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Engineers to Test Flood Defenses In New Orleans
Work continues on the outflow pipes that would be used when the floodgates are closed at the London Avenue Canal in New Orleans.
(By Alex Brandon -- Associated Press)
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But there are differences on significant issues and, as a National Research Council panel noted, those differences could affect how the city's new flood defenses are designed.
In its most recent review of the Army Corps' Katrina report, the Research Council panel noted the differing views of the "primary failure mechanism[s]" in the canal walls and warned that "the proposal of a single failure mechanism could lead future designers to focus on narrowly drawn conclusions, leading to neglect of other, equally plausible failure modes."
Robert G. Bea, one of the leaders of the Berkeley team of investigators, has called for an 8/29 commission "to truly understand why these failures developed" because "the Corps is still not designing things safely enough."
But Ed Link of the University of Maryland, who directed the Corps-sponsored report, said that although "there's lots more to learn and lots more analysis to be done, I don't see a lot of benefit in rehashing what's already being done. Fundamentally, there's a lot of agreement" about what happened in Katrina.
The canal walls that are the subject of the upcoming test, and ones like them, are relatively simple structures: concrete supported by steel sheet pilings driven deep into the soil.
The tricky element of their construction is the generally weak soils into which the walls and their supports are set -- part marsh, part clay, part sand. Much of the uncertainty regarding the walls' strength has to do with the varying soils.
"This is the Mississippi Delta, built up by hundreds upon thousands of years of sediments and the like being deposited," said Col. Jeff Bedey of the Army Corps, who is overseeing the canal experiment. "It's not simple."
Bedey noted that the canal walls are less critical to the city's protection than before because they are now a second line of defense. New floodgates are supposed to block a storm surge from entering the canals.
The strength of the canal walls remains important, however, because the canals will still be used to pump floodwater out of the city into Lake Pontchartrain. The Corps currently estimates that the canals can safely hold water to a level of four feet. Corps engineers are hoping, however, that the walls can sustain more than that so more pumping may be done.
Last week in the Gentilly neighborhood, as residents still rebuilding their homes watched construction workers section off a portion of the canal for the test, many alternately wondered and worried over whether such certainty is achievable.
"I worry that there's even a need for the test," said Virginia Bouvier, 42, a nutrition counselor who is fixing her home beside the canal. "We should have known about these walls before Katrina."
