Stresses caused by the sinking of highly unstable soil beneath about five miles of floodwalls in St. Bernard Parish could bend the pilings that support them and undermine the defenses, according to a new study from the U.S. Army Corps of Engineers that supports concerns raised by local levee officials since before the floodwall project was started.
The issues with subsidence are not cause for immediate concern and do not mean the floodwalls would not be able to withstand a storm today, Corps officials said Monday. And, with many questions about exactly how much the ground is sinking and how that will affect the structures, the Corps is putting in place a monitoring system aimed at keeping a close eye on the system.
The issue could mean the walls will need to be fixed or replaced before the end of their supposed 100-year life, though when it could be an issue is not known. And should that happen, it is unclear who would be on the hook to fix or replace defensive structures that cost hundreds of millions of dollars to erect.
So far, the levees beneath the floodwalls don’t show signs of settling at the rates predicted by the Corps analysis, which could be a sign that there won’t be a problem in the future, officials said. But the agency is stepping up the way it monitors the structures to get a better understanding of how the highly unstable soil on which they sit is settling, with an eye toward catching any potential issues before they become a threat, said Col. Richard Hansen, the Corps’ New Orleans District commander.
“There’s a knowledge gap the monitoring will help us fill,” Hansen said.
The floodwalls facing additional scrutiny run along the northeastern edge of the parish, tying into the Lake Borgne Surge Barrier at Bayou Bienvenue and ending at the Bayou Dupre Floodgate. Those were identified in a Corps study, released Monday, as potentially seeing enough settlement to cause problems with the pilings that support them.
The Southeast Louisiana Flood Protection Authority-East, which is responsible for hurricane protection in the New Orleans area, and the state Coastal Protection and Restoration Authority have been raising concerns about that possibility since before ground was broken on the floodwalls.
Flood Protection Authority Executive Director Bob Turner said he had not had time to review the Corps’ reports, which cover the subsidence issue as well as other concerns raised by local authorities about the design of the floodwalls. But, he said, more monitoring is a good step.
“We still have concerns,” Turner said. “I’m glad to see the Corps is moving forward with trying to get to the bottom of the issues.”
The $1 billion floodwall project includes defensive structures that run for about 23 miles on the edge of St. Bernard, from Caernarvon around the eastern side of the parish to Bayou Bienvenue. They are a part of the $14.5 billion in upgrades to the flood protection system in the New Orleans area that were installed after the region flooded as a result of breaches and failures during Hurricane Katrina in 2005.
The Corps’ studies examined levees and floodwalls throughout that stretch, though most did not appear to have problems with subsidence. Subsidence is of concern in the northeasternmost stretch of the floodwalls, which cost more than $300 million, because of the soil composition beneath the levees, which themselves are made of material that is up to Corps standards for subsidence.
Some of the most dramatic failures during Katrina were in St. Bernard, where large stretches of the levees breached and were washed away. That allowed surging water from the Mississippi River Gulf Outlet to flood into the parish.
In the aftermath of the storm, the Corps erected a line of levees around the parish as a stopgap measure. Those levees eventually were topped with floodwalls in a project that was completed in 2011. Together, the levees and floodwalls reach up to 32 feet high, tall enough to provide protection against a storm that has a 1 percent chance of occurring in any given year.
The concrete structures, known as T-walls because they resemble the letter turned on its head, sit on top of the levees rather than being embedded inside them. T-walls are preferred in many flood protection projects because their wide base can prevent them from being knocked over by a storm surge, as happened to several walls without that platform — known as I-walls — during Katrina.
The walls are further supported by pilings installed at an angle into the levees below. The angle provides additional support to prevent the structure from being toppled by storm surge.
But that angle also means the pilings are subject to downward pressure from the levee, something that is exacerbated as the soil beneath the levee settles.
That could eventually cause the pilings to bend, which would prevent the floodwalls from working as they are designed to do.
“The critical thing is the monitoring program be conducted in such a way that (any problem) is detected years in advance,” Turner said.
Exactly what is happening beneath the floodwalls — and how it might affect the structures themselves — is unclear. The study, based on a software model, predicts there will be enough pressure put on the pilings due to subsidence to cause problems.
However, there have been no indications that settlement is occurring at that rate. In fact, officials had expected a greater amount of settlement than they’ve seen so far, Hansen said.
But that fact underlines that much is unknown about how the soil in the area will handle the weight that has been placed on top of it and does not necessarily indicate that it will continue to follow an expected pattern, he said.
If a problem were to occur, it could be decades down the line, Hansen said. Or it could never become an issue at all, he said.
With little known about the specifics of the soil in the area and the way it subsides, the monitoring project will be “advancing the science,” said John Bivona, acting chief of the Corps’ engineering division in New Orleans.
The Corps first installed equipment to monitor the project in 2012 and added more stations in 2013. The newly proposed monitoring, which will cost $1 million to install, will involve installing additional equipment to check on strain and settlement along the entire length of the system around the parish, as well as placing pieces of steel in six locations within the levees that can be monitored for rust without having to dig through to the pilings themselves.
The Flood Protection Authority will be responsible for the roughly $50,000 a year it will cost to gather data from that system.
“It is certainly the minimum that should be done, and then, of course, what happens from there will give us a better indication of whether there’s additional steps,” Flood Protection Authority President Stephen Estopinal said.
In addition to the concerns about subsidence, the Flood Protection Authority also has raised warnings that corrosion could weaken the pilings.
Pilings are often coated with material to prevent corrosion, particularly when they are in wet environments. However, when building the floodwalls, the Corps chose not to coat the pilings. Instead, the pilings were made slightly larger than the design called for, with the expectation that some of the steel would be “sacrificed” to rust and leave enough uncorroded material to meet the requirements.
The Corps on Monday released another study focused on corrosion that officials said showed that even with the rust that would occur, the pilings would be able to perform up to the standards set for the project.
How any problems that are detected would be handled would likely depend on exactly what was discovered. The most radical solution would call for a full rebuilding of sections of the levee and floodwall, something officials do not believe there would ever be the funding to accomplish.
And, Estopinal said, it is unlikely the federal government would provide funding to the Corps to fix any issues that arise after already paying for the full hurricane protection system. That would leave it to the local levee authority or the state to come up with the money for any remediation that was necessary.
“Hopefully, it may not manifest itself in the way that we’re concerned about,” Estopinal said. “But hope is not good engineering design.”
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