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October 08, 2005

NOLA levees lacked firm foundation

Initially, officials attributed the flooding of New Orleans to overtopped levees. Allegedly, key levees were overwhelmed when huge post-Katrina waves sloshed over them. Now, there's an emerging consensus that overtopping wasn't the problem.

Floodwall Overtopping May Not Be to Blame

NEW ORLEANS, Oct. 7 -- The system of levees and concrete walls that was supposed to protect the New Orleans area from flooding was breached in dozens of places, investigators said Friday, a finding that indicates that the failures were far more widespread than originally thought.

Engineers probing the failures said they are increasingly convinced that floodwaters did not overtop two key floodwalls that collapsed on Aug. 29, swamping large portions of the city. Instead, evidence suggests that the floodwalls were weakened by the shifting soil beneath the structures, according to a team of experts from the American Society of Civil Engineers (ASCE) and the National Science Foundation (NSF). [WaPo]

Overtopping was a convenient excuse for the Army Corps of Engineers and others responsible for maintaining the levees. Any levee system can be overwhelmed by a big enough wall of water. New Orleans' levees were only designed withstand a surge from a Category 3 hurricane. Katrina made landfall as a Category 4, but with a storm surge closer to that of a Category 5 storm.

Closer investigation reveals that drowning of New Orleans probably had more to do with negligence and neglect than with the sheer power of the storm surge.

Concern about the inferior quality of the soil beneath the floodwalls is not new. In the early 1990s, a New Orleans-based contractor filed a legal claim against the Corps alleging that the soil beneath the floodwall on the 17th Street Canal was poor. A judge dismissed the contractor's complaint in 1996.

The teams investigating the floodwall failures say that a thin band of soft, peatlike soil lay more than 20 feet below the walls at both the 17th Street and London Avenue canals. But because the layer was deep and narrow, the crews that initially built the walls did not discover it, the engineers said. [WaPo]

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Comments

Get to the important stuff. Figure out a way to blame Bush for it.

You write: "New Orleans' levees were only designed withstand a surge from a Category 3 hurricane. Katrina made landfall as a Category 4, but with a storm surge closer to that of a Category 5 storm."

Katrina did make landfall as a Category 4, and may have had a storm surge closer to that of a Category 5 storm, but that landfall was a good ways to the east of NOLA. Remember, NOLA was not hit head-on by Katrina - at first everyone thought she dodged this bullet. You have to look east to the coast of Missippi for the effects of a catagory 4 storm and accompanying storm surge. NOLA was hit by the equivalent of a category 2 or at most 3 storm - and still the levees didn't hold. The White House storyline has been all along that the levees were designed to withstand a catagory 3 storm. Well, a category 3 storm, at most, is what hit NOLA and the levees failed.

The scandal is that the levees weren't even ready for the equivalent of a catagory 3 storm. The Corps of Engineers had requested funds to maintain and upgrade the levees for years. Unfortunately that money went to Iraq and tax cuts.

"a thin band of soft, peatlike soil"

In other words, swampland. Bottomless, boggy swampland. Refer to the description of the construction of Swamp Castle in "Monty Python and the Holy Grail", and the exploits of young Billy Mahone.

Just a note about the headline: levees and floodwalls are not the same. A levee is a mound of dirt, perhaps fifty feet wide at the base, with gently sloping sides and a broad crest and riprap or a retaining wall on the water side. You can easily walk up one. A floodwall is a concrete structure, maybe two feet wide at the visible base, with an inverted T- or L-shaped underground base for stability. Sometimes floodwalls are built on top of levees.

A levee can be overtopped or washed out, but not toppled. A floodwall will collapse if pressure and leakage destabilize the base.

The levees were not up to the task, this is true. But the levees are a bandaid almost as much as an adequate and ready means of evacuation and national guard strength...New Orleans had the benefit of NONE OF THESE. The scandal goes back much farther. As mentioned, by the mid 90's the engineers were all crying out for desperately needed remediation of conditions that, year by year, left New Orleans more exposed. A decades old policy of dredging shipping channels at the expense of not preserving the Mississippi delta's barrier islands should have been reversed the minute there was agreement on its impact on New Orleans vulnerability...that was 1998 or earlier. I pointed to a readable SciAm article explaining all this, published in 2001. Sound science and engeering simply do not have the influence they should in the Bush admistration's allocation of national resources.

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