The fear of the river levees being overtopped near New Orleans is unprecedented in modern history. And while the U.S. Army Corps of Engineers points out forecasts never predicted the river would spill over the levees and into the city itself, the agency is now looking at ways to better estimate and communicate the possible threats should the situation recur.
“The silver lining of it all is that it started this conversation,” said Jean Vossen, chief of the engineering division for the Corps’ New Orleans office. “This awareness of the combination of high river and surge — what is the appropriate modeling, the appropriate communication? Having been through Barry, it's part of our discourse now, and we’ve got to prepare for it.”
Barry eventually struck near Morgan City, with few impacts on metro New Orleans. But as the storm was developing, forecasters predicted it might substantially raise the level of the Mississippi River, which was already far higher than it normally is during the summer.
Some of the estimates had the river nearly reaching the heights of the levees that protect the area, although the projected impact was further complicated by disagreements over the exact heights of those levees.
Though still high for this time of year, the river is finally on an extended fall after more than nine months at above-average levels, so the chances the New Orleans area will face such a threat again this year have diminished.
Barry itself was the product of a confluence of events that, even with climate change driving more extreme weather, may not be replicated anytime soon. In short, a historically long stretch of strong storms in the vast Mississippi River basin — which kept the river high far past the date when it usually subsides — was combined with a storm system that took an unusual path out of Georgia and into the Gulf before becoming a hurricane. That made its path and effects difficult to predict.
“We had a high river in July with a storm that came the wrong way,” said Heath Jones, chief emergency manager for the Corps office in New Orleans. “We may never see this happen again.”
Then again, we might. And so the Corps has begun working on ways to more quickly and accurately convey the potential threat should another similar situation arise.
Throughout the storm, the Corps and other agencies worked to develop predictions on how high the river would rise and what its impact would be on the parishes along its banks. Meanwhile, sandbags and other methods were used to shore up a series of low points along the river levees, including the area around the Corps' headquarters in the Riverbend area of New Orleans.
Early in the storm’s approach, federal and local officials agreed that if projections showed water rising above the top of the levees, they would issue evacuation orders.
“Every one of those local parish (emergency managers) and parish presidents said if there’s any threat of overtopping, we’re calling an evacuation,” Jones said.
That ended up being unnecessary almost everywhere.
“Initially, there were predictions of overtopping from the river into Plaquemines Parish, which didn’t happen,” Vossen said. “In the metro area, we were in good shape.”
However, the scare has left people in the metro area with questions about exactly how high a river could be contained by the levees.
Corps engineers last week confirmed the accuracy of the information in a national database of levee heights that other Corps officials had disputed as Barry approached.
In addition to a low spot at the Corps’ headquarters on Leake Avenue, that database shows areas in the Lower 9th Ward and St. Bernard Parish where the levees are below the 20-foot height normally considered to be the minimum in the New Orleans metropolitan area.
As Barry approached, some forecasts suggested the river might rise to 20 feet — potentially, but not necessarily, high enough to overtop the levees in low spots.
It turns out that figuring out how high the river must be to spill over its banks is tricky. The first complication is a basic mismatch in how officials talk about the heights of the river and the levees.
Those two measurements are essentially taken from different starting points. At the Carrollton gauge, which is used to measure the river in the New Orleans area, the river’s height is measured from an elevation that’s almost a foot lower than the starting point for determining the height of the levee.
That means when forecasters say they expect the river to crest at 20 feet, they actually expect it to rise only to what would be considered 19.2 feet up the levees.
Add in another factor: The river’s height gradually decreases as it moves toward the Gulf of Mexico, where the river eventually reaches equilibrium with the sea — 0 feet. The rate of that drop depends on a number of factors related to the river’s height at Carrollton.
Meanwhile, scientists agree that a surge of hurricane-driven water up the river would tend to get less severe as it tries to push upstream against the mighty river's weight and current. But precisely how those countervailing forces interplay is still not well understood.
To get a better idea of the heights as the river progresses toward the Gulf, the Corps is asking for permission to install additional gauges, possibly at Belle Chasse and Pointe a la Hache.
Top Corps officials have endorsed the idea of creating a guide that would show what would happen if storms struck southeast Louisiana at different river levels. Those predictions would still be estimates, but they would likely give officials more confidence when determining the potential impact of a particular storm, Vossen said.
Such a guide will not be ready this season but could help determine future efforts, she said.
“Headquarters is encouraging us to put this together and get it to the public,” she said. “Something that’s not in the middle of the fire drill for a specific storm.”
Eventually, the Corps is preparing to build river levees that will also be designed to handle storm surge.
A study looking at how much the levee system in the New Orleans area will need to be raised by 2023 to maintain its ability to handle storms with a 1% chance of occurring each year will also take the river levees into account, Vossen said.
Even without Barry, the so-called “crossover point” marking the farthest-downriver point where levees would have to handle only river flooding, not storm surge, has been expected to move upriver.
“The assumption all along was that over time it would move upriver, just like our 1% assumptions rise over time from subsidence and sea level rise,” Vossen said.
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