VICKSBURG, Miss. – Heavy rain in the Mississippi Valley is necessitating the U.S. Army Corps of Engineers to open the Bonnet Carré Spillway in St. Charles Parish, Louisiana, March 8.
The decision to open Bonnet Carré is the responsibility of Maj. Gen. Richard G. Kaiser, commander of the Corps’ Mississippi Valley Division and president of the Mississippi River Commission, in Vicksburg, Mississippi.
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 March 8. Current forecasts indicate the spillway could be operated for two to four weeks. Operation of the structure relieves pressure on local levees, lowers river stages, and reduces the velocity of the river current from the spillway southward.
“Louisiana has a robust, well-trained and experienced local, state and federal team of emergency responders who are working together with the Corps to safely pass this high water,” said Kaiser.
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 Water Control Manual.
Other factors that affect the decision to operate the spillway are the overall condition of levees, 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, Missouri, 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.
“The Mississippi River and Tributaries system of levees and structures is designed for these types of high water events. In 2011, operation of the system helped result in prevention of 234 billion dollars in damages and no loss of life in the state of Louisiana,” added Kaiser.
The Corps works with agency partners to observe the effects of opening the spillway on environmental resources, including monitoring the water quality in Lake Pontchartrain, the introduction of endangered and threatened species (including the pallid and shovelnose sturgeon) into the lake, and the impacts of the spillway on fisheries such as oysters.
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 ensuring that the river does not exceed the design capacity for levees in 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. 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 anticipated at this time.
Bonnet Carré was first opened during the flood of 1937; since then, it has been operated 10 other times, during high water in 1945, 1950, 1973, 1975, 1979, 1983, 1997, 2008, 2011 and 2016.