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NEW ORLEANS - The engineers who designed the floodwalls that collapsed during Hurricane Katrina did not fully consider the porousness of the Louisiana soil or make other calculations that would have pointed to the need for stronger levees with deeper pilings and wider bases, researchers say.
At least one key scenario was ignored in the design, say the researchers, who are scheduled to report their findings at a congressional hearing today: the possibility that canal water might seep into the dirt on the dry side of the levees, weakening the embankment holding up the floodwalls.
"I'd call it a design omission," said Robert Bea, a University of California at Berkeley civil engineering professor who took part in the study for the National Science Foundation.
The research team found other problems in the city's flood-control system, including evidence of poor maintenance and confusion over jurisdiction.
Bea also questioned the margin for error engineers used in their designs, saying the standards - which call for structures to be 30 percent stronger than the force they are meant to stop - date to the first half of the 1900s, when most levees were built to protect farmland, not major cities.
"The center of New Orleans is certainly not protection of farmland, so the factor of safety was incredibly low," Bea said. "We're talking about thousands of families without homes and shutting down a commercial infrastructure that's pretty darn important to the United States."
While surging waters from the Gulf of Mexico flowed up and over levees east of the city, flooding in central New Orleans and parts of downtown was caused by breaches at barriers along the 17th Street and London Avenue canals, both of which have been built since the late 1980s.
Floodwaters eventually inundated as much as 80 percent of New Orleans and had to be pumped out over weeks because of the city's saucer-like topography.
The UC team is one of three independent teams working with the U.S. Army Corps of Engineers, the agency responsible for the levees' design and construction, to determine why the barriers failed and make recommendations to repair them.
Corps officials have said the barriers were never intended to withstand a storm as powerful as Katrina. Congress instructed them to build a network of levees and floodwalls that could withstand a Category 3 storm similar to Hurricane Betsy, which flooded New Orleans in 1965. Katrina was a Category 4 hurricane when it came ashore Aug. 29.
But since Katrina's center passed to the east of New Orleans, there is debate as to whether the city experienced more than the equivalent of a Category 3 storm.
Steel-sheet pilings driven into the ground are meant to stop seepage from the wet side of the levee to the dry side and serve as an anchor for the levees' protective, concrete walls. But a number of engineers have said the pilings were not driven deeply enough into the loose, porous soil endemic to southern Louisiana.
The result: Water seeped into the ground and destabilized the soil, causing the walls to collapse.
Bea said the flood-control system has many jurisdictions involved, and the resulting confusion leaves "no one minding the store."