Can nature-based alternatives to seawalls keep the waves at bay?
Areas across the US staring down a greater risk of flooding due to extreme weather and sea level rise are turning to a stark solution: building a wall. In Miami, a 20ft seawall has been proposed. In Charleston, South Carolina, a $1.1bn plan to build a seawall around the city’s downtown awaits sign-off from Congress. And in New York City, a 2.5-mile seawall under construction along the shoreline of lower Manhattan is expected to be finished by 2026.
Seawalls have long been used to protect people and property against coastal flooding.
But by solving one problem in one place, these concrete slabs can cause a myriad of issues elsewhere: scientists caution that seawalls can intensify surges at neighboring beaches and destroy nearby ecosystems. As a result, rather than representing a surefire way to reach flood resiliency, these structures may actually open the door to greater climate vulnerability.
However, more and more places are using nature-based solutions to protect against flooding and storm surges.
“Sea level rise is inevitable,” said Danielle Bissett, the director of restoration for Billion Oyster Project. “We have to allow the shoreline to adapt and be resilient rather than put up seawalls, which are obstructive.”
The non-profit hopes to restore 1 billion oysters to New York Harbor by 2035, in an effort to improve the area’s flood resiliency.
The organization also works with Living Breakwater, a nature-based green infrastructure in the works along the Staten Island coastline, to cultivate the region’s shellfish habitat. Overseen by New York state governor’s office of storm recovery, this $107m effort to mitigate storm surges through living barriers has installed two breakwaters – a series of rock piles that blunt the impact of waves – off the borough’s coast. A total of eight breakwaters are planned.
“We can’t build single-purpose infrastructure any more,” said Pippa Brashear, project manager for the Living Breakwaters. The structure that comprises granite rocks and eco-concrete, along with the biological activity that will latch on to and grow out of these structures are intended to work together. “We’re making something that’s not only a structure that knocks down waves, it’s a structure that supports life,” Brashear said.
Staten Island had been experiencing at least a foot of erosion a year since the 1970s. “By restoring the reef and the marine life which we’re already seeing moving back into the area, not only does the breakwater protect further erosion it attenuates the waves,” said Katie Brennan, executive director at the governor’s office of storm recovery.
Further north, researchers in Boston are experimenting with other alternative paths to rigid infrastructure. The “Emerald Tutu”, a system of interlacing floating mats with growing vegetation is under development to protect city coasts from sea level rise and storms.
“We’re not creating a solid barrier, but we’re using a network of these little units around the shoreline to achieve wave energy dissipation,” said Julia Hopkins, assistant professor in civil and environmental engineering at Northeastern University.
The first ever US seawall was constructed by the army corps of engineers in 1901 in Jamestown Island in Virginia. Since then, the agency developed its own guidelines, regulations and standards to build hundreds of seawalls along the US coastline. But with the nature-based solutions to storm surges, there is no similar blueprint.
“We need to look beyond traditional civil engineering,” Hopkins said. “We’re entering into a climate era that is unknowable, and we need solutions that are flexible.”
Such solutions vary from coast to coast. In the Chesapeake Bay, for example, the restoration of mangrove is under way. These trees can provide shelter for marine life and slow down waves. In Florida, marshlands and wetlands play a key role in reducing the storm surges. And in California, coastal dunes that form when wet sand is washed ashore can become obstacles to the wind that carries the bulk of storm damage.
“Marshes and mangroves and other coastal vegetation really do a great job of helping to prevent erosion and trapping sediment so that you’re not eroding the edge,” said Chela Zabin, marine ecologist at the Smithsonian Environmental Research Center who’s working on oyster reef restoration at Living Shoreline project, which stretches along the San Francisco Bay.
Anytime a wave hits something that has any kind of texture and ridges, it causes turbulence and that takes wave energy out and allows fine particles to settle out. “So you get both the wave protection and also sediment buildup,” Zabin says.
But not every place is well-fitted for a soft defense against floods. In downtown San Francisco, for example, the waterfront was built on filled land that required artificial protection from the start. And because it never had natural defenses, a hard, concrete barrier may be necessary to protect it.
“You have this heavily urbanized area right up against the water and a working waterfront, there’s not a whole lot of other options but a seawall or a bulkhead,” Zabin says.
With intensifying sea level rise, coastal erosion and ecosystem damage, living shorelines and nature-based approaches are now showing up in different forms across the country.
“I would say the thing everyone should strive for is working as much with nature, not against it, because nature always wins,” Bissett said.