Land grab.
Boxes represent new land that would be created by diversion structures in Barataria Bay and Breton Sound.

Robert R. Twilley

A Grand Diversion in Louisiana

BOSTON–Decades of draining, dredging, and other mistreatment have taken a severe toll on Louisiana wetlands. Hurricane Katrina focused a spotlight on the need to restore the marshes, which can lessen the risk of water surging toward New Orleans. But any such plan would have to compete for money with other flood-protection projects, such as raising levees. Results of a new model, unveiled here yesterday at the annual meeting of the American Association for the Advancement of Science (ScienceNOW's publisher), may help win support for repairing the wetlands.

Louisiana wetlands face extreme challenges. Sea level is rising and the ground is sinking, both of which threaten to drown the marshes. Normally, sediment deposited by the Mississippi River helps build up the delta, allowing the marshes to stay above water. But engineering of the river to make it better for shipping has caused much of its sediment to flow into deep water.

One proposal to fix this is to divert river water out of the main channel so that it flows closer to shore. As sediment gets dumped there, it would build land that would turn into wetlands. It's never been clear exactly how much land would be created, however. That makes it harder to sell the concept to politicians, says ecologist Robert Twilley of Louisiana State University in Baton Rouge.

So Twilley teamed up with geoscientist Christopher Paola of the University of Minnesota, Minneapolis, and others. About 15 years ago, Paola helped design a computer model that predicts how rivers create deltas. It has been used extensively to help mining companies figure out how sediment from tailings will fill up their storage ponds. With conservative assumptions about sea level rise and the subsidence of the area, two diversion structures--large concrete sluices placed in the river embankment--would create about 700 square kilometers of land over 30 years, said Gary Parker of the University of Illinois, Urbana Champaign. If sea level rises more slowly and land sinks less, the structures would build up 1000 square kilometers off the present coast.

"This is the largest and most aggressive element of a comprehensive plan," Twilley says. It would cost perhaps $500 million a year to build and operate. He hopes that the new analysis will persuade state legislators of the importance of the project. "They are going to have to put this in the top basket. I feel confident that the state will do that."

Denise Reed of the University of New Orleans in Louisiana agrees that the project is a good idea and that the new analysis may help win support for it. "This is pretty crucial information," she says.