The U.S. Army Corps of Engineers closed the final floodgate to the Morganza Spillway last Thursday.
The Corps started opening the floodway, located about 35 miles northwest of Baton Rouge, on May 15 — for the first time since 1973. The number of gates open on the Morganza peaked at 17.
The gates, built in 1954, were designed to divert water from the flooding Mississippi River into the Atchafalaya River Basin, which empties into the Gulf of Mexico.
The Bonnet Carre Spillway was also opened to protect New Orleans from floods May 9, diverting river water into Lake Pontchartrain and relieving pressure on levee systems downriver.
Ultimately, 330 of the spillway’s 350 bays were opened before the Corps began closing them June 12. The last 20 bays were closed June 20.
The Mississippi’s water levels reached record heights of 45 feet in Baton Rouge in May. This was three feet below the primary levee line but five feet above what is considered a “major” flooding stage.
It was predicted that the levees around Baton Rouge could not withstand this pressure. The Old River Control Structure was built after the Great Mississippi Flood of 1927 to withstand water pressures in Baton Rouge of 1.5 million cubic feet/second. Officials feared pressure would top this, forcing them into action.
The Corps debated three scenarios. Regardless of choice, the Atchafalaya Basin had to be flooded to ensure the safety of levee systems in Baton Rouge and New Orleans.
In May, Governor Bobby Jindal predicted that 2,500 residents and 2,000 structures would be impacted by the opening of the spillway.
Local officials ordered mandatory evacuations for hundreds of residents in the Atchafalaya Basin, but flooding in those areas was significantly less severe than initially expected.
The Corps estimated water would flow from the gates at 114,000 cubic feet/second at the Morganza Spillway when it was first opened.
Though results were less dramatic than anticipated, the state faced several setbacks because of the flood.
According to The Associated Press, the Bonnet Carre Spillway carried the Mississippi’s fresh water into brackish Lake Pontchartrain, and from there into the Gulf, for six weeks. The fresh water could cause problems for oyster harvesters, whose coastal beds depend on salt water. The beds have been fragile since last year’s BP oil spill.
Another byproduct of the spring flood has been the deposits of silt carried down from the midwest and deposited near the river’s mouth south of New Orleans. Dredges have been working to keep the channel clear for shipping.
The port complex between Baton Rouge and New Orleans is among the world’s busiest. The silt caused an oil tanker to run aground near the mouth of the river in June, but it was eventually freed from the muck.
 
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Contact Laura Furr at [email protected]