Floodwall Overtopping May Not Be to Blame
Saturday, October 8, 2005
NEW ORLEANS, Oct. 7 -- The system of levees and concrete walls that was supposed to protect the New Orleans area from flooding was breached in dozens of places, investigators said Friday, a finding that indicates that the failures were far more widespread than originally thought.
Engineers probing the failures said they are increasingly convinced that floodwaters did not overtop two key floodwalls that collapsed on Aug. 29, swamping large portions of the city. Instead, evidence suggests that the floodwalls were weakened by the shifting soil beneath the structures, according to a team of experts from the American Society of Civil Engineers (ASCE) and the National Science Foundation (NSF).
In the early days after the storm, accounts offered by the U.S. Army Corps of Engineers and others focused attention on a few of the most prominent breaches. But the team of engineers working with the Corps said the places where water broke through were much more numerous.
"This place was ripped to shreds. I was amazed," said Peter Nicholson, a University of Hawaii professor of civil engineering who is part of the investigating team. "There were dozens and dozens of breaches."
Engineers from ASCE and an NSF-funded group at the University of California at Berkeley have been poking through the wreckage of the levees to determine what went wrong.
Many miles of earthen berms and concrete walls are supposed to prevent the low-lying city from being inundated by the Mississippi River, Lake Pontchartrain and other nearby waters. The engineers said Friday that the system failed in a number of ways.
At two key breaches where huge volumes of water inundated the city -- at the 17th Street Canal and the London Avenue Canal -- the quality of the soil supporting the flood walls appears to have been a problem.
At the 17th Street Canal, they said, a section of the levee embankment moved back 35 feet. There is evidence of a similar "soil mass movement" at one of two London Avenue sites. The engineers speculate that either the pressure on the walls pushed them back against the soft soil or water seeping beneath the walls softened the soil, weakening the wall's support. "The soil moved," said Paul Mlakar of the Army Corps of Engineers. "The exact mechanism is not known at this time."
The soil in the area, composed of sand, silt, clay and peat is "compressible and not very strong," said Raymond Seed, a professor of civil engineering at UC-Berkeley.
An extensive analysis at the two canal locations has virtually ruled out overtopping as the cause of the failures, the engineers said. Overtopping occurs when rising waters spill over the top of a floodwall. The analysis shows that the water levels in both the London Avenue and 17th Street canals missed the top of the floodwalls by at least two feet, Nicholson said.
Many levees and flood walls did overtop. In some cases, catastrophic failures followed the overtoppings because the rushing floodwaters wore away the ground supporting the walls and the walls fell over.
"Some were simply overwhelmed and largely destroyed," Nicholson said. "However, many miles of levees performed satisfactorily, even many that were overtopped."
Concern about the inferior quality of the soil beneath the floodwalls is not new. In the early 1990s, a New Orleans-based contractor filed a legal claim against the Corps alleging that the soil beneath the floodwall on the 17th Street Canal was poor. A judge dismissed the contractor's complaint in 1996.
The teams investigating the floodwall failures say that a thin band of soft, peatlike soil lay more than 20 feet below the walls at both the 17th Street and London Avenue canals. But because the layer was deep and narrow, the crews that initially built the walls did not discover it, the engineers said.