Design flaws may have doomed levees
The Washington Post
NEW ORLEANS — Within a space of 15 hours on Aug. 29, three massive, concrete floodwalls in separate parts of the city suddenly fractured and burst under the weight of surging waters from Hurricane Katrina. The breaches unleashed a wall of water that swept buildings from their foundations and transformed what might have been a routine hurricane into the costliest storm in U.S. history.
Now, eight weeks after the storm, all three breaches are looking less like acts of God and more like failures of engineering that could have been anticipated and prevented.
Investigators in recent days have assembled evidence implicating design flaws in the failures of two floodwalls near Lake Pontchartrain that collapsed when soil beneath them became saturated and began to slide. They also have confirmed that a little-used navigation canal helped amplify and intensify Katrina's initial surge, contributing to a third floodwall collapse on the east side of town. The walls and navigation canal were built by the U.S. Army Corps of Engineers, the agency responsible for defending the city against hurricane-related flooding.
The preliminary findings — based on physical evidence, Corps documents, and hydrodynamic models run through a Louisiana State University (LSU) supercomputer — are the work of three teams of engineers and forensic experts conducting separate probes. The investigations are shedding light not only on the cause of the failures but also the scale of the rebuilding effort: The discovery of major flaws in the design of the levees and floodwalls could add billions of dollars to the cost of New Orleans' recovery.
Investigators have rejected the initial explanation offered by Corps officials after the hurricane that massive storm surges had overtopped floodwalls on the 17th Street and London Avenue canals on the north side of town. The new findings, for the first time, point to a human role in all three of the major floodwall failures, which left about 100,000 homes underwater and caused most of Louisiana's approximately 1,000 hurricane deaths.
Experts now believe Katrina was no stronger than a Category 3 storm when it roared into New Orleans, and Congress had directed the Corps to protect the city from just such a hurricane.
"This was not the Big One — not even close," said Hassan Mashriqui, a storm-surge expert at LSU's Hurricane Center. He said Katrina would have caused some modest flooding and wind damage regardless, but human errors turned "a problem into a catastrophe."
The National Science Foundation (NSF), the American Society of Civil Engineers (ASCE), and the state of Louisiana are all conducting investigations of the floodwall breaches. The Corps has offered data and other assistance to the independent inquiries but has declined to speculate on the causes of the tragedy.
John Paul Woodley, the assistant Army secretary overseeing the Corps, said it is still too early to cast blame. But he said the Corps intends to learn from the Katrina investigations and use the lessons to build stronger protections for the city.
"I'm not afraid of finding out the truth," Woodley said.
The independent investigations have pointed to two failures in the infrastructure maintained by the Corps that were critical factors in the destruction in New Orleans.
In 1965, the Corps completed the 76-mile-long, 36-foot-deep Mississippi River Gulf Outlet. The outlet — known locally as MRGO, or "Mr. Go" — created a navigation shortcut to the Port of New Orleans, but also what amounted to a funnel that would accelerate and enlarge any storm surges headed for the city's levees.
Mashriqui concluded the outlet's "funnel" intensified the initial surge by 20 percent, raising the wall of water about three feet. It also increased the velocity of the surge, which Mashriqui believes contributed to the scouring that undermined the levees and floodwalls along the outlet and Industrial Canal.
In the case of the 17th Street and London Avenue canals, the independent investigators believe it was the floodwalls themselves that were the problem. The reason was the naturally soft soil made up of river silt and swampy peat.
Investigators believe the walls collapsed when soft soil beneath them became saturated and began to shift. Newly released documents show the Corps was aware years ago that a particularly unstable layer of soil lay beneath both floodwalls.
"These levees did not overtop, yet they failed anyway," said Peter Nicholson, an engineering professor at the University of Hawaii at Manoa and leader of the ASCE investigating team.
In the 1980s, the Corps began constructing concrete floodwalls on top of older earthen levees to give the city's northern neighborhoods better protection from storm surges from Lake Pontchartrain. Soil tests in the 1980s detected trouble 20 feet below the surface: a thick layer of spongy, organic soil called peat. Soft and highly compressible when dry, peat becomes even weaker when saturated.
A 1988 document reveals Corps officials took careful measurements of the peat layer and tested the soil in a laboratory to calculate its relative strength, according to Robert Bea, a professor of engineer at the University of California at Berkeley and a member of the NSF investigating team. Based on those calculations, the Corps designed a concrete-and-steel floodwall anchored to the earth by steel pilings driven to a depth of 20 feet.
Meanwhile, because the canal was dredged to an even greater depth, water penetrated the peat layer from the inside, investigators said.
"Water was able to get around or through those pilings to the other side and start weakening the structure. There was a gap where water could get through," said Ivor Van Heerden, the deputy director of the LSU Hurricane Center and the leader of the Louisiana forensic investigation.
Hours after Katrina hit, water poured into the canals from Lake Pontchartrain and added enormous strain to the walls and levees. According to a scenario developed by Bea and other investigators, the already-saturated peat was the path of least resistance, allowing water to burst through the wall from underneath. At the 17th Street canal, truck-sized chunks of the old earthen levee were heaved 35 feet on a carpet of sliding soil.
Corps officials aren't yet convinced. "It is important not to jump to conclusions," said John Grieshaber, chief of the engineering division in the Corps' New Orleans district office.
A proposal for rebuilding the floodwalls, however, has set far tougher standards than existed 15 years ago. And the steel pilings now must be driven through the peat layer to 40 feet, twice as deep as before.
Copyright © 2005 The Seattle Times Company