The U.S. Army Corps of Engineers is using a variety of measures, including opening three massive river diversion features—the Bonnet Carré, Birds Point-New Madrid and Morganza floodways—to relieve pressure on the Mississippi River watershed.
Water spewed skyward at the rate of 10,000 cu ft per second on May 14 as the first vertical-lift gate was opened on the Morganza Floodway. It was the Corps' third big control measure in the flood fight and marked the first time that three main control structures on the lower Mississippi were opened at the same time.
The Corps' first move was blasting apart an earthen levee to open the Birds Point-New Madrid Floodway on May 2. That dropped river levels at Cairo, Ill., where the river gauge was at 61.7 ft and forecast to rise to 63 ft. The action diverted an estimated 550,000 cfs, significantly lowering river gauges in a matter of days, says Jim Pogue, public-affairs chief at the Corps' Memphis District.
On May 9, with the flow rate reaching 1.25 million cfs near New Orleans, Maj. Gen. Michael Walsh, Mississippi Valley Division commander, opened the second stopper—the Bonnet Carré Spillway, just above New Orleans—to relieve pressure on the Mississippi River levees and protect the metropolitan area.
By May 16, with 330 of 350 bays open, the Bonnet Carré was diverting 311,000 cfs to Lake Ponchartrain, exceeding the spillway's 250,000 cfs design capacity.
“Twice in its history, the structure has passed more water than 250,000 [cfs]safely,” says Lee Mueller, a Corps spokesman. “We are watching the guide levees and making sure we maintain them according to freeboard.”
But despite the opening of those relief valves, by May 14, the flow past the Red River Landing, near Baton Rouge, had reached 1.5 million cfs, the Corps' trigger for opening the Morganza Floodway.
At the time, Col. Edward Fleming, the New Orleans District (NOD) commander, said only one of Morganza's 125 bays would be opened on the first day and that other bays would be opened systematically to avoid scouring on the back side of the structure and environmental impacts as well as to allow for the evacuation of an estimated 25,000 people downstream. By the end of the second day, the Corps had opened 15 bays to divert 150,000 cfs, says Ricky Boyette, Corps spokesman. “The structure has the potential to be open for the better part of three weeks,” Walsh says. Morganza's capacity is 600,000 cfs.
Levees can sag or fail because of extended periods of saturation and the drawdown when a flood recedes. “We had major sloughing in 1973, so we're keenly aware of that threat,” says Tom Holden, Corps' NOD deputy director of project management. “In regards to the Mississippi levees, we are having the same stage floods [as] in 1973, but the duration is longer.”
In 1973, the Corps constructed setback levees and rock dikes to combat sloughing. “We will have to be vigilant going forward,” Holden says. “We have more than doubled our normal efforts.”
The NOD emergency operations center is on a 24-hour schedule, performing daily levee inspections, says Rachel Rodi, spokeswoman. Engineer-inspectors walk levees, monitoring sand boils and seepage. The Corps has distributed “hundreds of thousands of sandbags and tens of thousands of Hesco baskets,” Fleming says.
“Public safety is our highest priority, and we are making sure we operate this system as a system,” Walsh says. “This is a marathon, not a sprint.”
The flood-control plan is designed to control the “project flood,” which is a flood larger than the record 1927 flood. Compared to the 1927 event, the project flood is 11% greater at the mouth of the Arkansas River and 29% greater at the latitude of the Red River Landing.
The Mississippi River levees are designed to confine flow to the leveed channel, except where it enters the natural backwater areas or is diverted purposely into the floodway areas.
