Coastal Landscape Battles Weather to Protect Mainland

Watermarks ^ | March 2006

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Coastal Landscape Battles Weather to Protect Mainland taking on the might and fury of these storms are the state’s barrier islands and coastal wetlands — a fragile, yet proven, line of defense. “Barrier islands and marshes can’t stop the full force of a category 4 or 5 hurricane,” says Jack Kindinger, associate director of the United States Geological Survey (USGS) Florida Integrated Science Center. “But because these natural buffers reduce the effects of daily wave action and winter storms on shorelines and levees, a coast protected by wetlands will fare better in a storm of any size.” Marshes Soak Up Surge As a hurricane moves inland, coastal marshes deprive it of the warm moisture that fuels the storm’s growth. Wetlands also diminish a hurricane’s destructive power by reducing storm surge and absorbing wave energy. “Well-vegetated wetlands absorb much of the surge of category 1 and 2 storms because of their elevation and the friction the grass provides,” says Kindinger. Against the strength and surge of category 4 and 5 storms, he says, wetlands have a more limited effect. T Louisiana’s coastline has been a top pick over the last century for major hurricanes making landfall.

How much protection do wetlands offer? Many variables determine how well marshes reduce storm surge, including the slope of the continental shelf and the speed and direction of storm winds. Studies suggest that it takes as little as one mile or as many as four miles of functioning wetlands to reduce storm surge by a foot.

Throughout the year, wetlands protect levees from the erosive effects of waves by reducing their height and intensity. “A levee protected by marsh will require much less maintenance than will a levee exposed to daily tides and waves,” Kindinger says. Storms Stumble Over Sandy Speed Bumps Delicate ribbons of marsh and sand, barrier islands seem too small and fragile to have any effect on a powerful hurricane hundreds of miles across. But the severe damage these islands endure — often losing as much as half their land area — is proof of their effectiveness. “It takes a lot of force to remove that much sediment,” says Greg Stone, James P. Morgan Professor at Louisiana State University’s Coastal Studies Institute and Depart-

Many factors determine how well barrier islands stand up to hurricanes, including storm size and strength and the islands’ shape and elevation. Wide, moderately tall islands, such as those along the Florida coast, tend to fare well, says LSU’s Greg Stone. “The sand transported from an island’s beach and dunes by storm surge needs someplace to go, and a wide island offers a U.S. Geological Survey

The Bigger the Bump, the Better the Barrier platform for holding that sand,” Stone says. “Because Louisiana’s barrier islands are typically low and narrow, sediment tends to be pushed over the islands and into the bays behind them.” If that sediment isn’t replaced, the island will eventually disappear, as is happening to many Louisiana barrier islands.

“That’s energy depleted from the storm’s surge and the wind-driven waves on top of it. Think of a barrier island as a ‘speed bump’ that dissipates hurricanegenerated power.” Barrier islands, particularly those close to the mainland, also protect coastal marshes. “Where barrier islands have weakened or disappeared, bays are exposed to higher wave energy, which accelerates wetland loss and makes the coast even more vulnerable,” says Stone. “Putting money into rebuilding and fortifying levees and improving flood protection is vital, but it must be done in conjunction with the restoration of barrier islands.