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Sediment Diversion to Rebuild Wetlands

Aerial view of Louisiana wetlands
Vast wetlands in Louisiana have been lost to both natural and man-made processes, but preliminary work is now under way to rebuild as much as 25,000 acres of wetlands using water and sediment diverted from the Mississippi River. USGS

Contracts were awarded recently on two projects to divert flow and sediment from the Mississippi River to offset the dramatic loss of coastal wetlands in Louisiana.

May 7, 2013—Preliminary work is under way on two ambitious projects aimed at building as much as 25,000 acres of wetlands within coastal Louisiana by diverting water and sediment from the Mississippi River. The projects seek to form this land by mimicking the natural processes that created such areas in the region before levees were built along the river to control flooding.

The projects are an attempt to offset dramatic land loss along the Louisiana coast. The U.S. Geological Survey estimates the state is losing a staggering amount of coastal wetlands—the approximate equivalent of one football field every hour.

Two contracts were recently awarded to Arcadis, headquartered in Highlands Ranch, Colorado, to perform roles on both projects. The company’s engineers will support the U.S. Army Corps of Engineers district headquartered in New Orleans in developing a diversion system as part of the White Ditch Freshwater and Sediment Diversion Project, and the company will also be providing engineering and design services to Louisiana’s Coastal Protection and Restoration Authority for the Lower Barataria Sediment Diversion Project.

Arcadis provided technical expertise to the Coastal Protection and Restoration Authority as the latter developed Louisiana’s 2012 Coastal Master Plan, which includes both of these projects.

“It is groundbreaking,” says Rudy Guichard, a senior vice president and operations director at Arcadis, who points out that the company knows of no other sediment diversion projects in the United States that have been attempted on this scale before. “That piqued our interest,” he says.

Modeling work is being done by the Water Institute of the Gulf, an independent research organization based in Baton Rouge, to find locations on the Mississippi River that are especially high in sediment content. The goal is to achieve a 50 percent sediment-to-water ratio, Guichard says. The work will include physical borings, remote sensing, and computer modeling.

“There are going to be places along the river where you are going to have sediment deposits,” says Falcolm Hull, a project director for Arcadis. “What you want to do is put a diversion structure in the vicinity of those sediment-rich areas so that as much sediment as possible can be caught when the gates are opened.”

The project will probably include an inflow channel that will deliver sediment-rich water to a control structure with gates that, when open, will feed the water into an outflow channel that will direct it to the desired locations. It is likely that over the 50-year span of the project the outflow channel will be realigned to direct sediment to various target sites.

“There will be an outfall management plan developed,” Hull says. “We will be monitoring where the sediment is going and also the rates [at which] the land is being built. These projects will be built so that you can adaptively manage [them]. You can make modifications if in fact you are not getting the results you anticipate.

“Remember that land is being lost at the same time you are trying to build it,” Hull adds. The initial goal is to offset that land loss, rebuilding wetlands as fast as they disappear.

Early challenges in the project will include designing the inflow and outflow channels so that they can efficiently transport both the sediment and the water to the sites. The systems will essentially operate at the mercy of river conditions. “Right now our criterion is when the river exceeds six hundred thousand cubic feet per second, we are going to try to generate fifty thousand cubic feet per second of diversion,” Guichard says. “And a lot of times, that river is not generating that capacity.”

The projects might come with the added benefit of reducing shoaling on the river. “The overall theory is that the higher the ratio [of sediment] going out of the control structure, the less likely there is going to be shoaling in the river,” Guichard says. “The more [that] goes out, the less sediment that will slow down and settle. It is possible that there will be less maintenance as a result.”

The projects will also face the challenge of a diverse group of stakeholders along the river who have vested interests in flood control as well as using the river for navigation and water supply. The projects will also face challenges arising from the diverse interests of stakeholders, some focused on flood control, others on navigation, and still others on water supply. The team anticipates that it will be three to five years before implementation begins, and during that time a formidable permitting process is expected.



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