Wetlands Break Waves, Quell Surge
Coastal Landscape Battles Weather to Protect Mainland
Louisiana’s coastline has been a top pick over the last century for major hurricanes making landfall.
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.”
The most powerful storm ever observed in the Gulf of Mexico, Hurricane Rita made landfall near Louisiana’s western border September 24, 2005. The huge weather system pushed high winds and drenching rains northward across states from Texas to Florida.
NASA image courtesy Jeff Schmaltz, from the MODIS Rapid Response Team at NASA GSFC
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.
Can a stalk of grass resist hurricane forces? Multiplied over hundreds of acres, wetland vegetation does moderate storm surge as well as wind and wave energy.
USDA NRCS Golden Meadows PMC
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 Department of Oceanography. “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 hurricane-generated 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 and the wetlands that lie behind them.”
The Bigger the Bump, the Better the Barrier
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 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. The following illustrations show the different effects of storm surge on barrier island sediment.
1
Impact Level 1: Storm waves reach no higher than the beach. The beach will erode, but over time normal wave action will return the sand to the island. Sand remains within the system.
2
Impact Level 2: Waves reach higher than the base of the dune. The island’s beaches and dunes will erode and retreat, perhaps permanently.
3
Impact Level 3: Waves overtop the barrier island. If waves reach over the dune, or if there is no dune, sand will be pushed landward. Over time, the island will migrate toward land.
4
Impact Level 4: If the storm surge is high and the elevation of the barrier island is low, the island can become completely inundated. Sand is removed and transported over the island into the bay behind it.
U.S. Geological Survey