Shifting forecasts of the Mississippi River’s flow rate means the U.S. Army Corps of Engineers won’t open the Morganza Spillway any earlier than Thursday, officials said Sunday.
The Corps had not determined whether opening the spillway would even be necessary, said Ricky Boyett, chief of public affairs of the Corps’ New Orleans District, on Sunday.
The spillway diverts water from the Mississippi River north of Baton Rouge into the Atchafalaya Basin.
If they do open the spillway, the Corps does not believe it will cause any additional flooding in the Atchafalaya Basin, Boyett said.
The Corps’ projections show water will hit about the same levels as the 2011 flooding reached in the basin regardless of whether water from the Mississippi is diverted through the spillway into that area, he said.
Flooding in the basin caused by high water on the Red River should subside just as water from the Mississippi begins pouring into it from the Morganza Spillway if it’s opened, Boyett said.
While water levels won’t be higher, opening the gates likely would prolong the duration of flooding in the basin, he said.
“Regardless of whether Morganza is operated or not, the water will be about the same,” Boyett said. “The water levels we’re seeing in our models are about the same with or without operating the Morganza.”
The trigger for opening the spillway in northern Pointe Coupee Parish occurs when the river level reaches 57 feet at the structure and there is a 10-day forecast that shows the river will be flowing 1.5 million cubic feet per second. Changing forecasts of the river’s flow rate have prompted the Corps to delay the possible opening to Thursday, Boyett said.
The Morganza Spillway has been opened twice — in 1973 and in 2011 — since it was completed in 1954.
The Mississippi River in Baton Rouge was at 40.4 feet Sunday and was forecast to rise to 43.5 feet by Jan. 18 before starting to recede.
Boyett said the Corps’ models continue to show flooding in the Atchafalaya Basin roughly in line with levels seen in 2011. Although conditions this year are different — including wet ground throughout the basin — Boyett said residents and property owners should use 2011 flood levels to help plan for flooding this year.
“If they were impacted in 2011, they need to plan for some impacts this year as well,” Boyett said.
The Atchafalaya River at Morgan City was at 6.6 feet on Sunday and is expected to slowly rise by Jan. 23 to 8 feet. When it reaches 8 feet, buildings on the river side of the protection walls in Berwick will flood and water will overtop the Rio Oil Co. dock in Morgan City, flooding buildings at the foot of Ann Street on the river side of the floodwall, according to the National Weather Service.
Upstream on the Atchafalaya, the river at Butte La Rose was at 18.5 feet Sunday and is expected to crest at 20 feet by Jan. 20.
Follow Bryn Stole on Twitter, @brynstole
Advocate staff writer Amy Wold contributed to this report.