Unstable flood walls 'protected' Orleans

NEW ORLEANS -- Along the Mississippi River and Lake Pontchartrain, great earthen levees were ample enough to hold off much of the surging waters propelled by Hurricane Katrina.

But concrete flood walls installed over the past several decades along the drainage and barge canals cutting into New Orleans, which collapsed in several places during the storm, were built in a way that by the Army Corps of Engineers' own standards left them potentially unstable in a flood, according to government documents and interviews.

An Army Corps engineering manual cautions that such flood walls "rarely exceed" 7 feet because they can lose stability as waters rise. But some of the New Orleans canal walls rose as high as 11 feet above dirt berms in which they were anchored.

In addition, as a result of federal budget constraints, the walls were never tested for their ability to withstand the cascades of lake water that rushed up to, or over, their tops as storm waves pulsed through the canals on Aug. 29, corps and local officials say.

Hurricane Katrina was the first serious test of the flood walls, said Stevan Spencer, chief engineer for the Orleans Levee District, and it "just overwhelmed the system."

Since the storm, Army Corps officials have said that there is a simple explanation for the devastation: Katrina was a Category 4 storm, and Congress only authorized a flood control system to handle a Category 3 storm. "Anything above that, all bets are off," said Al Naomi, a senior project manager in the Corps' New Orleans district.

But federal meteorologists say that New Orleans did not get the full brunt of the storm, because its strongest winds passed dozens of miles east of the city. While a formal analysis of the storm's strength will take months, the National Hurricane Center said the sustained winds over Lake Pontchartrain only reached 95 mph, while Category 3 storms are defined by sustained winds between 111 and 130 mph.

This raises a range of questions about how the walls that failed were designed and constructed, as well as whether the soil in some spots was too weak to hold them. Investigations by federal engineers and outside experts are just now beginning.

One factor could be height, said Robert G. Bea, a former Army Corps engineer and professor at the University of California, Berkeley, who is part of a National Science Foundation inquiry into the flood controls failures. The higher the wall, he said, the greater the risk it could tip under the ever-greater pressure of rising waters.

The 2000 edition of the Army Corps of Engineers manual "Design and Construction of Levees" says that the height of flood walls built on levees is an important factor in their ability to withstand a flood. For that reason, the manual says walls like those used in New Orleans "rarely exceed" 7 feet. But on two of the three canals where breaks occurred -- the 17th Street and London Avenue canals -- the concrete sections rise 11 feet above the dirt berms. Each wall resembles a row of teeth set in a jaw. Individual slabs are anchored to a continuous steel sheet buried in the dirt, giving the wall its strength. Above a short foundation, the slabs are only linked by rubbery gaskets that allow the concrete to expand and contract without cracking.

Hassan S. Mashriqui, an engineering professor at Louisiana State University and an expert on storm surges, said the segmented nature of the walls could be an additional problem, since any weak point can cause a catastrophic failure.

"Since they're not tied together, you get a little bit of a gap, and that's what water needs to make it fail," Mashriqui said.

Other questions surround the walls' design, known as an "I-wall" for its slim cross-section that fits easily into densely developed areas.

The Corps manual for flood-control construction suggests a different design for walls higher than 7 feet -- walls shaped like an inverted T, with the horizontal section buried in the dirt for extra stability.

But that option was never considered, Army Corps engineers said, because "T walls" were more expensive, required a broad base of dense soil for support and were not necessarily stronger.

The Army Corps and local levee authorities also never tested whether the chosen I-wall design could survive if water flowed over the top and cascaded onto dirt embankments below.

Corps officials said they were proscribed from considering stronger wall designs for the canals both by the tight quarters and by federal law, which requires that they seek and study only the level of flood control authorized by Congress.

"Our hands are tied as to looking at higher-level events," said Naomi.

Naomi said the recommendations in the flood control engineering manual were "general guidance," and that conditions at a particular site could justify deviations.

He defended the walls, saying, "The floodwalls have functioned over the years very successfully and without incident. The design works. It has worked in other locales. And will likely continue to be used as long as you do not subject it to pressures that it was not designed to handle."

The broken walls, which were long seen as a second choice to earthen levees, are testament to 40 years of fiscal and political compromises made by elected officials, from local levee boards to Congress and several presidential administrations, as they balanced costs and environmental concerns with the need to protect a city that lies largely below sea level and still subsiding.

Ever since Hurricane Betsy flooded parts of New Orleans in 1965, the federal government has funded a hurricane defense system designed to guard against an equivalent storm. But as the threat of a more intense hurricane became better understood in recent years, government funding for flood prevention in New Orleans did not keep pace with a growing alarm among many local residents, scientists and even the Corps' own engineers.

Standing next to the shattered remains of one of the concrete walls last week, Cynthia Hedge-Morrell, a New Orleans councilwoman, said: "In my opinion, they were playing Russian roulette with people's lives."

"Do you realize that if those walls had held, we'd have just had a little cleaning job?" said Hedge-Morrell, whose district between downtown and the lakefront was covered with 10 feet of water from the floodwalls breaks. "We would not have this massive loss of life and destruction."

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