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Investigators Link Levee Failures to Design Flaws
Three months before Katrina, Mashriqui told a room full of emergency managers that the outlet was a "critical and fundamental flaw" in the Corps' hurricane defenses, a "Trojan Horse" that could amplify storm surges 20 to 40 percent.
With the help of a supercomputer, Mashriqui has now concluded that the effect was even worse than he predicted.
The analysis shows that the outlet's "funnel" intensified the initial surge by 20 percent, raising the wall of water about three feet. But it also increased the velocity of the surge, which Mashriqui believes contributed to the scouring that undermined the levees and floodwalls along the outlet and Industrial Canal. He found that Katrina's surge moved through nearby Lake Borgne at less than 3 feet per second. But the rate was about 6 feet per second at the mouth of the funnel, and as much as 8 feet per second in the funnel.
Mashriqui also found that in the areas where the outlet had wiped out marshes and other wetlands, levees and floodwalls were much more likely to fail. In areas where the natural buffers remained, the manmade defenses held, even when they were overtopped.
"Without MRGO, the flooding would have been much less," he said. "The levees might have overtopped, but they wouldn't have been washed away."
Corps officials declined comment on the results of the modeling. But Corps spokesman Jason Fanselau said the agency's own data still point to a massive surge that exceeded the height of the Industrial Canal floodwall by more than a few feet.
"Katrina flat-out overwhelmed the system," he said. "There was a huge wall of surge that obliterated entire sections of the floodwall."
In the case of the 17th Street and London Avenue canals, the independent investigators believe the floodwalls themselves were the problem. The reason was the naturally soft soil made up of river silts and swampy peat that has been the bane of builders here for two centuries.
Investigators now believe the walls collapsed when the soils beneath them became saturated and began to shift under the weight of relatively modest surges from the lake. And newly released documents show that the Corps was aware years ago that a particularly unstable layer of soil lay beneath both floodwalls.
"These levees did not overtop, yet they failed anyway," said Peter Nicholson, an engineering professor at the University of Hawaii at Manoa and leader of the ASCE investigating team. "It's important that we find out now exactly what went wrong, because the Corps is already starting to rebuild."
Documents given to investigators by former Corps contractors have shed some light on what government engineers knew about the weak soils and how this knowledge affected their decisions.
In the 1980s, the Corps began constructing concrete floodwalls on top of older earthen levees to give the city's northern neighborhoods better protection from storm surges from Lake Pontchartrain. Soil tests in the 1980s detected trouble 20 feet below the surface: a thick layer of spongy, organic soil called peat. Soft and highly compressible when dry, peat becomes even weaker when saturated with water.