Since it was completed in 1962 the Old River Control Structure has been the head of the Atchafalaya River and is designed to siphon 30% of the flow of the Mississippi into the Atchafalaya, no matter how big the flood is flowing down the Mississippi. It is also head of the West Atchafalaya Floodway. A pair of levees, set about 17 miles apart define the edges of the Atchafalaya Floodway.On the west the protection levee stretches for 74 miles from Bayou Courtableau south to Bayou Teche. On the east the protection levee extends from Morganza 90 miles south to Morgan City. Short guide levees direct flood water from the Old River Control Structure and the Morganza Structure to the east and west floodways.
A second pair of levees bracket the Atchafalaya itself for 77.9 miles on the west and 51.9 miles on the east. South of the end of these levees, the flooded Atchafalaya can inundate the whole floodway from protection levee to protection levee. Ring levees protect the cities of Simmesport, Melville, and Morgan City.
The U.S. Army Corps of Engineers defines the Project Flood as one that flows past Red River Landing at 3 million cubic feet per second. The Old River Control Structure, the West Atchafalaya Floodway and the Morganza and East Atchafalaya Floodways, if fully opened, would siphon off 1.5 million cfs and leave 1.5 million cfs to flow south to the Bonnet Carre, where another 250,000 cfs are siphoned off, leaving the rest, 1,250,000 cfs to stream past New Orleans to the Gulf of Mexico. Water drains out of the floodway along the Atchafalaya River and through the Wax Lake Outlet to the west of the river.
The Corps of Engineers opened the Bonnet Carre Spillway on Monday, May 7th. The engineers will consider opening the Morganza Structure over the weekend of May 13th.