NOLA History: The Bonnet Carre’ Spillway

Every few years, heavy spring thaws in the north combine with showers and storms to create high-water conditions along the Mississippi River. Experts from the U.S. Army Corps of Engineers monitor measurement gauges placed up and down the river to know when the floodways should be opened. When the “Carrollton Gauge,” which marks the height of the river near Uptown New Orleans, hits 17′, the talk of opening the Bonnet Carre’ Spillway, located nineteen miles to the north, gets serious.

The story of the Bonnet Carre’ spillway begins with the Great Flood.

The Great Flood of 1927 was one of the triggers of the Great Migration of African-Americans from the rural states of the South to urban centers in the Northern United States. The aftermath of the flood, in which levees along the Mississippi River were blown up in attempts to relieve the flood-stage conditions threatening the city of New Orleans, also heralded a more robust plan to protect New Orleans from its greatest benefactor, the river. The defense plan included raising the river’s levees to unprecedented heights, as well as building a controlled diversion for the river’s water north of the city.

Construction on Bonnet Carre’ began in 1928 and the spillway was completed in 1931. It has two components: a series of floodgates (originally 285, later expanded to 350) running parallel to the river for approximately 1.5 miles in St. Charles Parish, and a floodway extending six miles north to Lake Pontchartrain. When opened, the spillway’s gates divert the excess waters of the river into its floodway, then into the lake. Lake Pontchartrain, which flows into Lake Borgne, then into the Gulf of Mexico, can easily absorb the flow of river water, allowing it to dissipate into the Gulf.

The Bonnet Carre’ Spillway was first opened in response to high-water flood conditions in 1937. It has been opened nine additional times, most recently on May 14, 2011. Most openings have lasted for about a month. The longest openings were 57 days in 1945 and 75 days in 1973. Unlike the Morganza Floodway, constructed in 1954 north of Baton Rouge, the Bonnet Carre’ has been opened enough times that no one has tried to establish permanent developments inside the floodway. With nothing in its path, the decision whether to open Bonnet Carre’ usually involves more discussion about the major incursion of fresh water into the brackish Lake Pontchartrain that occurs when it’s opened. That much fresh water in the lake has a major impact on shrimp and oyster production for some years after flood conditions pass.

In-between openings of the Bonnet Carre’ Spillway, the floodway is a recreational and commercial wonder. Families, scout troops and other groups go camping, boaters and fishermen can launch their boats from facilities in the floodway, and hikers can walk the floodway, exploring wildlife. Hauling companies also come in and scoop up truckloads of the rich silt left over from when the waters recede, providing lawns and gardens across the New Orleans metro area with an incredible boost in fertility.

Visitors can see the ultra-high waters of the river when flying into the city, at the Moonwalk in the French Quarter, or at Audubon Park. They don’t have to fear flooding, however. Even Twain would be impressed at just how well the Corps of Engineers has managed to get the river to “change its mind” when Bonnet Carre’ is opened.

“The Mississippi River will always have its own way; no engineering skill can persuade it to do otherwise.” — Mark Twain.

Edward Branley is the author of New Orleans: The Canal Streetcar Line, and Brothers of the Sacred Heart in New Orleans, both books in Arcadia Publishing’s Images of America series. He is partner at @Yatmedia and on Twitter as @YatPundit.

 

 

Tagged as: Bonnet Carre' Spillway, Mississippi River, New Orleans flood protection, New Orleans History