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Desperate Measure Yields Insights for Delta Restoration

Bathymetric map of the Mississippi River near the Bonnet Carre Spillway

Bathymetric map of the Mississippi River near the Bonnet Carre Spillway (BCS). The 90 degree bend upstream of the spillway has no alluvial cover and the channel bed here is consolidated clay. An underwater sand bar covers this clay on the interior bend adjacent to the BCS.

In late spring 2011, one of the largest pulses of water in recorded history traveled down the Mississippi River, threatening the ports, industries, farms and towns of the river’s lower reaches, including New Orleans. To avert disaster, the U.S. Army Corps of Engineers opened the Bonnet Carré Spillway and diverted from 10 to 20 percent of the total river flood discharge into Lake Ponchartrain.

Jeffrey Nittrouer, a National Science Foundation post-doctoral researcher at the University of Illinois, studied the impacts of the diversion shortly after the floodwaters subsided. Surprisingly, he found that 31-46% of the total sand load carried by the river during the 6 weeks the spillway was open was carried through the spillway. The location of the spillway was not intentionally chosen for delta restoration. But Nittrouer determined its land building power came from the fact that it was located on an inside bend of the river downstream from the river-bend apex. This key insight could help scientists and engineers choose the best sites for land-building diversions.

“A tremendous amount of sediment made its way out from this diversion indicating that if the sites of diversions are properly located, there’s the real opportunity to produce much more land than any of us are predicting right now,” says David Mohrig, Nittrouer’s former PhD adviser at the University of Texas at Austin. “It was a very special case in terms of the location, but the net effect of it was they got about as much sand out from the diversion as is estimated to be in the total load of the Mississippi for a whole year.”

This fall, Nittrouer will join the faculty of Rice University.

Read the article “Mitigating land loss in coastal Louisiana by controlled diversion of Mississippi River sand” (Nature Geoscience, July 22, 2012)

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