Bonnet Carré Spillway

The Bonnet Carré Spillway is a flood control operation in the Lower Mississippi Valley. Located in St. Charles Parish, Louisiana - about 12 miles (19 km) west of New Orleans - it allows floodwaters from the Mississippi River to flow into Lake Pontchartrain and thence into the Gulf of Mexico. The construction of the spillway was completed in 1931.

Overview

The Bonnet Carré Spillway consists of two basic components: a control structure along the east bank of the Mississippi River and a floodway that transfers the diverted flood waters to the lake. The control structure is a mechanically controlled concrete weir which extends for over a mile and a half parallel to the river. When opened, the control structure slightly restricts the flow of the river (at the structure’s location) toward its main channel, thereby causing it to rise in elevation just high enough to flow into the diversion channel; and, with sufficient elevation (or head), to carry the overflow volume into Lake Pontchartrain. The lake’s opening to the gulf is sufficient to absorb and dissipate any conceivable volume of flood flow. Thus, the flood surcharge portion of the water from the Mississippi is divided between the main river and the diversion channel; with the surcharge bypassing the New Orleans metropolitan area, resulting in the Mississippi being lower (through that area) than it could have been; and reducing the stress on the area's levees that line the river. Confined by guide levees, the floodway stretches nearly six miles (10 km) to Lake Pontchartrain, with a design capacity of 250,000 cu ft/s (7,100 m3/s). The spillway is crossed by U.S. 61 and Interstate 10.

The spillway was built in response to the Great Mississippi Flood of 1927 that inundated much of the Mississippi River basin. It was first opened during the flood of 1937, and nine times thereafter through 2011 to lower river stages at New Orleans. The most recent opening began on May 9, 2011, when river levels in New Orleans approached the flood stage of 17 feet (5.2 m).

The spillway is part of the United States Army Corps of Engineers' multi-state plan, called the Mississippi River and Tributaries Project (MR&T), providing flood protection for the alluvial valley between Cape Girardeau, Missouri and the mouth of the river near Venice, Louisiana. Due to the wide expanse of the project and the complex problems involved, the plan contains an array of features. The MR&T Project provides for levees to contain flood flows, floodways such as the Bonnet Carré to redirect excess flows away from the Mississippi and has other aspects such as channel improvement and river bank stabilization for efficient navigation and protection of the levee system. It also involves reservoirs and pumping plants for flood control drainage.

2007 expansion

In 2007, extraction operations began on a 42-acre (170,000 m2) square section of spillway property. The 7,600-acre (31 km2) spillway, designed to divert rising water out of the Mississippi River during flooding seasons, long has been a major source of sand and clay that is used in construction projects throughout the area. But in the post-Katrina era, clay extracted from the spillway will be crucial to raising the levees in St. Charles Parish and east Jefferson Parish.

Once the excavation is complete, the pits will be filled with water and stocked with bluegill, largemouth bass and other fish to enhance the area's secondary function: a recreational haven for boaters, hikers, hunters and fishers.

History of openings

Years when the spillway was opened, number of days it remained open, and peak number of bays opened

  • 1937 48 days, 285 bays opened
  • 1945 57 days, all 350 bays opened
  • 1950 38 days, all 350 bays opened
  • 1973 75 days, all 350 bays opened
  • 1975 13 days, 225 bays opened
  • 1979 45 days, all 350 bays opened
  • 1983 35 days, all 349*** bays opened ***(one broken)
  • 1997 31 days, 298 bays opened
  • 2008 31 days, 160 bays opened
  • 2011 42 days, 330 bays opened
    Main article: 2011 Mississippi River floods

Media

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