Report casts doubt on New Orleans' levees

WASHINGTON — The force of surging high water from Hurricane Katrina bent back a key New Orleans floodwall and splintered its foundation, an investigating panel said Friday 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 floodwall 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 floodwall near Lake Pontchartrain collapsed Aug. 29 without ever being overtopped by Katrina's storm surge, according to the panel appointed by the Army Corps of Engineers.

Corps officials said the findings prompted an immediate reassessment of 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 floodwall left much of central New Orleans under water. 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 floodwalls as the primary reason for the collapse. Friday'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 floodwalls 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 gave way.

"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 New Orleans has dozens of miles of similar floodwalls that were weakened but not breached by Katrina.

"It raises questions about the stability of all the other walls," said David Daniel, leader of an engineers' advisory board overseeing the task force's work.