Coastal Louisiana gets that sinking feeling
Subsidence in Bayou may lead to biggest - and riskiest - wetland restoration in history.
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Nowhere in the world is a coastal region losing losing wetlands and protective barrier islands faster than Louisiana. The region is sinking as layers of river-borne muck hundreds of feet thick shift under their own weight.Skip to next paragraph
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Now, state, local, and federal officials are pulling together a $14 billion plan to slow the losses, in what would rank as the most ambitious wetlands restoration effort in history. As they do, scientists are raising new questions about how quickly the subsidence is occurring. It's a question whose answer could determine the shape of the political debate over the direction the program takes, or perhaps whether it moves forward at all.
"There's a lot riding on this," says Jeff Williams, a researcher with the US Geological Survey's coastal and marine geology program at Woods Hole, Mass. "Has Louisiana passed some sort of threshold beyond which it's not possible to restore the coast or maintain the conditions we have?"
If the answer is yes, he adds, "It doesn't mean you have to write it off," but it would significantly affect the shape of the program. Current notions of trying to mimic nature's processes for rebuilding river deltas through dredging and moving silt, rechanneling streams and river tributaries, and pumping water from one location to another could shift toward solutions such as the Netherlands' system of dikes, dams, and seawalls. Depending on loss-rate estimates, planners might be faced with the extreme measure of moving millions of people to higher ground.
It's a daunting prospect for a region already engaged in an economic life-or-death struggle against geophysics. For 20 years, the US Army Corps of Engineers has been trying to prevent the Mississippi River from changing course and heading down what is now the Achafalaya River. All that stands between the Mississippi and disaster are three dams that regulate the flow of water down the Achafalaya. If the river were to break through and shift course, Baton Rouge and New Orleans would become economic backwaters instead of bustling maritime and industrial centers, while the region along the Achafalaya would be inundated.
For their part, the state's coastal barrier islands and wetlands represent the first line of defense against hurricane storm surges for cities and towns in the region, as well as for critical oil and gas pipelines, rail lines, and highways. They also play a key role in sustaining the coast's fisheries. Yet these natural defenses are vanishing as the earth supporting them subsides by roughly an inch per decade.
Much of this subsidence occurs naturally as the sediment under the region compresses under its own weight, depressing the crust underneath it. The depression can be measured along a vast stretch of the Gulf Coast and well up into the Bayou state, says Louisiana State University geologist Roy Dokka, who dubs the process "a slow disaster."
Humans have accelerated land loss by building dikes and levees that have prevented the silt-laden Mississippi River from replenishing the delta. In addition, oil, gas, and water extraction have, in effect, knocked props out from under the sediment, causing further sinking.
Whatever the mix of causes, "an inch a decade is a horrendous rate," says Wilson Shaffer, with the National Weather Service lab responsible for developing tools to forecast hurricane storms. "It means that less intense storms can cause more damage."
It also means saltwater intrusion into once-thriving freshwater ecosystems.
Last week, researchers at Louisiana State University and the National Oceanic and Atmospheric Administration released the results of a study suggesting that parts of coastal Louisiana and Mississippi could lose up to a foot of elevation over the next 10 years. Over the next 70 years, they estimate, current rates of subsidence could cost southern Louisiana 15,000 square miles.
The results, released at a hurricane conference in New Orleans, have triggered a flurry of e-mails and phone calls among scientists who have been working on the region's subsidence problem. They note that this rate of change far outstrips the estimates of roughly an inch-a-decade established by previous studies. They say they are hard-pressed to explain the new numbers, based on what they know about the region's geological processes and the impact of human activities.
Indeed, a team led by US Geological Survey scientist Bob Morton published a study last April suggesting that the rate of subsidence may be slowing, because oil, gas, and water are not being extracted as furiously as they once were.
Researchers say that if the latter is true, it would imply that efforts to mimic natural processes in restoration efforts may be enough. If the higher rates hold, however, they could argue for more expensive - and perhaps socially disruptive - solutions.
Researchers are scheduled to meet in New Orleans next month to try to sort out the issue. All this might appear to be a tempest in a scientific teapot if it weren't for the high stakes for the future of the state as well as for the various federal agencies vying for a piece of the restoration pie.
Moreover, it may be a harbinger of things to come if forecasts for sea-level rise resulting from climate change pan out.
The generally accepted sea-level changes affecting Louisiana are "close to what climate-change models forecast for sea-level rise," Dr. Williams notes. What happens in Louisiana, he says, could hold lessons for low-lying regions worldwide that may confront an oceanic onslaught.