The Mississippi River has been above flood stage for the longest recorded time in history and now Tropical Storm Barry will bring heavy rain and a storm surge of 3 to 6 feet to Southeast Louisiana.
The swollen Mississippi River is already above 16 feet in New Orleans. Some levees in the Lower 9th Ward, St. Bernard Parish and Algiers are less than 20 feet high, according to data from the U.S. Army Corps of Engineers, which maintains 10 percent of the nation’s levees.
In New Orleans, where 50 levees failed in 2005 during Hurricane Katrina, Mayor LaToya Cantrell has declared a state of emergency. Some Louisiana parishes are under mandatory evacuation orders.
"The river is already more than 10 feet higher than it normally would be," Dr. Scott Hagen, director of the Louisiana State University Center for Coastal Resiliency, told weather.com. "So it doesn't take a whole lot of surge to put us in a more precarious position. Even without any event at all, we are in a challenging situation—such high flows for such extended periods of time along hundreds, literally thousands, of miles of levees along the Mississippi River.
"The official forecast is showing a three foot rise in New Orleans," National Weather Service hydrologist Jeff Graschel told weather.com. "We're currently at 16 feet and we're showing a three foot rise." That forecasted river rise in New Orleans for Saturday, Graschel said, accounted for an expected one to three foot storm surge plus 10 to 20 inches of rain.
A USACE representative disputed the USACE data showing levee heights below 20 feet, according to NOLA.com, without accounting for the disconnect between their maps and statements.
"The information came from a database that had old data," John Rahaim, director of Homeland Security in St. Bernard Parish, told weather.com. "All the levees in St. Bernard Parish are up to where they should be. We don't feel we have any issues with the levees with any overtopping at this time."
Ricky Boyett, U.S. Army Corps of Engineers spokesman, told weather.com on Friday that the Mississippi levee system actually ranges between 20 to 25 feet high. "We're not showing overtopping in the New Orleans area," he said.
John Monzon, who manages all of the levees on the West Bank of New Orleans, told weather.com, "Everybody's pushing this 20 foot magical number." He stressed that river height and vulnerability are not uniform along the lower Mississippi.
Monzon did not confirm that all levee heights on the West Bank are at least 20 feet high. He stressed that "20 feet" was an arbitrary number.
Alternately, "What we should be focusing on is freeboard," the amount of space between the top of the river and the crown of the levee, Monzon said on Thursday. "All our levees on the West Bank as of 2 p.m. yesterday still had 6 feet of freeboard." Monzon went out Wednesday and measured the freeboard at multiple gages.
"Five to six feet of freeboard is there right now," Hagen said. In other words, outside of a 6-foot storm surge, levees in New Orleans should be able to contain the river.
Sandbagging efforts were underway Thursday in vulnerable stretches of levees, Derek Boese, chief administrative officer of the Southeast Louisiana Flood Protection Authority East, told weather.com.
"With all of the improvements that have been done since Katrina, the system is still in very good shape," Boese said. But he added about the storm, "People should be concerned."
The USACE feels confident that Barry won't bring the Mississippi over the banks of New Orleans' levees. That confidence comes from an intimate understanding of the levees' fortifications since Hurricane Katrina.
"After Hurricane Katrina, Congress provided us $14.5 billion to build a system that will defend against a one percent storm surge event," Boyett of the Corps told Weather.com Friday. "We are armoring it so the system continues to get stronger."
Other flood protections are working quietly upriver. The massive Old River Control Structure allows over 20 percent of the Mississippi to flow into neighboring the Atchafalaya River. And the Bonnet Carre Spillway, which was opened twice this year, additionally relieves the Mississippi, passing 100 million cubic feet of water per second into a protected floodway, according to Boyett, and ultimately putting a cap on the amount of water that can reach the highly populated cities of Southern Louisiana.
Perhaps counterintuitively, the high river itself protects against storm surge. An angry ocean surging into the coast has to battle with a massive, swollen river traveling in the exact opposite direction. However much storm surge reaches New Orleans 100 miles up the coast is how much a storm has won over the river.
But the bloated river itself is a reality check against the amount of storm surge it can fend off. The Mississippi is entering hurricane season 10 feet higher than average, only four feet below the levee tops. And the Mississippi valley is seeing a new normal, and human-caused climate change is a notable factor.
“Rainstorms are getting more intense,” Barry Keim, Louisiana’s state climatologist told weather.com. “There’s a pretty good indication that we’re going for longer periods without rain, but when we do get rain these short bursts are producing more rainfall than they used to.”
climate scientists have attributed some extreme events along the Mississippi to human activity. A 2016 study attributed a three-day extreme rainfall event in south Louisiana, which brought 30 inches of rain to parts of the state, to human-caused climate change.
Not only is climate change to blame for the intensity of that event, the study additionally concluded that a similar event hitting the Central Gulf Coast region “is now expected to occur at least 40 percent more often than it was in our pre-industrial past.”
Boyett said, "We want to get through this event without water in the city, but we know that as soon as this event passes, we're still in a flood fight. And we're going to be in a flood fight for a little while longer."