System Failures Seen in Levees

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.

The storm surge swept over the top of the industrial canal and eroded its foundation. But the water was more than two feet below the tops of the walls on the 17th Street and London Avenue canals, Seed said. As a result, the loads were well-within the wall’s design, he said.

“The wall sections were designed to carry water to a higher level than we saw,” he said. “The wall should not have failed.”

Construction defects also may have played a role. Analysis of concrete samples from the 17th Street Canal shows that the levee fractured in ways that suggest the material was substandard.

The defects in design and construction might have been offset had the Corps of Engineers used higher safety margins. In basic terms, the walls were weak and unsafe, Bea said.

Once it calculated the maximum loads a hurricane could impose on the levees and walls, the corps applied a margin of safety 30% higher than the maximum load, according to guidelines published in 2002 and throughout other Army engineering documents.

Such a margin is far below the level engineers typically set for highway bridges, dams, offshore oil platforms and other public structures, Bea said.

A more typical approach would have doubled the wall strength over the maximum expected loads. Such a margin of safety is used for two reasons: uncertainly about the loads and the strength of the wall’s construction. “This margin of safety was incredibly low,” Bea said.

Engineering expert Ron Hamburger said most engineers for decades had used a more sophisticated design approach called probabilistic design analysis, a field pioneered in part by Bea. This method tries to estimate the probability of failure over time.

Public safety structures now are designed to last an estimated 10,000 years without failure. By contrast, the New Orleans levees may have been designed to withstand 50 to 100 years of natural forces, Bea said.

Beyond the immediate causes of the levee failures, Seed said, investigators are finding that the flood protection system in New Orleans is overseen by a tangle of local, state, multi-state and federal organizations that do not work in a coordinated way.

Along a single levee in one section of New Orleans, Seed said, investigators have found seven overlapping lines of government and private authority, including road agencies, levee boards, railroads and the Corps of Engineers. Such confusion has led to designs that don’t always make sense, he said.

“They should rethink the entire flood protection system of New Orleans,” Seed said. “It is a real hodgepodge of authority. If there was a coordinated effort, more could be done with less money.”

In addition to examining the levee breaches, investigators are looking into whether railroad companies failed to shore up gaps in storm walls where tracks pass through. Those gaps are unprotected and are supposed to be plugged with sandbags during a hurricane. Preliminary evidence suggests they were left open, allowing water to pass unobstructed into the 9th Ward.

The Army Corps and local officials had warning signs that the levee system had shortcomings.

In the case of the soil defects, at least two contractors had warned that the soil conditions were weaker than the corps realized.

But the federal officials failed to heed the warning signs, Bea said, citing a culture that has the same flaws that investigators found in NASA after the Columbia space shuttle accident.

Bea said the corps had “normalized deviance,” meaning the corps had accepted as normal deviations that should have warned of impending disaster.