As coastal flooding surges, ‘living shorelines’ seen as the answer
Salt marshes and oyster reefs prove more resilient than seawalls when big storms hit
On August 27, 2011, Hurricane Irene crashed into North Carolina, eviscerating the Outer Banks. The storm dumped rain shin-high and hurled three-meter storm surges against the barrier island shores that faced the mainland, destroying roads and 1,100 homes.
After the storm, a young ecologist then at the University of North Carolina at Chapel Hill named Rachel K. Gittman decided to survey the affected areas. Gittman had worked as an environmental consultant for the U.S. Navy on a shoreline-stabilization project and had been shocked to discover how little information existed on coastal resilience. “The more I researched, the more I realized that we just don’t know very much,” she explains. “So much policy and management is being made without the underlying science.” She decided to make shorelines her specialty.
What Gittman found was eye-opening. Along the hard-hit shorelines, three quarters of the bulkheads—typically concrete walls about two meters high that are the standard homeowner defense against the sea in many parts of the country—were damaged. Yet none of the natural marsh shorelines were impaired. The marshes, which extended 10 to 40 meters from the shore, had lost no sediment or elevation from Irene. Although the storm initially reduced the density of their vegetation by more than a third, a year later the greenery had bounced back and was as thick as ever in many cases.
Gittman’s study confirmed what many experts had begun to suspect. “Armored” shorelines such as bulkheads offer less protection against big storms than people think. By reflecting wave energy instead of dispersing it, they tend to wear away at the base, which causes them to gradually tilt seaward. Although they still function well in typical storms, they often backfire when high storm surges overtop them, causing them to breach or collapse, releasing an entire backyard into the sea.
In a later study, Gittman and other researchers surveyed 689 waterfront owners and found that the 37 percent of properties protected by bulkheads had suffered 93 percent of the damage. And bulkhead owners routinely had four times the annual maintenance costs of residents who relied on nature instead. Salt marshes looked soft but did not break.
In recent years, more scientists and policy makers have come to believe that “living shorelines”— natural communities of salt marsh, mangrove, oyster reef, beach and coral reef—can be surprisingly effective in a battle coastal residents have been losing for years. U.S. shores are disintegrating as higher seas, stronger storms and runaway development trigger an epidemic of erosion and flood damage. Every day waves bite off another 89 hectares of the country. Every year another $500 million of property disappears. Overall, some 40 percent of the U.S. coastline is suffering ongoing erosion. In some places, the rate of loss is breathtaking. Go to Google Earth Engine’s Timelapse feature and watch Shackleford Banks melt away like ice cream on a summer sidewalk.
Historically, almost all money spent on coastal defense has gone toward “gray” infrastructure: seawalls, bulkheads, levees and rock revetments. That is beginning to change as researchers become more sophisticated in measuring the long-term impact of “green” coastal defenses. Insurance companies and governments are finally taking notice and might actually turn the tide toward living defenses.
Wetlands outperform walls
Around the time that Hurricane Irene was bowling up the East Coast, Michael W. Beck, a research professor at the University of California, Santa Cruz, and then lead marine scientist for the Nature Conservancy, was initiating a collaboration with the insurance industry that today may begin to change coastal conservation. “A lot of people were saying that ecosystems worked for flood protection, but the evidence was thin,” Beck tells me at his Santa Cruz office. The physical mechanisms were clear: oyster and coral reefs limited erosion and flood damage by acting as natural breakwaters (offshore seawalls), dispersing wave energy with their corrugated surfaces. Salt marshes and man- groves, with their earthen berms and friction-generating forests of stalks, could rake more than 50 percent of the energy out of storm surges in less than 15 meters of territory.
But although scientists understood the physics, no one had put it into a form that could be used easily by policymakers. Beck set out to rectify that. “If I want to change practices, I can’t bring my ecosystem model to FEMA or the U.S. Army Corps of Engineers,” he explains. “I have to look at their risk model and put ecosystems into that.”
Beck and his colleagues began collaborating with Lloyd’s of London, Swiss Re and others in the insurance industry, which have some of the best data and models in the world on assets and risk. When he plugged data on coastal ecosystems into their risk models, it became clear that living shorelines were excellent defenses. And, he notes, “when I tell the Corps, FEMA and the development banks that these are the numbers from the insurance industry, I automatically have a different level of credibility.”
The first study focused on damages from Superstorm Sandy, which clobbered New York and New Jersey in 2012. Working with Risk Management Solutions, a leading risk-modeling firm, the scientists showed that wetlands prevented $625 million of flood damage from the storm, which was surprising given that the coasts in the region had already lost 60 to 90 percent of their protective wetlands over time. In areas that flooded, the few remaining wetlands lowered damage by 11 percent on average. As important was the ability to buffer garden-variety floods: in one local study, properties behind marshes suffered 16 percent less annual flood damage than properties that had lost their marshes. “That’s well within the range for which you could expect [insurance] premium reductions,” Beck points out.
He and his partners then turned their economic and risk-management models on the Gulf Coast, from Texas to Florida, which is regularly battered by big storms. They did an exhaustive analysis of the annual expected benefits and costs of all types of infrastructure. The team estimated that the coast would suffer $134 billion of losses over 20 years if no preventive measures were taken. Elevating homes could prevent $39.4 billion of those losses, but it is incredibly expensive. At an average of $83,300 per house, it would cost $54 billion to prevent that $39 billion in damages. The six-meter-high dikes being built in Louisiana were a worse option; at $33,000 per meter, they were an absurdly expensive way to protect a relatively limited amount of property, returning just $1 in savings for every $4 of expense. Smaller levees built on land in front of many low-lying coastal communities prevented much more damage for almost the same cost.
In terms of bang for the buck, sandbags were the best investment, saving $8.4 billion of damages for a mere $0.84 billion in expense. Natural defenses ranked high as well. Wetlands restoration, which could prevent $18.2 billion of losses, would cost just $2 billion. Oyster-reef restoration could prevent $9.7 billion in losses for $1.3 billion. Barrier island restoration offered $5.9 billion of prevention for $1.2 billion. And “beach nourishment” (replenishing depleted beaches with sand dredged from the sea floor) in the eastern Gulf could save $9.3 billion for $5.5 billion.
That last one surprised many people because replacing beach sand year after year is often seen as a fool’s errand. “If the only choices you gave me were beach nourishment versus fully gray infrastructure,” Beck says, “I’d choose the former as the lesser of two evils.”
Overall, the research found that $57.4 of the $134 billion could be prevented cost-effectively, almost all of it through green infrastructure.
One type of restoration that was not part of the study is large-scale diversion of the Mississippi River. Diverting sediment-laden water through a gap in the river’s levees and letting that sediment filter into struggling marshes can restore their health and elevation, but the region is subsiding so quickly that not even the famously muddy Mississippi can save it from the encroaching sea. “It is going to be expensive to recreate an entire ecosystem,” Beck says, “and it is better and cheaper to start earlier.”
Cost-effective restoration may be tricky on long, sandy coasts, too. Beaches and barrier islands are by nature transient. Planting grasses to rebuild dunes can help keep beaches in place but only temporarily in many cases. At some point, residents will have to move back from the receding shoreline.
Beck is quick to point out that built infrastructure is still incredibly important and that cost-effectiveness is not the only consideration. “Anywhere you’ve got significant people and property,” he says, natural solutions will “be used together with some form of built infrastructure.” Metropolitan areas, ports and other places where the risk tolerance for a major flood would be extremely low need seawalls, even if such structures are not cost-effective. Still, Beck says, certain populated areas can benefit from a hybrid approach. “Even if you’re building levees, they can be shorter if they have marshes in front.”
Shoreline trials and errors
One reason living shorelines are becoming an economically viable approach for coastal defense is that researchers and municipalities are getting better at rebuilding them. Early marsh-restoration designs, which followed forestry science and gave each plant plenty of space to avoid competition, were actually counterproductive. It turns out that in bare mud flats, “when marsh plants are together, they share oxygen, so their growth rate is twice as high,” says Brian Silliman, an ecologist at Duke University. Root them in large clumps, and the growth rate of each individual plant can triple. Add blue crabs, which eat the snails that eat the salt-marsh grasses, and the plants do even better. Scientists are also finding that marshes do best when they have a protective sill—a linear berm that fronts the seaside edge of the grass and stands. Made of hard material such as shell, stone or concrete, its height and position are typically chosen so that water covers it at high tide, but it is exposed during low tide. The sill takes the brunt of wave energy but also traps sediment behind it, allowing the grass to thrive and marsh floor to retain its elevation or even rise.
Almost any hard material can make a successful sill. Large shoreline-stabilization projects use big boulders or stackable concrete blocks, a practice that has been criticized by some experts who say that these structures are living shorelines in name only. But many lower-profile restorations integrate sills more seamlessly into the natural habitat. In the Southeast and Gulf Coast regions, marshes historically possessed a natural sill in the form of an intertidal oyster reef. Many of those reefs were overharvested long ago, ruining the sill and exposing the marshes to erosion.
In these warm, oyster-friendly waters, new sills can be formed by placing a hard substrate along the low-tide line at the front edge of the marsh for baby oysters to set on. Some sites with lots of wave action have used small, hollow concrete structures or plastic mesh “onion bags” studded with shell and lashed together. When successful, these artificial materials are quickly covered by oysters and disappear into the interstices of the growing reef. But the concrete often remains visible for years, and the bags have been criticized for breaking and scattering plastic through the environment.
Gittman, now at East Carolina University, is testing an alternative material called Oyster Catcher that is made of jute cloth dipped in Portland cement and rolled into various hollow configurations. It hardens with extensive surface area to recruit larval oysters. In addition to being light and flexible, it holds together just long enough to get a reef established, then disintegrates. The product received its first big test when Hurricanes Florence and Michael struck North Carolina last fall. Michael tossed shell bags up into the marshes, but the Oyster Catcher reefs didn’t budge. The showing was encouraging, but Gittman worries that conservation groups may oversell the potential. “A living shoreline can’t save your house from a Category 5 storm,” she says. “Although neither can a bulkhead.”
Gittman and Beck both stress the need to tailor living shorelines to local conditions. One reason oyster restoration is so cost-effective in the Gulf and the Southeast is because there have been plentiful wild oysters to seed new reefs with babies. That is not the case in most of the country. Chesapeake Bay, for example, was long the poster child for futile oyster restoration. Oyster populations in the bay had fallen to less than 1 percent of historical norms, and decades of effort and tens of millions of dollars barely budged the needle.
“Conceptually, Chesapeake Bay was not our best model,” Beck says. “It put oyster-reef restoration back because it made it look so difficult and expensive. Well, when you’re working in a system where you’ve only got 1 percent left, guess what? It ain’t easy. When you’re in the Gulf of Mexico and you’ve still got 50 percent of your reefs left, it’s a different story. If you build it, the oysters will come.”
Beck extends that lesson to coral reefs, the most underappreciated of natural defenses. “Coral reefs are the single most effective ecosystem for flood-risk reduction,” he says. Corals, which have evolved to take a daily pounding that would destroy most other living things, form natural seawalls exactly where you want them—just offshore, in front of resorts, beach towns, coastal roads and other pricey assets. When healthy, they make remarkably effective breakwaters, reducing wave energy up to 97 percent. They are also affordable: reef restoration averages about $1,300 per meter, versus $20,000 for artificial breakwater construction. The insurance industry’s assessment for mitigating risk from climate change in the Caribbean found that reviving reefs and mangroves was an order of magnitude more cost-effective than seawalls or breakwaters.
Even though reefs do not line a lot of shorefronts, the annual expected benefits they generate are significant—more than $100 million a year in the U.S. alone and more than $400 million a year each in Mexico, Malaysia, Indonesia, the Philippines and Cuba.
Of course, many coral reefs are not healthy, and losing just a single meter of reef height doubles the direct damages from flooding. For that reason alone, Beck believes reef-restoration projects will multiply. Although the science of coral restoration is young, the potential is enormous—so long as a reef has not already collapsed. “Some of these corals actually grow pretty fast,” Beck says. “For example, in places in Indonesia where there’s still good reef habitat and lots of healthy corals around small sites that have been destroyed by blast fishing, reefs can turn around pretty quickly.”
Rising tide of support
Coastal restoration may finally be getting the attention it deserves. “Things are really beginning to change,” Beck says. The Army Corps, which for decades has favored hardscape solutions, has launched an Engineering With Nature initiative—something many planners thought they would never see. The National Oceanic and Atmospheric Administration has made living shorelines a centerpiece of its coastal-resilience blueprint. Hundreds of projects have been completed or are underway around the country, ranging from shoreline stabilizations in Maryland to levee removal in Puget Sound. Most are small, community-based efforts, but larger ventures are becoming more common.
Stimulus funding that flowed after the American Recovery and Reinvestment Act of 2009 increased the size of some projects significantly. Kilometers of oyster-reef projects now line Alabama, Texas and Louisiana. The flagship is Coffee Island, off the Alabama coast. The shoreline had receded up to 100 meters. The Nature Conservancy placed a three-kilometer-long line of shell bags and concrete balls about 30 meters offshore, paralleling the island. The reef immediately blocked wave energy, allowing the marsh to rebuild. Within two years approximately 200 baby oysters per square meter had colonized the structure, covering it and attracting fish, crabs and birds.
Outside the Gulf Coast and the Southeast, restoration projects may be more challenging. California, for example, is a tough challenge. “In San Francisco Bay,” Beck says, “we’ve lost more than 90 percent of the natural marshes, so you have to go in and recreate an environment wholesale in and around a hell of a lot of people.”
Yet where there is a will—and local money—there is a way. The San Francisco Bay Clean Water, Pollution Prevention, and Habitat Restoration Measure, passed by Bay Area voters in 2016, raises $25 million a year for 20 years through a parcel tax. That $500 million will be used to build 40,000 hectares of wetlands — the largest shoreline restoration undertaken in the U.S.—using various techniques. The most novel is horizontal levees. Instead of a high, narrow mound that lines the shore, horizontal levees are broad mud flats, marshes and grasslands that gradually rise from the water’s edge for hundreds of meters back onto the land. They are graded with vast amounts of earth (often repurposed from building projects) and planted with starter plugs. They can be lower and 40 percent less costly than a traditional levy because the depth absorbs floodwater. The configuration also gives marsh communities space to retreat as seas rise.
Another encouraging sign is the Living Shorelines Act, introduced in the U.S. House of Representatives by Frank Pallone, whose New Jersey district was devastated by Superstorm Sandy. The bill would designate $20 million in grants a year to living- shoreline projects. A Senate version was introduced by Chris Murphy of Connecticut and Kamala Harris of California. Their prospects in the current Congress were uncertain at press time, but their existence shows that living shorelines are gaining ground.
North Carolina’s Coastal Resources Commission recently approved a new process that will make it as easy to obtain a living-shoreline permit as that for a bulkhead. Maryland has an even stronger law in place, requiring a homeowner to prove why a bulkhead is needed instead of a natural shoreline. Other states may follow these leads.
The most promising indication of all may be the 2018 agreement made by the Nature Conservancy, the reinsurance industry and the Mexican state of Quintana Roo to create a trust fund to protect the Mesoamerican Reef, off the coast of Cancún and Puerto Morelos. The deal will include the first insurance policy ever taken out on a natural ecosystem. If the reef is damaged by a storm, insurance funds are released to rebuild its natural capital.
For living shorelines to become an important part of any long-term coastal defense plan, policymakers in government, insurance and development will have to start improving and installing them before bad storms hit—and funding the next round of projects through post-disaster spending. That requires good science and good economic numbers—which now exist—as well as good proof in the form of demonstration projects, which are increasingly common.
The first significant examples of post-disaster spending on natural infrastructure could occur as FEMA and other agencies look to spend more than $100 billion in recovery funds from recent hurricanes. Although FEMA’s traditional hazard-mitigation investments have focused on tactics such as buying out damaged coastal homes or elevating them, the agency has adjusted its new “benefit-cost analysis” policy to favor investment in natural infrastructure. Beck expects this change in emphasis to result in federally funded projects of unprecedented scope in Florida, Puerto Rico and the Gulf Coast. Other large-scale development may soon follow worldwide as governments, disaster-risk managers, businesses, banks and insurers look to mitigate their risk exposure as cost-effectively as possible. When that happens, it will mark a moment when society realizes nature is not a luxury. It is the future.