Sediment capacity lowered

Planned since 2004, the study lately has taken on new urgency, as a result of evolving insights into the river’s capacity.

A 2011 study found that much of the sediment and water the river carries into Louisiana never make it past New Orleans, disproving the assumption that most of what the river gathers from its vast drainage system eventually is sluiced out the river’s mouth and into the Gulf of Mexico.

In fact, only 19 percent of the total suspended sediments, including just 1.4 percent of the suspended sand that passes the mouth of the Atchafalaya River north of Baton Rouge, exits the mouth of the Mississippi, the study found.

A significant portion of the Mississippi River sediment downstream of the Atchafalaya is trapped on land between the riverbank and the levees during times of high water. Even more drops to the bottom of the channel before it reaches New Orleans.

That last finding was attributed to one of the study’s most-surprising and significant discoveries: Less than half the water the Mississippi carries past the Atchafalaya makes it to the Gulf — 46 percent. Since lower water volume equates to less power for moving sediment, the finding casts new light on the potential of man-made and natural spigots drawing from the main channel.

“The results of the present sediment budget suggest that only a relatively small proportion of the upstream sediment load is available for coastal restoration approaching the Gulf,” the study’s authors concluded.

The study was conducted between 2008 and 2010 by some of the scientists now involved in the new hydrodynamics study, including Mead Allison, director of physical processes and sediment systems at The Water Institute of the Gulf, the Baton Rouge-based research organization that assists the state Coastal Protection and Restoration Authority.

“We were not saying there isn’t still enough in the river for coastal restoration — this is a huge system and there’s still plenty enough mud to work with,” Allison stressed.

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