LSU Scientist Warns Of LMR Course Change

A hydrologist at Louisiana State University (LSU) presented research at the American Geophysical Union’s fall meeting in New Orleans, La.,  earlier this month on how sedimentation near the Old River Control Structure combined with a major flood could result in the Lower Mississippi River changing course above Baton Rouge, La. Old River is located roughly halfway between Baton Rouge and Natchez, Miss.

The doomsday scenario is not new. John Barry’s 1997 book Rising Tide, which chronicles the Great Flood of 1927, ends at Old River: “Many engineers believe that sooner or later, no matter what man does, the Mississippi will shift its channel to the Atchafalaya. And a finger of the sea will climb north past New Orleans, north to Baton Rouge.”

In fact, a Google search of headlines from 2011 during the last major flood on the Lower Mississippi River unearths an ominous string of headlines related to Old River, including “America’s Achilles’ heel: the Mississippi River’s Old River Control Structure” from Weather Underground, “As the Army Fights the Mississippi River, Who Is Winning?” from Popular Science, and “Is This The Year The Atchafalaya River ‘Captures’ The Mississippi?” from Forbes.

The Atchafalaya is a shorter and steeper course to the Gulf of Mexico than the Mississippi’s current route. From Old River, the distance to the Gulf of Mexico is only 142 miles by way of the Atchafalaya compared to 315 via the Mississippi, according to the U.S. Army Corps of Engineers’ Old River fact sheet. Starting in the late 1800s, the Atchafalaya River began capturing more and more of the Mississippi River. Large floods, like the Great Flood of 1927, sent a higher volume of water down the Atchafalaya, scouring that channel, thus making it an even more efficient route. By the 1950s, the Atchafalaya River was capturing around 30 percent of the Mississippi River. Realizing a course change was imminent, Congress directed the Corps to construct a barrier to maintain the Mississippi on its current path. The result was the Old River Control Structure.

Old River is made up of four components—the Low Sill, Overbank and Auxiliary structures and a hydroelectric station—which together regulate the Mississippi River as it flows past the Atchafalaya River. The four structures work in tandem to maintain a 70-30 split between the Mississippi and Atchafalaya.

The Low Sill and Overbank structures were completed in 1962. The Low Sill Structure came close to failing in 1973, though. The Corps began construction on the Auxiliary Structure in 1981, with construction complete in 1986. The power plant began operating in 1985.

LSU Research

The new research was conducted by LSU hydrologist Yi-Jun Xu, whose specializations include hydrologic modeling and riverine nutrient and sediment transport. Xu looked at sediment accumulations in the Mississippi River from Old River all the way to the Gulf of Mexico. Most interestingly, the research found that some 36 million metric tons of sediment has accumulated below the Old River Control Structure, elevating the river floor and narrowing the river channel in the vicinity by a half mile.

Sediment accumulations below Old River would reduce the Mississippi River’s ability to pass water during a flood event, and Xu believes a potential outcome would be the river overwhelming the structures at Old River. Xu concluded there is already enough sand below Old River to trigger a course change in the event of a major flood. Not only that, Xu’s study points out that some weather forecasts believe the Mississippi’s flow will increase by 11 to 60 percent by the end of the 21st century. The increased flow could be exacerbated by rapid runoff caused by urbanization all along the Mississippi River Basin.

“When a mega flood comes, it will overpower the Old River Control Structure, if the river floor elevation continues,” Xu said.

The result of a course change would be catastrophic for South Louisiana and the nation as a whole. The increased flows down the Atchafalaya would disrupt communities along the river like Simmesport, Krotz Springs and, near the Gulf, Morgan City. It remains to be seen what a permanent, dramatic increase in flows on the Atchafalaya River would mean to those communities. What is more certain is the effect a course change would have on the Mississippi below Old River.

Below Old River, the former Mississippi River channel would become a salty tidal inlet of the Gulf of Mexico. For communities that draw their drinking water from the Mississippi River—New Orleans, Metairie, Kenner, Houma and Thibodaux—the course change would be a public health disaster. Baton Rouge currently draws its drinking water from an aquifer separate from the Mississippi River.

What’s more, industry between Baton Rouge and New Orleans would suffer from the lack of fresh river water. Refineries and petrochemical facilities draw fresh water from the river. In short, the people and industries that drive Louisiana’s economy—and provide energy to much of the nation—would be in jeopardy.

“If that happened, it would directly affect the lives of nearly 2 million Americans as well as the multi-trillion-dollar petrochemical assets along the Lower Mississippi River,” Xu said.

Ricky Boyett, a spokesman for the New Orleans Engineer District, said that, while the Corps has not dredged the Mississippi River in the vicinity of Old River simply because the river there is a shallow draft channel, the agency has monitored sediment accumulations there. The channel, based on Corps observations, is “getting shallower as sediment accumulates in that area,” Boyett said. That sedimentation is due to several processes, he said, which all affect one another.

“These effects include long-term variations in sediment supply from the watershed; adjustments in river geomorphology resulting from cut-offs, revetments, and other infrastructure constructed over the past century; and the effects of the Old River diversion itself, both the natural channel that existed before 1962 and the complex as it has operated since,” Boyett said.

Boyett added the Corps is undertaking an assessment of the Mississippi River to investigate how the channel itself is changing over time.

“We cannot discount that there may also be unknown processes at work, so our knowledge of what is occurring will be better informed through further study,” Boyett said.

To combat sediment buildup at the Old River structures and on the Atchafalaya River side of Old River, the Corps periodically increases flows through the structures to flush out that part of the channel. The Corps plans to do just that December 27 and 28 during daylight hours. The Corps released a notice to mariners last week warning of increased stages and velocities on the Atchafalaya River as far down as Simmesport and on the Mississippi River near Old River.

“Mariners should exercise extreme caution when navigating through these areas during the above-mentioned time period,” the statement read.