Posted on February 6, 2018
By Steve Hardy, The Advocate
If you drive out to the middle of nowhere and keep going, you can see where the U.S. Army Corps of Engineers is battling with the forces of nature.
At the confluence of the Mississippi, Atchafalaya and Red rivers, the Corps has erected towering gates that bend the flow of the water. Without human intervention, the current channel of the Mississippi River would slow to a trickle while the Atchafalaya would swell.
In the 1973 flood, currents swept away a 67-foot concrete wall used for river control, and the entire structure that took four years to build came perilously close to being washed out overnight. That would have set off a chain of catastrophes ending with the crippling of America’s agricultural and petrochemical industries, the loss of New Orleans’ drinking water and the drowning of Morgan City.
The Corps has fortified its river control structures since the 1970s, but research published last year by an LSU professor warns that the river is still one bad flood away from forever altering the landscape of Louisiana. Other experts, including those at the Corps, say the threat isn’t so imminent, but they agree that officials need to seriously examine how they are reining in the water.
Today’s Mississippi River has existed for only a few hundred years — a blip in geological time. The river is constantly eroding channels in some places and dropping sediment in others, causing its path to wander. Over the millennia, the river has reached the Gulf through St. Bernard Parish and via what are now the bayous Cocodrie, Teche, and Lafourche. It’s been in its current channel only since the Middle Ages. Left to its own devices, the river would shift west toward the Atchafalaya channel.
In 1900, around 5 percent of the water from the Red River and the upper Mississippi was going down the Atchafalaya; by the 1950s, the Atchafalaya was collecting about 30 percent, said Tulane University professor Mead Allison, director of physical processes and sediment systems at the Water Institute in Baton Rouge.
Had the trend been allowed to continue, the Atchafalaya would have become the predominant path of the Mississippi River by the 1970s or 1980s, Allison said.
That would dry up the docks from Baton Rouge to the coast where farmers, oil refineries and chemical plants load their goods. Additionally, much of southeastern Louisiana, including New Orleans, relies on the river for drinking water. If the water level dropped, it wouldn’t be able to push out the Gulf of Mexico, so saltwater from the coast would creep into the drinking supply, said James Barnett Jr., author of “Beyond Control: The Mississippi River’s New Channel to the Gulf of Mexico.”
Faced with the prospect of relocating so many residents and infrastructure, it was easier to take on the gargantuan task of hemming in the river.
In 1959 the Corps opened the first components of what’s now known as the Old River Control Structure, which takes its name from a stretch of river bypassed in the 19th century with a man-made canal.
The facility regulates the flow of the rivers, sending prescribed percentages of water down the Atchafalaya and the Mississippi every day, but it is not designed to mitigate floods.
The complex now includes three separate control facilities, levees, canals, and a lock/dam. A private hydroelectric power station operates nearby.
The control structure “stopped time” on the Mississippi River, said Army Corps public affairs officer Ricky Boyett. The Red and Mississippi rivers continue to send 30 percent of their combined flow down the Atchafalaya, while the lower Mississippi claims the remaining 70 percent, just as in the 1950s.
But the Mississippi isn’t just water. It also carries sediment that is piling up on the riverbed and constricting the downstream flow, which causes water to back up during a flood. Every cubic yard of silt means that much less water can fit in the channel, and the accumulation has built up over the years. Left to its own devices, the sedimentation would have been one of the factors driving the Mississippi flow into the Atchafalaya, before the Corps locked in the 70-30 split.
The riverbed downstream from the control structure has risen 30 feet since 1992, LSU hydrology professor Yi-Jun Xu said. Some sandbars have tripled in size, according to a summary of his research provided for the American Geophysical Union, where Xu presented his findings in December.
The professor believes a sufficiently large river swell could overwhelm the control structure, sending the Mississippi careening down the Atchafalaya basin.
Other river researchers generally defer to the Corps on the matter, saying that the matter deserves investigation but pointing out that the federal agency is performing a Mississippi River study to be finished in coming months that will inform how they move forward.
Boyett said the Corps never says never, but that the Mississippi does not show signs of overwhelming the control structure any time in the near future, especially since the Corps built the new auxiliary control structure after the dangerous flood of 1973. The Corps anticipates having at least a decade or two until it needs to start worrying.
Xu pushes back against that assessment. It would take a “huge” flood or a major hurricane, he allowed, but one of those could strike any year. In 2011, the water in the Mississippi rose high enough that the Corps had to take the rare precaution of opening the nearby Morganza spillway, a flood control structure that tamed the water.
Scientists don’t yet know what precise weather conditions could overwhelm the Old River complex, but anything stronger than the 2011 event would present a “real risk,” and the risk will continue to increase as more sediment is deposited in the channel, Xu said.
“These changes diminish the river’s capacity to carry water on its current course. When sections of the river’s floor rise to a sufficient point, a sudden increase in flow — perhaps from a flood — could drive the Mississippi River to overwhelm the control structure and adopt a new path, potentially causing the Mississippi to be captured by the Atchafalaya,” his research summary states.
U.S. Rep. Garret Graves, R-Baton Rouge, who chairs the House subcommittee that oversees Corps projects, also doesn’t believe the control structure is at risk of failing anytime soon. Still, he sent Xu’s findings to them and has asked that they respond.
“It definitely is a concern,” he said.
For example, the 2011 flood was able to push the foundation of the control structure. It was a small amount, but enough that people took notice, Graves said.
During that flood, Corps officials realized they were going to have to open the Morganza spillway, which relieves pressure on the Mississippi during high water by opening gates along the river to allow some of the flow to drain into a leveed area of the Atchafalaya Basin. It’s a rare maneuver. Since it was built in the 1950s, the spillway has been opened only in the 1973 and 2011 floods.
The spillway opens just a bit downstream of the control structure, and the same 36-member crew operates both sites.
In 2011, the Corps threw open the spillway gates before it normally would have. Historically, the Corps has made decisions based on the amount of water flowing down the Mississippi each second. During the 2011 flood, the river got dangerously high, and the Corps had to open the spillway before it hit the normal flow threshold to keep the Mississippi from overtopping the levees, assistant operations manager Kayla LeBlanc said.
“So something had changed in the river. … That operation manual has been rewritten,” she said.
The accumulation of sediment could have been a contributing factor, LeBlanc said.
The Corps is aware of sediment build-up and is taking several steps to address it. When the river is moving quickly, the Corps sends it through the older “low sill” control structure because it’s a straighter path, so less sediment drops.
When the Mississippi is sluggish and dropping silt anyway, the Corps runs it through the auxiliary structure that was built to beef up the site’s capacity after the 1973 flood. That means that sediment builds up beyond the auxiliary structure, so the Corps occasionally sends extra water down the route to flush it out. A good flush can dislodge about 250,000 cubic yards of sediment, LeBlanc said.
Corps officials say they are mindful of the need to come up with long-term solutions. The agency’s Mississippi Valley Division is currently conducting a technical assessment of the river and its tributaries, rolling in data from the 2011 flood. The New Orleans office, which oversees the Old River Control Structure, expects the results within a year and will use the findings to determine what strategy is best.
Experts said that could take a variety of forms, including dredging the river, being more flexible with the 30/70 split, redesigning the control structure, and looking for solutions farther north. All would require money and time.
Meanwhile, the river is vulnerable, Xu argues. No one can predict when the big flood will happen; it could come in a hundred years or this spring.
“There’s a real risk,” he said.
Scientists must perform more research to figure out precisely what conditions could overwhelm the control structure, but Xu warned that anything above the level of the flood seven years ago could bring ruin.
Allison, of the Water Institute, however, cautioned it would probably take a “catastrophic flood of epic proportions.” Such a flood is technically possible, he said, and the matter deserves study, but the state and the Corps are investigating, and he urged the public not to panic.
Authorities also wondered whether a megaflood would necessarily divert the course of the river and whether the Corps could get it back on its current track before it permanently settles into the Atchafalaya channel. Allison said it could depend on the particular flood but that he was not aware of any models that examined where the excess water would go in a river disaster.
A worst-case scenario channel switch would reverberate worldwide, said Robert Twilley, executive director of Louisiana Sea Grant. All the grain that floats down the Mississippi from the Midwest that would lose access to ports, he said. Then there are the oil and gas companies, plus all the other goods that move through river ports.
The Corps estimates an economic loss of $14 million per day should the structure fail.
Then there’s the human cost.
The Atchafalaya doesn’t have enough room to receive all the water that would be forced upon it. “You don’t have that depth,” Twilley said.
“Basically that whole region becomes inundated. … Morgan City is gone, pretty much.”
The state’s coastal master plan would have to be scrapped as the Mississippi and Atchafalaya are both so heavily tied to the encroachment of the Gulf of Mexico, he said.
Twilley said he didn’t have the research background to discuss the odds the structure might fail but remarked that it “is hard to even think about. It’s daunting to think of that scenario.”
Barnett, the author and historian, said some people near his Natchez home are already seeing the fallout from the rising riverbed. Folks who own land in the batture — the space between the river and the levees — have long grown timber. The trees can handle a bit of high water now and then, but the Mississippi has crept closer and closer to the levees, and now the land is underwater so often that the trees drown and die, Barnett said.
The most extreme proposal he’s heard is to allow the Mississippi River to adopt the Atchafalaya channel, but to use tools like the control structure to make sure the switch happens in a gradual, scheduled way. Barnett emphasized that he is not advocating that path, though it would put an end to the Corps’ “arms race with mother nature.”
Hurricane Katrina soured a lot of people on the Corps, observed Nicole Gasparini, a Tulane earth and environmental sciences professor. But without the federal agency “there would be no New Orleans. … There would be no Mississippi River.”
Societies thrive and die by their rivers. Just look at Chinese history, where dynasties rose and fell based on the course of the Yellow River, Gasparini said. Yet the Corps’ levees and control structures have been keeping the Mississippi in place for nearly half a century after it would have jogged west if left to itself.
Nevertheless, the river demands vigilance, especially as changing weather patterns threaten to increase rainfall, or at least make it harder to predict how the water will behave.
“We’re being proactive with the unique dynamics of the Mississippi River,” LeBlanc, of the Corps, said.
While the sediment has grown over the years, the problem does not come as a surprise.
“Back in September 1952, consultant Lorenz Straub had voiced his concern that the Old River outflow channel might fill with sediment and be unable to clear itself. According to the 2011 report from the St. Louis District’s Applied River Engineering Center, Straub’s apprehension was coming true,” Barnett wrote in “Beyond Control.”
“(Ivan) Nguyen and colleagues reported that the low-sill and auxiliary structures ‘could become totally buried with sediment during an extreme event.’ … By coincidence, the ink was barely dry on (the study) when the Mississippi produced an ‘extreme event,’ ” he wrote, referencing the 2011 flood.
However, Corps officials have expressed confidence that it will be able to clear the sediment or otherwise address the issue before it becomes a pressing concern.
Any change on the river will require consideration of ecology, economics and other factors on both the Mississippi and the Atchafalaya, added Boyett, the Corps spokesman. Academic studies have their place, he said, but putting a new operational plan in place requires thinking about every angle and securing public support.
“There is nothing easy about water management in south Louisiana,” Boyett said.
Source: The Advocate
