Bonnet Carré Spillway

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Bonnet Carre Spillway on opening day 2011.
The Bonnet Carre Spillway diverting excess Mississippi River water.

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[edit]

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.[1] The control structure is a mechanically controlled concrete weir that 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).[2] 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).

Structure[edit]

The floodgate inlet is offset into the Mississippi River bank, at a location that had seen natural flooding for at least two centuries. The concrete structure has been designed to reduce settling into the riverbank. The opening and closing of the spillway is done by lifting out up to 7000 wooden 'needles' (8" x 12" wooden beams) arranged in 350 linear bays. This is done by two rail-mounted gantry cranes atop the spillway. The number of needles removed determines the water flow into the spillway. Gaps built into each bay allow limited seepage anytime the river level reaches the bottom of the bays. Removal of all 7000 needles takes about 36 hours but is usually done over a period of several days.

History of openings[edit]

Years when the spillway was opened, number of days it remained open, peak number of bays opened, percent of bays opened, and ideal flow capacity. Ideal flow capacity is calculated by ideal rate of flow for each bay multiple by number of bays opened; remember that the spillway has a design capacity of 250000 cu ft/s and there are 350 bays making the ideal rate of flow for each bay 714.3 cu ft/s.[3][4]

Year Days Bays Opened (%) Opened Ideal flow capacity
1937 48 285 81.4% 203,571 cu ft/s
1945 57 350 100% 250,000 cu ft/s
1950 38 350 100% 250,000 cu ft/s
1973 75 350 100% 250,000 cu ft/s
1975 13 225 64.3% 160,714 cu ft/s
1979 45 350 100% 250,000 cu ft/s
1983 35 350 100% 250,000 cu ft/s
1997 31 298 85.1% 212,857 cu ft/s
2008 31 160 45.7% 114,286 cu ft/s
2011 42 330 94.3% 235,714 cu ft/s

[5]

Kugler and Kenner Cemeteries[edit]

There are two graveyards located in the spillway. The cemeteries containing the bodies of both free and enslaved African-Americans are under several feet of sediment. When the Spillway gates are opened, the cemeteries are flooded with up to 30 feet of water diverted from the Mississippi River into Lake Pontchartrain. According to investigative reporter, Shonna Riggs, "The location of the gravesites was a mystery until the 1970's when the US Corp of Engineers was attempting to excavate a ditch in the spillway. During that process, a tombstone and casket were discovered. In 1986, the corps ordered a historical study of the spillway and discovered a second cemetery. The two cemeteries were placed on National Register of historic places on 10/16/1987". According to the Louisiana Historical Preservation Society, there are 14 graves located in the Kugler Cemetery and 144 in the Kenner Cemetery. The artifacts discovered during the 1986 study included coffin furniture, coffins, grave markers, cultural remains and human remains. The archeological district consists of two discontinuous, but historically associated, historic cemeteries. The cemeteries, named Kenner and Kugler, are Black burial plots which appear to date from the early 1800s to 1929. The sites are located on former adjoining nineteenth and early twentieth century sugar plantations in St. Charles Parish. According to oral histories, both cemeteries were dedicated burial plots on the back side of their respective plantations. The Kenner Cemetery, located on the former Roseland Plantation, was reported to be marked by iron and wooden crosses during its period of use. No such markers were confirmed archeologically, but one granite headstone was recovered from the site. The Kugler Cemetery, located on the former Hermitage Plantation, was reported to contain iron crosses and a metal fence. One such iron cross and remnants of what could have been the fence were recovered during the 1986 archeological investigations. At present the sites are indistinguishable from the surrounding landscape.[6]

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.[7]

Natural Resource Management[edit]

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.

The spillways secondary function is as a recreational area. It is used for off-road vehicles, biking, boating, hiking, hunting and fishing. It is designated as a protected wildlife management area by the Louisiana Department of Wildlife and Fisheries.[8][9] To enhance fishing in the spillway, the clay extraction area pits were filled with water and stocked with bluegill, largemouth bass and other fish.

See also[edit]

Notes[edit]

  1. ^ http://www.mvn.usace.army.mil/pao/bcarre/bc3.gif Army Corp of Engineers: Bonnet Carré Spillway
  2. ^ http://www.mvn.usace.army.mil/pao/bcarre/bc6.jpg Army Corps of Engineers: Annotated aerial view of spillway
  3. ^ Matt Scallan (11 April 2008). "Previous Spillway Openings". New Orleans Times-Picayune: A–1. 
  4. ^ "Corps Finishes Closing Bonnet Carre Spillway". WDSU/AP. Retrieved 2008-05-08. 
  5. ^ "U.S. Army Corps of Engineers Team New Orleans". Retrieved 05-13-201. 
  6. ^ "Corps to check status of watery graves". Saint Charles Parish St. Charles Herald Guide. Retrieved 2008-05-15. 
  7. ^ http://www.mvn.usace.army.mil/pao/bcarre/bc4.gif Army Corps of Engineers: Louisiana MR&T structures
  8. ^ "Wildlife Management Areas". Louisiana Department of Wildlife and Fisheries. Retrieved 2014-01-18. 
  9. ^ "Bonnet Carre Spillway". Louisiana Department of Wildlife and Fisheries. Retrieved 2014-01-18. 

External links[edit]

Coordinates: 30°0′8″N 90°26′29″W / 30.00222°N 90.44139°W / 30.00222; -90.44139 This article incorporates public domain material from websites or documents of the United States Army Corps of Engineers.