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VICKSBURG, Miss.– Heavy rain in the Mississippi Valley is prompting the U.S. Army Corps of Engineers to open the Bonnet Carré Spillway on Friday, April 11, 2008, for the first time in 11 years.
The Corps will open the spillway to keep the volume of Mississippi River flows at New Orleans from exceeding 1.25 million cubic feet per second (cfs), which current projections indicate will occur on April 11, 2008. The spillway may be open for an estimated two to four week period, during which time the Mississippi is expected to crest at about 17 feet at New Orleans, without operation of the spillway. Operation of the structure will relieve pressure on local levees, lower river stages, and reduce the velocity of the river current from the spillway southward.
The decision to open Bonnet Carré is the responsibility of Mississippi River Commission President Brig. Gen. Michael J. Walsh, commander of the Corps’ Mississippi Valley Division in Vicksburg, Miss.
Environmental, hydrologic, structural, navigational and legal considerations all bear on the decision to open Bonnet Carré. Essentially, the spillway is only operated when existing conditions, combined with predicted discharges, reach the operational level as prescribed in the approved Bonnet Carré Spillway Operations Manual and the Mississippi Valley Division Operations Plan 2007-02 for Floods.
Other factors that affect the decision are the overall condition of the levees and the ability of the river to pass flows, and the effects high water and river currents may have on vessels navigating the river, including the risk of vessels losing control and colliding with levees.
Bonnet Carré, located 28 miles above New Orleans, is a vital element of the multi-state Mississippi River and Tributaries (MR&T) system, which uses a variety of features to provide flood protection to the alluvial Mississippi Valley from Cape Girardeau, Mo., to Head of Passes. MR&T features include levees and floodwalls to contain flood flows, floodways (such as Bonnet Carré) to redirect high flows out of the Mississippi River, reservoirs and pumping plants for flood control drainage, and channel improvement and stabilization features to protect the levees and improve navigation of the river.
Bonnet Carré is the southernmost floodway in the MR&T system. Located on the east bank in St. Charles Parish, it can divert a portion of the river’s floodwaters via Lake Pontchartrain into the Gulf of Mexico, thus allowing high water to bypass New Orleans and other nearby river communities. The structure has a design capacity of 250,000 cfs, the equivalent of roughly 1,870,000 gallons per second.
The Bonnet Carré structure consists of a control structure and a floodway. The control structure is a concrete weir that parallels the river for a mile and a half. It consists of 350 gated bays, each holding 20 timber “needles,” for a total of 7,000 needles. When needles are removed, river water flows into the floodway and is conveyed nearly six miles between guide levees to the lake. Operation of the structure is relatively simple. Two cranes, moving on tracks atop the structure, lift timbers from their vertical position in the weir and set them aside. A complete opening of all 350 bays is not planned at this time.
Bonnet Carré was first opened during the flood of 1937; since then it has operated seven other times, during high water in 1945, 1950, 1973, 1975, 1979, 1983, and 1997. The flood of 1997 was the last time the spillway was operated.
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