The Mississippi River’s rise in recent weeks is prompting the U.S. Army Corps of Engineers to perform a test run of the Bonnet Carre Spillway on Monday, preparing for the possibility that it will have to be opened for the first time in five years.
Current projections show water levels right around the trigger for at least a partial spillway opening. The river has been rising over the past couple months, which is common for the spring, as rainfall from storms in the Midwest and South trickle down to Louisiana.
Forecasts issued Friday morning say significant rises will occur on the lower Mississippi for the next two weeks, with the river cresting in New Orleans at 16.5 feet on April 27.
The Bonnet Carre opens when the river flow rate reaches 1.25 million cubic feet per second, which usually translates to around 17 feet on New Orleans’ Carrollton gauge, or roughly 17 feet above sea level. At this point in the flood cycle, however, higher flows are correlating with slightly lower elevation, said Matt Roe, a spokesman for the New Orleans district of the Army Corps. This could mean opening the spillway at 16.5 or 16.6 feet.
Aerial of Interstate 10 looking west as it crosses Lake Pontchartrain at the Bonnet Carré Spillway, seen Friday, September 13, 2024, in Kenner, La.
“Operation of the spillway is still in the forecast,” said Roe, but the agency is waiting to see if the forecast pans out as projected before making a recommendation to open it.
If a spillway opening does happen, it is not likely to be long, as forecasts indicate river levels will gradually fall a few days after cresting.
“We’re still monitoring the forecast daily and talking to the National Weather Service daily to get a feel for how close the forecast is going to be to actual conditions,” Roe said.
The test run
The Bonnet Carre Spillway, located in Norco, links the river to Lake Pontchartrain through floodways and bays that are opened at high river levels. Opening the spillways alleviates pressure on the levee system in South Louisiana, but the rush of fresh water into the lake can harm commercial shrimp and oyster fisheries. The spillway has not been opened since 2020.
The “test opening” on Monday is something the Army Corps tries to conduct every year when the water gets high enough, regardless of whether they plan to open the Bonnet Carre.
“It’s just a real-world experience for the crews to get out and open the structure,” Roe said.
A train travels along a track near where the Bonnet Carre Spillway and Lake Pontchartrain meet, Tuesday, October 24, 2023. (Photo by Brett Duke, The Times-Picayune)
The Bonnet Carre was built following the flood of 1927, and the technology at the structure is largely unchanged, Roe said, making the opening a “very labor-intensive process” for the crews.
Opening the structure requires using cranes to individually lift a certain number of timber beams called “needles” from the bays of the control structure. When this process is initiated, water is typically already seeping through the structure, straining against the timbers due to the high river levels.
“When the river is high enough like it is going to be on Monday, we do have water passing through the structure, and so it will very closely emulate an actual operation,” Roe said.
During the test run, crews will open a few bays and then close them that day.
