Spillways 101

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Once again, the New Orleans region serves another purpose for the middle 2/3 of the United States: to manage the water flowing through the lower part of the Mississippi River, so  it does not exceed 1.25 million cubic feet per second in New Orleans, allowing the port to remain active, the population to stay in place and reduces the potential  for the loss of property.

The opening was  praised during a 10 a.m. news conference by New Orleans Mayor Mitch Landrieu for its role in keeping the city and neighboring areas safe. “What we’re witnessing right now is really an engineering miracle,” Landrieu said. “So many of us for so long wanted to make sure our homes and our lives were protected by creating a levee system. What you’re about to see is a levee system that is managed as a risk reduction (system), making sure we do what we’re supposed to do and when we’re supposed to do it, to protect lives and protect homes.”

The multilayered flood-control system is formally known as The Mississippi River & Tributaries Project. Administered and built by the Mississippi River Commission and the U.S. Army Corps of Engineers, the project was launched after the disastrous Mississippi River Flood of 1927. The project was authorized by Congress in 1928. Before the 1927 flood, levees were the only safeguards used to combat river flooding.

 

Bonnet Carré Spillway

Studies to determine the best location for a spillway along the lower river had identified one at the site of the 19th century Bonnet Carré Crevasse, about 33 river miles above New Orleans. Between 1849 and 1882, four major crevasses had occurred at this location. In fact, during the flood of 1849, a 7,000-foot-wide crevasse at Bonnet Carré flowed for more than six months .

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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 spillway was built in response to the Great Mississippi Flood of 1927 that inundated much of the Mississippi River basin.

Distance above New Orleans 32.8 river miles
Length of Weir Opening 7,000 feet
Number of Bays 350
Width of Bays 20 feet
Creosote Timbers 20 per bay
Floodway Design Capacity 250,000 cfs (cubic feet per second)
Length 5.7 miles
Width at River 7,700 feet
Width at Lake 12,400 feet
U.S. Lands 7,623 acres
Frequency of Operation (est.) 10 years
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

 

Corps New Orleans District commander Col. Richard Hansen said Tuesday that he expects all 350 bays of the spillway’s weir to be opened. Each bay has 20 creosoted timber “needles” that must be pulled by cranes moving on a rail atop the weir.

There are two graveyards located in the spillway. The cemeteries containing the remains of both free and enslaved African-Americans are under several feet of sediment. 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.  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 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 19th- and early 20th-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. 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. The January opening marks the earliest that the 5.7-mile-long spillway connecting the Mississippi to Lake Pontchartrain has been opened, and its 11th opening, since it was completed in 1931.

 

Morganza Spillway

The Morganza spillway stands between the Mississippi and the Morganza Floodway, which leads to the Atchafalaya Basin and the Atchafalaya River in south-central Louisiana. Its purpose is to divert water from the Mississippi River during major flood events by flooding the Atchafalaya Basin, including the Atchafalaya River and the Atchafalaya Swamp. In an extreme flood event, a major release of water from the Morganza Spillway into the Morganza Floodway and Atchafalaya Basin inundates not only the floodways themselves (between their levees), but extensive additional areas of southern Louisiana throughout the Atchafalaya Basin.

At risk in the Atchafalaya Basin are Morgan City (population 13,500), various smaller populated places, many farms, thousands of oil and gas wells, and considerable swampland. Inhabitants know that the region is a natural floodplain, and the Corps of Engineers issues written notices annually to all interests reminding them of the possibility that it might open the spillway and flood the area. Any decision to open the spillway must be carefully planned to give ample warning and protect life and property. Part of that planning process includes the Corps’ preparation of maps known as “inundation scenarios” so that interested parties can discuss how much water, if any, should be allowed through the spillway.

 

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About DW

New Orleans resident, writer, activist.

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