Katrina: Sea change on the Gulf Coast – Part 3 • Environment

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."

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