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Coastal communities built atop drained salt marshes and leveled mangrove forests had little natural flood protection from hurricanes Irma and Harvey, exposing the need for better planning against future storms and sea-level rise, marine scientists and environmental engineers told Bloomberg BNA.
“As they say, floods are natural events, disasters are human-caused events,” Bruce Stein, vice president for the National Wildlife Federation’s conservation science and climate science adaptation program, said. “It is where and how structures and people are put into the path of flooding, or other natural hazards, that determines the extent and cost of property damage.”
Harvey’s historic rains created storm surges that turned the concrete urban sprawl between downtown Houston and Galveston Bay into a flowing river, and inundated cities like Rockport and Port Arthur along the Texas Gulf Coast.
Irma’s 130 mile-per-hour winds created storm surges of up to 10 feet that overwhelmed what little is left of natural defenses posed by mangroves and sea marshes in the Florida Keys, Miami Beach in southern Florida, Tampa Bay along the Gulf Coast, and other cities in central Florida.The flooding and property damage could have been mitigated in some communities had Florida and Texas done more to protect coastal wetlands, mangroves and salt marshes—or even added more man-made levees and sand dunes as storm barriers—Stein said.
Coastal wetlands play an important role in reducing wave energy, and a smaller role in mitigating storm surges of the kind that Irma unleashed in South Florida, said marine scientist Molly Mitchell at the College of William & Mary’s Virginia Institute of Marine Science.
But the resiliency of coastal communities can be improved if multiple lines of defense are employed by using wide swathesof salt marshes or mangroves that are backed by engineered levees, Mitchell said. “The natural structure will help break up the wave energy, while the engineered structure will provide the defense against the wave elevation,” she said.
“Ideally it would have to be be combined with some hard infrastructure for protecting critical facilities like nuclear power plants,” Thomas Wahl, assistant professsor for coastal risks and engineering at the University of Central Florida, said.
But coral reefs, which are far more efficient than wetlands at reducing wave energy, can’t be grown everywhere, Wahl said, and “you can’t build a dike on a sandy beach in Florida. The water would seep right through.”
But there are steps that can be taken.
Mitchell pointed to the Netherlands, where some 227 kilometers (141 miles) of dikes protect the Dutch mainland and barrier islands.
But the Dutch wanted a solution that would be more responsive to the natural conditions of the coast, as well, and in 2015 decided to adapt its flood defenses along the Wadden Sea coast to include salt marshes in response to growing concern over sea-level rise caused by climate change.
In the U.S., a federal law enacted in 1982 doesn’t bar risky development on barrier islands—but reduces incentives for such activity by making these properties ineligible for financial assistance, including federal flood insurance.
With the Trump administration expressing widespread skepticism about human-caused climate change, renewed federal action seems unlikely in the coming years. Still, some states are acting more aggressively.
Louisiana’s 2017 coastal restoration plan offers a 50-year blueprint for the state to restore a coastline that has been devastated by hurricanes, land subsidence and other factors, said Gerald Galloway, a research engineering professor at University of Maryland.
The plan calls for a mix of projects, including restoring salt and freshwater marshes, diverting sediment, constructing breakwaters, and rebuilding barrier islands.
Between 1932 and 2010, the Louisiana coast lost more than 1,800 square miles of land, including more than 300 square miles of marshland that was lost to hurricanes Katrina and Rita in 2005, and Gustav and Ike in 2008, according the U.S. Geological Survey.
The key to coastal resilience is to develop a comprehensive, regional long-term plan that combines natural features of the coast along with engineered solutions that mimic nature, said Galloway, who was appointed in 2010 to Louisiana’s advisory committee on Coastal Protection, Restoration and Conservation.
But engineered solutions can be overwhelmed, Galloway said.
In Houston, for example, ponds and dams filled to capacity and overtopped when Harvey lashed the Gulf Coast with record rains.
“What really nailed Houston was not a large storm surge, but rather an extraordinary amount of rainfall,” Stein, of National Wildlife Federation, said.
Harvey was a rainfall-dominated event, but what if there was a Katrina-like-event where a wall of water moved in, Galloway asked.
“It would wipe out refineries, wipe out homes between Galveston Bay and Houston,” he said.
A form of wetlands known as coastal prairie potholes are native to Houston, “but now all you see is pavement,” Stein said. The eight-county Houston area has built over most of its freshwater wetlands so the flood-absorbing capacity of its natural systems has been compromised, he said. These freshwater wetlands are the headwaters for virtually all of the water bodies feeding into Galveston Bay.
Since 1992, the Greater Houston metro area lost about 5.5 percent—or 24,600 of the 447,949 acres—of natural freshwater wetlands mapped by the U.S. Fish and Wildlife Service’s National Wetland Inventory, according to a 2014 study by the Texas A&M University’s Texas Coastal Watershed Program that attempted to quantify wetland loss in the city.
The greatest losses occurred in Harris County, which includes the city of Houston itself, and where almost 30 percent of the freshwater wetlands that had existed in 1992 were gone by 2014.
In Florida, the problem lay in development that has continued to the water’s edge, encroaching into the mangroves and barrier islands, Stein said, pointing to Miami Beach, which was developed on a barrier island that normally would have protected against storm surges.
Florida has lost about half of its mangrove forests just since the mid-1900s.
The state has about 469,000 acres of mangrove forests today, which provide critical habitat for adult fish, stabilize shorelines and help prevent storm surge and erosion damage to coastal property, according to the Florida Department of Environmental Protection.
While storms and hurricanes, notably Andrew in 1992, have damaged mangroves, “human destruction of mangrove habitat by shoreline development is their greatest threat,” contributing to the removal of nearly 500,000 acres between 1943 and 1970, a University of Florida fact sheet notes. In 1996, Florida passed a law restricting trimming of mangroves.
Loss of mangrove habitat has been severe in Florida’s largest estuaries. Since 1900, Tampa Bay, home to one of the largest ports in the nation, has lost nearly 50 percent of mangrove forests and salt marshes. Charlotte Harbor has lost nearly 60 percent. And on Florida’s East Coast, construction of mosquito ditches and empoundments have destroyed or compromised 85 percent of mangroves in the Indian River lagoon.
Both Galloway and Stein said city planners need to reconsider their past policies of allowing people to build homes in flood-prone areas, or encourage those already living along the coast to relocate to higher ground, and find ways to preserve natural habitat.
“It is a challenge to get people’s cooperation,” Galloway said, but he added it can be done. After the Red River flooding in 1997, North Dakota relocated whole communities away from the river banks.
Other solutions include the use of engineered techniques that mimic nature, such as permeable pavements, strategically planted wetlands and vegetation to capture downpour and filter it through the ground naturally.
In Houston, which is completely built up, the goal should be where it’s possible to remove impervious driveways and install driveways that allow stormwater to filter through, Alisha Renfro, coastal scientist with the federation’s Mississippi River Delta Campaign, said.
Stein pointed to the two-mile stretch in Pensacola, Fla., where oyster reefs have been constructed to provide shoreline protection.
A report coauthored by Stein cites a number of states that are taking steps to enhance natural defenses.
Alabama placed the western spit of Dauphine Island, a barrier island located three miles south of Mobile Bay, under the protection of the Coastal Barrier Reduction Act, a 1982 law signed by President Ronald Reagan that discourages development by making properties ineligible for federal flood insurance. Alabama aligned its local and state zoning laws, making at least the edge of the island that is most prone to storms off development.
A recent study quantified the storm surge risk reduction benefits of retaining coastal wetlands at $625 million in averted property damages along the New Jersey shoreline that was caused by Hurricane Sandy in 2012, according to Michael Beck, the study’s author and a senior scientist with the Nature Conservancy.
Beck said his study was the first to put an economic value on the benefits of coastal wetlands, but—like all other engineers interviewed by Bloomberg BNA—he cautioned that nobody is saying coastal wetlands weren’t a silver bullet to prevent storm surges and flooding.
Still, he said, “we should be investing more in natural resiliency because it does offer protection.”
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