Katrina: Sea change on the Gulf Coast – Part 3 • Environment
PECAN ISLAND, LA.
From inside a small motor boat, this glimpse into the future of southern Louisiana's hurricane protection isn't much to look at.
It's an embankment, rising a mere foot and a half above open marsh water here in the Rockefeller State Wildlife Refuge. Cordgrass, just starting to flower, sprouts along its edges like hair on a monk's tonsure.
"We planted those in the early part of this year," says Darryl Clark, with the US Fish and Wildlife Service (FWS), who eases in an aluminum outboard for a closer look. "They're doing very nicely."
The 1,000-foot-long barrier, one of several that form a sergeant's-stripe pattern in the open water, are a small part of a vast effort to rebuild the region's natural hurricane defenses, restore the ecological vitality of its wetlands, and perhaps slow the alarming rate at which the region is sinking.
The coast's watery descent may be a key early test of how other coastal communities around the world might deal with rising oceans and stronger tropical cyclones. Many scientists say these are likely results of global warming.
At best, southern Louisiana would become a model for how to build sustainable and resilient seawater defenses that help keep coastal populations where they are, some specialists say. At worst, they add, the region's enhanced defenses could buy time – perhaps a century or two at most – for residents to figure out whether they need to pull up stakes and head north to higher, drier ground.
"We have so much of this state at or below sea level. You have high rates of subsidence. You get concentrated populations, such as New Orleans and other coastal cities. You have a high incidence of high-magnitude events – hurricanes and winter storms ... it's like putting the whole system on fast forward" compared with other delta regions, says Gregory Stone, a professor in the department of oceanography and coastal sciences at Louisiana State University in Baton Rouge. "A lot of engineers and scientists are looking to see how we're going to come out of this ordeal."
Experts are not at a loss for ideas. For example, in July, the US Army Crops of Engineers served up the outline of an ambitious, long-term plan to cut the risk from Category 5 hurricanes in a report to Congress. Rather than relying principally on levees that hug urban areas, the Corps is considering a triple-layer defense.
Extensively restored barrier islands would be the first line of defense, absorbing the first shock from the surge of high water that hurricanes bring. Revitalized wetlands would form the second line of defense. Armored levees and other forms of barriers stretching from Texas to Mississippi would form a final defense, a Maginot line often a few miles inland. These final barriers would be designed to allow boat traffic to move back and forth. By some accounts, those levees could reach 30 to 60 feet high.
Such an engineering project would take years and tens of billions of dollars to complete. In the near term, improving the region's natural hurricane defenses would fall to those who are trying to restore its wetlands and barrier islands. They also hope to stem the sinking – or subsidence – that is allowing the Gulf of Mexico to creep farther northward.
The region, is "ground zero for three ecosystems in a state of collapse," explains Russell Watson, who like Mr. Clark, is with the FWS office in Lafayette, La.
He says these include: the once "outrageously productive" lower Mississippi River Valley; the equally fecund coastal wetlands, which are disappearing under water at an average rate of 24 square miles a year; and offshore waters, which each summer host a "dead zone" of oxygen-poor water, smothering commercially valuable fish.
In this context, he says, sustainable recovery means ecosystems that maintain themselves in a healthy state without constant massive human intervention. Engineers are pretty good at the "dump and pump" approach of rebuilding wetlands and barrier islands with water, silt, and sand.
"We know how to do that," he says, "but is it sustainable to expect to dump and pump from now until forever?"
New results from a Louisiana State University (LSU) study are helping to quantify the beneficial effect wetlands restoration could have on hurricane protection.
As early as the mid-1960s, the US Army Corps of Engineers tested the notion that wetlands could provide a buffer against storm surges. It concluded that a storm surge would shrink one foot for every 2.75 miles of wetlands it crossed. The number represented an average taken from several storms. But over the years, it took on a life of its own as the expected return on investments in wetlands, researchers say.
After hurricanes Katrina and Rita plowed into the region, LSU researchers Hassan Mashriqui and G. Paul Kemp took a new set of measurements and used them in modeling experiments. They found that when a surge encountered coastal wetlands at least 100 miles long and 25 miles deep, the surge indeed dropped one foot for each three miles inland it traveled. Where dredged channels were present, however, the storm surge traveled up to six miles before it dropped a foot. They also found that when the surge encountered a 100-yard-long phalanx of trees, the waves riding atop the surge lost 95 percent of their energy.
While trees don't reduce the height of a surge, the team found, they can reduce dramatically the erosive punch of waves against levees. The researchers' goal is to come up with a viable range of numbers that engineers can use as they design their layered defense.
Indeed, what storms take away – some 200 square miles of coastal wetlands between Katrina and Rita – they also give.
Marshes at Cocodrie in southern Terrebonne Parish "are in great shape, and many parts of the coast are like that," says Denise Reed, a University of New Orleans geologist. "They get thick pulses of sediments from hurricane surges to keep things going." She is combing through data on last year's storms to see how much sediment they delivered to wetlands.
State, federal, and local planners have identified a common set of wetlands restoration projects they'd like to accomplish by 2050. The goal is to restore the system's ability to replenish itself at a rate at least equal to the region's subsidence rate by then. Some projects are finished; others, like the one the FWS's Clark oversees, are works in progress. Still others are but gleams in planners' eyes and await federal funds in order to proceed.
This is not just about "bugs and bunnies," says Dr. Reed. "The bottom line is protecting people."