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Cuts to Mississippi levees could build new land in sinking delta

21.10.2009
Diverting sediment-rich water from the Mississippi River below New Orleans could generate new land in the river's delta in the next century that would equal almost half the acreage otherwise expected to disappear during that period, a new study shows.

For decades, sea-level rise, land subsidence, and a decrease in river sediment have caused vast swaths of the Mississippi Delta to vanish into the sea. The anticipated buildup of new land in a portion of the delta, as simulated by a computer model, could compensate for a large fraction of the expected future loss, protect upriver areas from storm surges, and create fresh-water habitat, the researchers say.

"What this model mainly shows is that we can, to a large degree, match future land loss by making these diversions," says David Mohrig, an associate professor at the University of Texas (UT) in Austin and a member of the research team. Wonsuck Kim, an assistant professor at UT-Austin led the study, which is reported in today's issue of Eos, the weekly newspaper of the American Geophysical Union (AGU).

The delta of the Mississippi River has been losing land to the sea at an average rate of about 44 square kilometers (17 square miles) per year since around 1940. The natural equilibrium between soil loss and sediment deposition has been altered by the levees that the U.S. Army Corps of Engineers built below New Orleans to prevent the Mississippi from flooding. The confined waters at the end of the river's course flow faster and drop their sediments over the continental platform, draining uselessly into the Gulf of Mexico.

"History recorded in the deposits of the river shows that the main channel of the Mississippi moved roughly every 1,000 years to a new lowland area," Kim and Mohrig note. "But the engineering of the levees has kept the river from entering lowland areas and depositing sedimentation."

The model looks at potential effects of an existing and contentious proposal to divert Mississippi River water through a pair of cuts made opposite each another in the levees 150 kilometers (93 miles) downstream from New Orleans. Nearly half of the river's flow would spill out through the cuts, taking sediment with it and depositing it to each side of the river channel.

Despite sea level rise, increased land sinking rates, and a drop in the river's sediment supply, the diversions would create an amount of new land equal to up to 45 percent of the area that would otherwise be lost to the sea in the coming century, the model predicts. Enough flow would remain in the main channel of the river to allow navigation there, the researchers report.

Other researchers studying coastal restoration had previously proposed creating these two diversions to allow water and sediment to exit the enclosed river and build two lobes of new land in adjacent shallow-water sections of Breton Sound and Barataria Bay.

But critics say that dams in the upper sections of the Mississippi River have reduced the water's sediment content so much that there isn't enough raw material left to rebuild the delta. Also, detractors argue that future sea level rise and the current high sinking rate of the delta would make restoration impossible.

"Until we put together this model, there was just a lot of debate that wasn't substantiated by anything but by intuition," says Mohrig. "We needed to move from having very soft impressions of what could be done to making predictions that can actually be tested."

The modelers use a conservative sediment supply rate, subsidence (i.e.,
sinking) rates from 1 to 10 millimeters (0.04 to 0.4 inches) per year, and rates of sea level rise that range from 0 to 4 mm (0.16 in) per year. In their calculations, the authors consider diverting only 45 percent of the water to ensure that the section of the river below the diversions remains open to navigation.

The model predicts that the two diversions would create between 701 sq km (about 271 sq miles) and 1217 sq km (470 sq miles) of new land over a century, partially offsetting land loss. Kim and Mohrig calculate the engineered new delta lobes would make up for 25 to 45 percent of the area expected to vanish throughout the delta between now and 2110.

"Diversions are really the only cost-effective way of building land that anyone is proposing," Mohrig says

The researchers verified their model by making it run a simulation of the evolution of another delta influenced by an existing diversion of the Mississippi River: the Old River Control Structures. These structures divert water from the Mississippi to the Atchafalaya River, which also empties into the Gulf of Mexico. The Atchafalaya River is currently building new land both in the Atchafalaya Delta and its subsidiary, the Wax Lake Delta, and Kim's model was able to predict the amount of land that has been built since 1980.

Torbjoern Toernqvist, an associate professor at Tulane University in New Orleans who has conducted research on the Mississippi Delta but not related to this study, says that the authors have been "too optimistic" in the sea level rise and subsidence rates they use in their model, since the current rates are already higher than the ones the researchers use in their best-case scenario.

"What they consider their worst case scenario should be considered the base case scenario -- the combination of 4 mm/yr of sea level rise and the subsidence rate of 10 mm/yr is the most likely scenario," Toernqvist says.

"That said, it is also true that even with the worst case scenario, [the model predicts] you can build substantial new land."

Kim and Mohrig acknowledge that there is debate about how fast the Louisiana coast is sinking. The highest rates of subsidence, caused by automobile traffic, pumping of underground oil and gas, and other human influences don't affect large swaths of the delta, they note."We're comfortable with the numbers" in the new study, Mohrig says.

He and Kim collaborated on the model with scientists from Louisiana State University, Baton Rouge; the University of Minnesota, Minneapolis; and the University of Illinois, Urbana-Champaign.

The National Science Foundation funded the research.

Peter Weiss | American Geophysical Union
Further information:
http://www.agu.org
http://www.agu.org/sci_soc/prrl/primages.html

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