River shoaling started before coastal project

• By AMY WOLD
• Advocate staff writer
• Published: May 21, 2009 - Page: 15A

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An area of the Mississippi River south of Venice experienced shoaling before — not just after — a coastal restoration project was built there in 2003, according to the state Office of Coastal Protection and Restoration.

During a presentation to the state Coastal Protection and Restoration Authority Wednesday, staff members used U.S. Army Corps of Engineers data on river depths in the area to show that shoaling in the area occurred before any project was built, said Robert Routon, the state’s project manager for the  project.

That information will be presented to the corps and other agencies today as part of an ongoing discussion about how much impact the West Bay Sediment Diversion project has had in putting additional sediment in the nearby Pilottown Anchorage.

The anchorage is used by river traffic for staging or when there are hazardous conditions on the river. However, the anchorage is filling in with sediment, making some of it unusable.

The issue of additional shoaling — or the build up of sediment on the river bed — in the anchorage came up last year at a Coastal Wetland Planning, Protection and Restoration task force meeting.

The task force’s original agreement in building the project called for maintaining the anchorage to pre-project depths by dredging.

The cost of dredging is estimated at more than $100 million, which prompted the task force to discuss either finding a cheaper solution or closing the project down.

The project was designed to allow 20,000 cubic feet per second of river water to pour through a cut in the natural levee and into marshes to the west of the river. Currently, the diversion is carrying between 25,000 and 27,000 cubic feet per second of water, said Luke Le Bas, project engineer.

The intent is to have the water to carry sediment into the shallow water areas and build new land — similar to what has been occurring in the Atchafalaya River delta at the Wax Lake Outlet.

This “sediment diversion” is the first of its kind in the state, but the state and federal agencies have tried to find a way to deal with the dredging costs.

That’s why the state decided to look into river depth information compiled by the corps to see how much shoaling was actually occurring in the area, and how much could be attributed to the project. What they found is that river bed is decreasing in width, but that had been occurring for years before the West Bay Diversion was built, Le Bas said.

“The river is filling in,” he said. “It’s been going on for decades and it will continue to go on.”

The sediment diversion project probably contributes to the shoaling, but it proved to be very complex to determine just how much, Le Bas said.