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Engineers to examine how well Old River structures keep Mississippi on the right track

September 4, 2023   The Advocate

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North of New Roads and Morganza, a roughly 60-year-old collection of dams, channels, locks and guide levees has kept the Mississippi River from jumping its course and taking the steeper, more direct route down the Atchafalaya River.


With climate change promising to put the Old River Control Complex to the test, the U.S. Army Corps of Engineers has wanted to have a close look and, potentially, make repairs to a piece of that complex that was severely damaged during historic flooding in 1973.


The construction firm McMillen Inc. of Boise, Idaho, has been awarded a $34.2 million contract to build a steel dam so the structure, known as the Low Sill Control Structure, can be blocked off and dewatered next year for the first time since 1987 for inspections and possible repairs.


"By keeping the Mississippi and Atchafalaya rivers on their current courses, the Old River Control Complex infrastructure is critical to both the region and the nation," Col. Cullen Jones, commander to the Corps' New Orleans District, said in a statement Thursday. "This effort to inspect and repair the Low Sill structure will help ensure the integrity and successful operation of the complex for years to come."


Without Old River, the Mississippi likely would have done what Nature has been seeking and already headed down the Atchafalaya, leaving Baton Rouge and New Orleans without enough river water for commerce and also, south of Baton Rouge, enough fresh water for 1.2 million people.


This summer's hot, dry spell that has kept Mississippi levels low, but it follows more than a decade of repeated and extended high water periods on the lower river.


Of the 15 times the Bonnet Carré Spillway has been opened upstream of New Orleans to protect the city, six have occurred since 2011 and four between 2018 and 2020, including twice in 2019.


The river hit an all-time record height near Morganza in 2011, forcing the opening of the Morganza Floodway for only the second time in its history. It helps protect Baton Rouge and New Orleans but floods 25,000 acres of farms on an overland path to the Atchafalaya River and past Morgan City.


Located in Concordia Parish, the Old River Control Complex sits north of both of those Corps of Engineers-operated flooding safety valves, where the Mississippi, Red and Atchafalaya rivers meet. Old River keeps their waters locked into their respective flows, circa 1950.


Consisting of three major features, the Low Sill, Overbank and Auxiliary structures maintain a 70/30 downstream split between the Mississippi and Atchafalaya from the combined upstream waters of the Mississippi and the Red.


In this area, the Red ends and the Atchafalaya begins just west of the Mississippi.


In 1973, high waters caused one of the Low Sill's upstream guide walls to collapse and scoured beneath a wide area of its foundation, down into its steel support piers.


Emergency repairs kept the Low Sill from collapsing then, Corps officials said. The Morganza Floodway also had to be opened to relieve pressure.


Even before more permanent repairs to the Low Sill were made in the late 1980s when the structure was last dewatered, Corps officials had known a fixed Low Sill wouldn't be able to handle as much water pressure as it could previously. Before those repairs, the agency built a then-new third structure, the Auxiliary Structure, to compensate.


Corps officials want this latest, steel structure finished so the 566-foot-long Low Sill, which continuously handles water from the Mississippi, can be dewatered in the late summer or fall of 2024 when the river is traditionally in a low-water period.


Underwater inspections are already done regularly, but the Corps plans to take a much more thorough look at the Low Sill.


Ricky Boyett, a Corps spokesman in New Orleans, said dewatering the Low Sill will allow officials to see parts of the structure they haven't been able to view previously and allow for direct testing of the Low Sill's foundation among other checks.


Repairs or other steps could be taken based on what those inspections find, he added.


"It's going to be multiple things happening once we dewater it. This contract is going to be the key to allow us to dewater it, if you will," Boyett said.


Last year, Corps officials suggested the new dam could be earthen, but the agency is going instead with the steel structure, which can be reopened temporarily if the river gets too high.


"Today we can't predict when the water will rise, but if we're in the middle of doing it and the water does rise, then we have to have a plan already in place of how are we able to secure the site, let the water come back in, but then as soon as it goes down, get back in there," Boyett said.


The operations of Bonnet Carré, Old River and Morganza during periods of high water have all triggered criticisms, in the past, from landowners, communities and also industries that rely on fisheries affected by the facilities, particularly Bonnet Carré.