Corps of Engineers calls levee failure unforeseeable

Blame is placed on slicing effect of water, weak soil behind levees

March 11, 2006|By NEW YORK TIMES NEWS SERVICE

NEW ORLEANS -- One of the city's major levees failed during Hurricane Katrina because of a combination of factors that had not been anticipated by the Army Corps of Engineers when it planned the city's flood protection, according to a new report that the corps released yesterday.

The report, based on the corps' own investigation of the levee failures, described a previously unknown set of circumstances during the storm: The floodwaters rising in the 17th Street canal sliced through the protective levee like a knife through a cake, sharply reducing the levee system's ability to resist the water's push. The levees then gave way because of the inherent weakness of the soils behind the levee and pushed into the adjoining neighborhood.

Up to now, none of the investigations into the breaches had coupled the downward slicing effect of the water with the adjacent soil's weakness as a reason for the failure at that site. When the levee breached, the waters that spilled out inundated well-to-do neighborhoods such as Lakeview and, with other breaches at the London Avenue canal, reached toward the center of the city.

"This was a mechanism that we didn't envision in the original analysis," said Walter Baumy, the deputy director of the corps task force that is rebuilding the hurricane protection system.

But other investigators who have been critical of the corps said yesterday that the corps should have anticipated the failure and designed levees that could stand up to the punishment of a storm like Katrina, even if the mechanism had not been laid out.

"They should have known," said Ivor van Heerden, a hurricane expert at Louisiana State University. "That's part of the suite of failure mechanisms in systems like this."

The one-two punch that destroyed the levee started with the water from the storm pushing against the concrete floodwalls that sit atop the earthen levees. The levee is a long mound of tightly packed earth; the flood-wall rises out of the top of the hill, anchored by steel sheets that have been driven into the soil.

When the waters of Katrina rose toward the top of the wall, the researchers said, the pressure pushed the wall outward, toward the houses on the other side of the levee. That movement opened a small space between the floodwall and the soil, allowing the raging waters to course vertically down the sheet pile and, effectively, cut the earth levee in two. That took away half of the protective effect of the levee on the canal side.

The second part of the one-two punch was the weakness of the soils beyond the levee, which did not have the strength of the compressed soils under the levee. With the pressure coming from the canals, the levee pushed sideways by about 45 feet, contorting the ground behind it into hummocks like a bunched-up rug.

This description of the levee failure was part of the second interim report of an investigation created by the Corps of Engineers that is made up of more than 150 experts from inside and outside of the organization. This new description differs somewhat from explanations that have been offered by other groups investigating the levee breaches, some of which focus on shortcomings in the original design, construction and maintenance of the system.

Without both factors working together against the system, the investigators said, the floodwalls would probably not have failed.

David Daniel, the president of the University of Texas at Dallas and chairman of an external engineering panel reviewing the new report for the corps, said during a news conference held by the corps yesterday that the failure mode was not "utterly unforeseeable," but was not one that "one would normally consider in this kind of embankment design." He mused: "Were the dots connected? That's still an open question."

Another independent investigator said that the corps should have known about the potential for this particular kind of failure because it had studied the phenomenon directly in 1988. Robert Bea, a professor of engineering at the University of California at Berkeley, said the 1988 study, which was conducted by the New Orleans district of the corps, found results of high water pressure very similar to what seems to have occurred during Katrina.

"Their contention that this is an act of God or an unforeseeable failure mode will not hold water," said Bea, who is investigating the levee breaches as part of an independent team financed by the National Science Foundation.

A leader of the corps investigation, professor Ed Link of the University of Maryland, said his researchers were well aware of the 1988 test and resulting report and used it in their analysis. "In fact, it was an important reference document for us," he said. But that is only "part of the picture," he said, and had to be taken together with the instability of the soil behind the levee.

Bea, the engineer from Berkeley, found that argument unconvincing. He said soil boring records from the time of the levee design that he has acquired in his investigation show layers of clay so toothpaste-soft that they fell out of the pipe used to extract them before they could be measured. "We know these soils are swamp guck," he said.

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