NEW ORLEANS (AP) A foundation problem — although not the one targeted by earlier studies — caused the 450-ft.-long (135 m long) break in a floodwall and levee on New Orleans’ western edge when Hurricane Katrina hit, the U.S. Army Corps of Engineers said.
A naturally occurring, 20-ft.-thick (6 m thick) layer of clay that helped support the floodwall was too weak for the job, according to a report released March 10 by a Corps task force set up to find out why the levees broke. Had the floodwall and levee held, much of the western half of the city would have escaped flooding.
Previous analyses by other groups had targeted the layer of sand and peat over the clay as a likely culprit.
“The failure plane was not in the peat. It was in the clay below the peat. That became the weakest part of the system,” said Ed Link, project director for the study by the Interagency Performance Evaluation Task Force.
Part of the levee was pushed 40 ft. (12 m) backward, and can be seen above the water — along with that section of floodwall — in an aerial photograph published as part of the task force’s second report.
High water pushed back the floodwall, which is set into the center of the earthen levee. Once water got between the floodwall and the front half of the levee, it effectively cut the levee in half lengthwise.
The floods then pushed the floodwall, and the half of the levee behind it, backward on a layer of soft clay below the surface, the report said.
The floodwall’s design didn’t include either the possibility that water could get between it and the levee or that the clay might be unstable, corps officials and others said in a news conference. The two factors combined created the breach, they said.
“We are incorporating the information into our current repairs, and incorporating it into our assessments for the future,” said Col. Lewis Setliff, who is in charge of the levee repairs that the Corps wants completed by the beginning of next hurricane season on June 1. “We are also evaluating the repairs we are making ... to see if we need a change of course.”
Setliff said the corps is looking at all the “I-walls” — vertical concrete barriers anchored by vertical sheet steel pile — in New Orleans-area levees.
The 17th Street Canal and two others that broke during and after Hurricane Katrina will be cut off at the mouth by new floodgates if a hurricane approaches this year, to keep high water in Lake Pontchartrain from stressing their levees and floodwalls.
And, wherever it can, the Corps is replacing I-walls with “T-walls,” which have a horizontal concrete base and are anchored by steel beams driven diagonally through the levee.
Those should be “significantly more resistant” to such failures Link said in a news conference after the 332-page preliminary report and more than 400 pages of appendices were released.
The final report is due June 1.