Looking for answers in broken walls around New Orleans

A Johns Hopkins professor leads engineers and researchers examining why the levee system did not hold up against Katrina

October 28, 2005|By FRANK D. ROYLANCE | FRANK D. ROYLANCE,SUN REPORTER

Weeks after Hurricane Katrina crashed ashore in Louisiana, there was no mistaking the destructive power of the water that overwhelmed New Orleans' levees and drowned the city.

During an inspection of the wrecked levees this month, Robert A. Dalrymple spotted sections of the heavy, corrugated sheet-steel piling that formed the base of flood walls. They had been stretched flat and knocked down.

Other sections of concrete and steel flood wall simply collapsed.

"It fell over in a big way," declared the Johns Hopkins University civil engineering professor. "Like, you could find the wall maybe a couple hundred feet from the canal. It was pushed out of the way."

Why the flood-protection system for New Orleans failed is officially undetermined. The agency responsible for that system, the U.S. Army Corps of Engineers, is still investigating.

But the evidence Dalrymple's team and others have found contradicts an early theory that the system failed because it wasn't built for a hurricane of Katrina's force.

Although Dalrymple avoids drawing conclusions, the evidence is mounting that design flaws, faulty construction -- or a combination of the two -- caused some sections of the levee system to fail. They simply couldn't hold back flood waters they were designed to stop.

Dalrymple, 60, is president of the Association of Coastal Engineers, chairman of its Coastal Engineering Research Council and an expert on water waves.

He was on a team sent to Thailand last winter by the American Society of Civil Engineering to study the impact of the Dec. 26 tsunami.

After Katrina, he got the call again -- this time to gather evidence for studies of the levee failures in New Orleans. His team's report, now in preparation, will be submitted to the ASCE's Coasts, Oceans, Ports and Rivers Institute.

The institute will publish a paper within the coming year with conclusions about the levee failures and recommendations for avoiding similar failures.

With Dalrymple on the inspection trip were three other prominent engineers, researchers from the U.S. Army Corps of Engineers and a group from the University of California at Berkeley, which hopes to apply lessons learned to levee systems in that state's Central Valley.

The ASCE team arrived in New Orleans on Oct. 2, and began visiting many of the breaches along the city's 350 miles of levees. "There are dozens," he said. "We didn't count them because there were just more than we could see."

But the failures were neither uniform nor universal. For example, none of the tall earthen dikes that line the Mississippi River was broken. Nor did they see any failures anywhere along the shore of Lake Pontchartrain.

East of the city and along the man-made Mississippi River Gulf Outlet, there were signs that some earthen levees were overtopped by Katrina's 20-foot storm surge: grass washed from the tops of the levees, and deep gouges were carved by high water cascading down onto the embankment on the landward side.

"If it's sufficiently gouged out the levee just washes away," Dalrymple said.

Some scientists blame the design of the Outlet for accelerating the flow of water from Lake Pontchartrain. Some of those failures might have been preventable, he said. For example, in the Netherlands, dike builders protect against erosion by adding "scour" protection -- perhaps six feet of paving or rock -- at the base of the landward side of the flood wall.

"It's a more expensive design, but it allows for overtopping," Dalrymple said. In New Orleans, "we didn't find evidence of scour protection."

Other major levee failures occurred along the Industrial Canal in the eastern part of the city, the London Avenue Canal in the center and the 17th Street Canal to the west -- navigation and drainage canals that reach deep into the city from Lake Pontchartrain.

Unlike the broad earthen levees elsewhere in the region, these are flood walls -- concrete walls much like highway sound barriers, poured on top of pilings driven into an earthen base.

Along the Industrial Canal, Dalrymple said, there was clear evidence the storm surge overtopped the flood wall, parts of which were swept hundreds of feet away. "There was a trench excavated at the base of the wall, and the wall failed," he said.

Precisely how the wall failed is uncertain. Water flowing over it may have simply removed enough of the embankment to expose the sheet piling, weaken it and allow it to be pushed aside. Or water may have seeped under the wall and eroded its support.

The force of the water that burst from the Industrial Canal was evident in the adjacent neighborhood, where many homes weren't just submerged but washed away -- leaving only a slab.

Whatever the precise mechanism, the levee along the Industrial Canal was clearly overtopped -- the storm exceeded the levee's design.

Not so along the London Avenue and 17th Street canals. Debris and water lines there showed that the surge stopped several feet below the top of the 14-foot barriers. Yet the flood walls failed.

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