Why New Orleans levees failed

That is one implication of studies released Wednesday by engineers and scientists investigating the failures of New Orleans' levees during hurricane Katrina. The lessons learned there could be useful to ocean- and riverside communities in other parts of the United States. For the Big Easy, those lessons need to be absorbed quickly, many of those researchers say.

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"We know a great deal about what happened and why," said Raymond Seed, an engineering professor at the University of California at Berkeley who is heading a National Science Foundation's investigation into the breaches. Noting that repairs currently are under way and that another hurricane season is only seven months off, "we have a hurry-up on our hands" in applying Katrina's lessons, he said.

Dr. Seed, testifying Wednesday before the Senate Committee on Homeland Security and Governmental Affairs, is one of many specialists urging the US Army Corps of Engineers to strengthen the levees as it moves to repair them.

For its part, the Army Corps is conducting a "lessons learned" study that it plans to complete by next July. The Corps' senior research scientist, Paul Mlakar, cautioned against drawing conclusions before all the evidence has been amassed and analyzed.

Roughly 80 percent of the city was flooded - with up to 20 feet of water in places - because of levee failures in Katrina's aftermath. The most likely causes, researchers said at the hearing, ranged from overtopping and seepage to improperly set pilings made from steel sheets.

After arriving at the scene a month after the hurricane moved on, researchers say they were surprised at the large number of breaks they found compared with what they had seen in media reports.

"We found dozens of breaches throughout the levee system," says Peter Nicholson, who leads the American Society of Civil Engineers' levee-assessment team.

Among the reasons for failure:

• Katrina's storm surge overtopped some sections. The cascade eroded soils from the base of the landward side of some levee sections, causing them to fail.

• In sections where the surge didn't overtop levee segments, water percolated under the sheet pilings through layers of peat, sand, and clay and bubbled up on the other side. Ivor van Heerden, a marine scientist at Louisiana State University, noted that these failures tended to occur where the pilings were driven only 10 or 11 feet into the ground. Where pilings were driven 25 feet, the levees kept the water at bay. Indeed, he expressed concern that this percolation may have weakened other sections of the levee system that appear to have survived Katrina.

• The junctions between different kinds of levees were often weak. "If it's earth versus concrete, the earth will lose," Dr. Nicholson noted.

• Levees made from fill or dredge material from canals were more likely to fail if they lacked significant patches of marshland in front of them to blunt the effects of the incoming storm surge.

Several researchers note that the Corps of Engineers is working diligently to repair the levee system, but add that they are concerned that the Corps is stretched far too thin. "You've got to ensure they have adequate resources," Seed said.