The massive failures of levees in New Orleans during Hurricane Katrina, which flooded the city and caused hundreds of deaths, resulted from flaws at almost every level in the conception, design, construction and maintenance of the region's flood-control system, according to the preliminary findings of investigators.
The Army Corps of Engineers, local levee boards in Louisiana and other agencies failed to grasp warning signs over the last decade that the levees were not as strong as expected, reflecting a cultural mind-set that did not pay enough attention to public safety, according to Robert Bea, an engineering professor at UC Berkeley who is part of a National Science Foundation investigating team.
The team is one of three high-level technical groups investigating the floods that began Aug. 29. A written preliminary report is to be released next week and then presented to a Senate hearing. Although the investigators' work is far from over, some important points have emerged:
* At least two, and possibly three, of the breaches that took down storm walls in the city during the hurricane resulted from design flaws involving weak soil conditions, according to Raymond Seed, a UC Berkeley engineering professor who is leading the investigating team.
* Levees also failed because they were designed and built in the late 1980s and 1990s without adequate safety margins, said Bea, a civil engineering expert. The safety margins, intended to give levees an extra measure of strength, were set far lower than the protective margins typically used for such critical projects as bridges, hospitals and dams.
* The overall architecture of the city's flood-control system, some of which dates back more than 100 years, has created unnecessary vulnerabilities. The long drainage canals that extend into New Orleans from Lake Pontchartrain "are inviting the enemy into the city's backyard," Bea said. The canals should be replaced by underground culverts and pumping stations located on the lake's edge, investigators said.
* Maintenance practices also were lax. The triggering event in the catastrophic failure of the 17th Street Canal may have been the fall of a large oak tree planted at the base of the levee, investigators said.
High winds during the hurricane may have knocked down the tree, causing a large root ball to heave up and undermine the foundation of the levee, according to photographic analysis and eyewitness accounts. The tree's falling started a chain reaction that took out several hundred feet of flood wall. A similar scenario may have played out on the London Avenue Canal.
"It was like uncorking a bottle," Bea said.
The U.S. Army Corps of Engineers also is investigating the levee failures, led by Paul Mlakar, a senior research scientist at the corps' engineering research and development center. Mlakar said his team also was examining whether the oak tree triggered the failure.
"It is a hypothesis that we are looking at," Mlakar said.
Mlakar said he was not ready to release any of his group's early thinking, although he does not take issue with the work done by the National Science Foundation investigators. A third team, organized by the American Society of Civil Engineers, has said little about its work.
Other poor maintenance practices were found along miles of other levees, where burrowing animals created large tunnels that undermined already weak foundations. Maintenance and inspection are the responsibility of local levee boards.
Levee board officials in New Orleans and neighboring Jefferson Parish insisted Friday that the flood walls in their regions were well-maintained before Katrina struck. And they reacted skeptically to the notion that trees growing near the flood walls might have contributed to the breaches during Katrina's violent storm surges.
Orleans Levee District Commissioner Allen H. Borne Jr. said that along the 17th Street levee, maintenance crews kept the barriers and the grounds nearby free of harmful vegetation.
"There were no trees on the levees anywhere," Bourne said.
He also said he doubted reports that nutria, a species of large rodent, might have undermined the levees by digging trenches beneath them. He acknowledged that the animals had been sighted on occasion near the levees, "but only a few here and there, not in the hundreds."
A surge of water estimated at 24 feet -- about 10 feet higher than at the levees along the city's eastern flank -- swept into New Orleans from a bay on the Gulf of Mexico, Seed said, and caused most of the flooding in the city. He said that surge from Lake Borgne resulted in the breaches on the Inner Harbor Navigation Canal -- known in New Orleans as the industrial canal.
The industrial canal breaches occurred first, about 9 a.m. on Aug. 29, the day Katrina hit. The second breach occurred at the 17th Street Canal about 4 p.m. The London Avenue levee did not fail until about midnight.