The failure of the levees in New Orleans did not come from the overflowing of Lake Pontchartrain over the tops of the walls, but rather from soil failures under the levees, the Los Angeles Times reports today. This is the second study that refutes the claims made in the days after Hurricane Katrina's destruction that the government should have foreseen the levee failure given the intensity of the storm:

The levee breaches along two major canals that flooded New Orleans after Hurricane Katrina resulted from massive soil failures under concrete storm walls, not from hurricane surges that sent water over the tops of the walls as Army officials initially said, according to teams of investigators who have examined evidence in the last week.

The findings appear to chip away at the simple story that the storm surge was much larger and higher than the walls were designed to handle, though investigators caution that it is too early to blame design or construction. ...

Immediately after the storm, the Army Corps of Engineers thought that a surge from Lake Pontchartrain had moved up drainage canals in the city and overflowed concrete storm walls, eroding foundations and leading to the breaches.

Investigators have found no evidence of such overflow and foundational scouring at the breaches in the London Avenue and 17th Street canals, two main failures behind the central New Orleans flooding. In fact, in one case, water marks are a full 2 1/2 feet below the tops of the walls.

Instead, investigators have found strong evidence that the soil structure was too weak for the pressure of the water, wind and waves.

The blame for the destruction, inasmuch as anyone can blame anyone for a hurricane, would then appear to fall onto the original designers of the levees -- and also create a much bigger problem than before. The first Corps report would have resulted in an extension of the existing levee system, raising the walls higher to contain larger surges than those caused by Level 4 hurricanes. Now we find out that not only would those changes have proven worthless, but would have likely added to the design deficiencies of the soil and the pressure.

This shows the foolishness of making snap judgments about fault in the immediate aftermath of a catastrophe. The experts, both locally and at FEMA, had concluded that based on water levels at the levees the city would be safe from a systemic failure and widespread flooding. That resulted in the cheery prognoses offered by Michael Brown, Ray Nagin, and others on the Monday of the levee failures. No one knew that the levees had base design deficiencies, nor could they tell until after the levees failed. That does not let Louisiana or New Orleans off the hook for not properly evacuating the city in accordance to its plans, but it at least shows why the concern dropped after Katrina passed through the area.

The bad news will come wheh the government gets the estimate for completely tearing out the existing levees and replacing them with a system designed to handle a Level 5 hurricane. That will cost billions and will displace a number of New Orleans citizens -- assuming they come back to the Big Easy after Katrina anyway.

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