Although diverting water and sediment from the Mississippi River is included within the state’s restoration and protection master plan, many uncertainties remain about how these restoration projects will function.
“Diversions are one important tool,” said Samuel Bentley, associate professor with the department of geology and geophysics at LSU. “But, we have very little data on what large diversions will do.”
Questions include how sediment is distributed though a large diversion, how much sediment can be supplied through a diversion, and how much can be retained to help eventually built land?
River diversions work to mimic the way the rivers initially built south Louisiana, through floods that carried fresh water and sediment into the marsh. That action of providing new sediment to the marsh stopped when levees were built along the Mississippi River after the 1927 flood.
Diversions get at least some of that sediment material flowing again into portions of the coastal marsh.
“It’s critically important, when talking about restoring the coast, to talk about how sediment moves through the system,” said Alex Kolker, assistant professor with Louisiana Universities Marine Consortium.
One of the more recent examples being studied is the West Bay Diversion, which was built in 2003 south of Venice in Plaquemines Parish.
This project of the federal/state Coastal Wetlands Planning, Protection and Restoration Act program involved cutting through the natural levee and allowing the river to flow into the West Bay area.
There are plans to close the diversion in 2013 because previous agreements to dredge a nearby anchorage have proven too costly for the program.
There are, however, some lessons to be learned from the multi-year operation of the project, researchers say.
After losing sediment from the water bottom for six years after construction, the West Bay Diversion area experienced about 6,000 acres of sediment accumulation after the 2011 Mississippi River flood.
Two things seemed to combine to change the direction of sediment loss, said Mitch Andrus, engineer with Royal Engineers and Consultants, LLC.
The first was the building of a 60-acre island in the bay area with the intent to slow the water flow and help sediment settle out of the water. The second was the large amount of water that flowed into the area during the 2011 Mississippi River flood, he said.
“About 6,000 acres of the bay experienced accretion (accumulation of sediment),” Andrus said. Looking back, he said, the island should have been created from the start of the West Bay project. The difference of sediment accumulation before and after the island was built indicates that future diversion designs should include ways to slow water flow for better sediment retention.
One of the big questions about diversions is how much sediment is available in the Mississippi River for restoration projects.
It’s hoped a study currently being conducted by the New Orleans District of the Army Corps of Engineers and the state’s Coastal Protection and Restoration Authority will answer that and other questions.
In addition, the study will help determine the location, size and possible operating scenarios for future diversions as they get designed. More information about this study — called the Mississippi River Hydrodynamic and Delta Management Study — is available online at http://www.lca.gov/.
Amy Wold covers environmental issues forThe Advocate. She can be reached firstname.lastname@example.org.