effectively mitigate flood risk. However, the marshes that once lined the San Francisco Bay have all but disappeared as the shoreline has been thoroughly developed.

"We've built cities and communities and our world under the assumption that these habitats will continue to protect us," Taylor-Burns said. "And yet we degrade them."

A natural upgrade

Expanding the height of the levees currently protecting the San Francisco Bay shoreline would be a very expensive undertaking. Taylor-Burns and colleagues, with support from the California Ocean Protection Council, set out to study whether reintroducing wetlands to the bay's shoreline, through the construction of horizontal levees, could provide a cheaper, effective alternative.

A horizontal levee is a gradual slope of sediment extending off of a traditional levee, producing an artificial shoreline, the surface of which can become a wetland ecosystem. The plants on the levee reduce energy from oncoming waves while providing the environmental benefits of a wild habitat.

"It can also sequester carbon, support fisheries and provide recreation benefits," Taylor-Burns said.

In order to compare levee designs, the research team began with a meticulously designed and tested numerical model of San Francisco Bay, developed by researchers at the United States Geological Survey (USGS). Patrick Barnard, Research Director for the CCCR and a co-author of the study, aided in adapting these models to the study of horizontal levees in his previous role as a scientist at USGS.

"This was a really nice synergy between more fundamental model research and development, and the applied, solution-oriented work that the Center has been building upon," Barnard said.

The team calibrated the model using data from China Camp Marsh. The researchers then ran a series of computer simulations, throwing various sea level and storm conditions at horizontal and traditional levee designs to compare their risk reduction benefits.

The horizontal levees performed well in the simulations, improving the flood protection of the traditional levees by as much as 30%. The team's simulations showed that wider, more gradually sloping levees performed best at flood prevention.

The research indicates that horizontal levees may provide a cost-effective solution to coastal flooding. "Horizontal levees could be a less expensive way to reduce the risk of levee failure with climate change, as opposed to increasing the height of the levees themselves," Taylor-Burns said.

This study builds upon a decade of research on the environmental risk to coastal regions and potential adaptation solutions from the lab of UC Santa Cruz professor Michael Beck, who was also a co-author on the study.

"We built on previous work on coastal hazard risks and the engineering of nature-based infrastructure," said Borja Reguero, a professor in the UC Santa Cruz Coastal Science and Policy Program, the coastal hazards and engineering lab at UC Santa Cruz, and the study's principal investigator. "And then we applied advanced, specific methods for marshes and levees in the Bay Area."