Rebuilt Wetlands Can Protect Shorelines Better Than Walls

Sourced from Scientific American

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 were damaged. The walls, typically concrete and about two meters high, are the standard homeowner defense against the sea in many parts of the country. 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 bent 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.

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