Report Casts Doubt on New Orleans Levees
Saturday, March 11, 2006
The force of surging high water from Hurricane Katrina bent back a key New Orleans flood wall and splintered its foundation, an investigating panel said yesterday in a report that sheds new light on the cause of the city's flooding while raising questions about the safety of the city's surviving levees.
The report contradicted earlier views about why the 17th Street Canal flood wall collapsed, but it also said that the failures were "not anticipated" by the levees' designers and that the system did not perform as intended. A 450-foot section of the flood wall near Lake Pontchartrain collapsed Aug. 29 without ever being overtopped by Katrina's storm surge, according to the panel, which was appointed by the Army Corps of Engineers.
Army Corps officials said the findings had prompted an immediate reassessment of ongoing efforts to rebuild 169 miles of Katrina-damaged levees. "We are incorporating the information into our current repairs," said Col. Lewis Setliff, who heads the rebuilding effort.
The collapse of the 17th Street flood wall left much of central New Orleans underwater. The wall, a concrete structure that sits atop an earthen levee, was designed by the Corps.
Previous studies by independent analysts pointed to weak, peatlike soils beneath the flood walls as the primary reason for the collapse. Yesterday's report by the Interagency Performance Evaluation Task Force, a group of 150 engineers and experts from government, academia and private industry, also implicated weak soils. But the panel said that was only one of several factors in an unusual, top-to-bottom cleaving of the levee that occurred hours after Katrina hit.
The split itself was caused by the high volume of water in the canal, which pressed the flood walls backward by several inches and tore a gash in the soil where the wall intersects with the earthen levee on which it rests. Water rushed into the growing crack, investigators said, and the walls finally gave way. Relatively soft, poorly compacted soils on the sides of the levees could not stand up against the strain, the report said.
"We've never seen this precise combination of factors," said Ed Link, director of the task force.
The findings were viewed as worrisome by some experts who noted that New Orleans still has dozens of miles of similar flood walls that were weakened but not breached by Katrina.
"It raises questions about the stability of all the other walls," said David Daniel, president of the University of Texas at Dallas and leader of an American Society of Civil Engineers advisory board overseeing the task force's work. "At a minimum we expect the Corps will reanalyze all the walls on the assumption that this same failure mechanism could occur elsewhere."