- Unique combination of stresses that engineers could not have predicted caused the 17th Street Canal floodwall to fail and flood thousands of homes and businesses during Hurricane Katrina, according to an interim report of the task force investigating the disaster for the Army Corps of Engineers.
- Evidence points to forces that came together in a combination unique to the science and thus could not have been anticipated by the system's design teams.
- Interagency task force members said experiments with sophisticated computer models show the 17th Street Canal floodwall came down in a four-step process:
A unique combination of stresses that engineers could not have predicted caused the 17th Street Canal floodwall to fail and flood thousands of homes and businesses during Hurricane Katrina, according to an interim report of the task force investigating the disaster for the Army Corps of Engineers.
The report also points to soil subsidence that left floodwalls and levees lower than design specifications as contributing to the other failures and breaches that helped flood 80 percent of New Orleans and killed more than 1,100 residents in August.
Although independent analysts have blamed the 17th Street Canal failure on faulty engineering, including flawed soil investigations by local firms, the Interagency Performance Evaluation Task Force, composed of experts from academia and industry as well as state and federal agencies, said evidence points to forces that came together in a combination unique to the science and thus could not have been anticipated by the system's design teams.
"I would say it's certainly going to come as a surprise to many people, if not most people," said Ed Link, University of Maryland professor and task force project director.
The group said the causes of the London Avenue canal floodwall collapses are not yet known and emphasized that its findings are preliminary.
Bob Bea, a University of California professor who is part of a National Science Foundation investigation into the failures, said the task force's explanation of the 17th Street Canal breach is lacking.
"It's our jobs as engineers to anticipate the failure points, and when that doesn't happen, breakdowns like this occur," Bea said, emphasizing that he is speaking only for himself and not the NSF team. "The corps has a documented history where they say, 'We couldn't have anticipated this, therefore it was an act of God.'
"An experienced engineer knows he can't accept that."
Four steps to hell
Interagency task force members said experiments with sophisticated computer models show the 17th Street Canal floodwall came down in a four-step process:
-- As water in the canal rose to 10 feet -- an unprecedented but not unplanned height -- the pressure from the water and wind-driven waves in the canal began to push, or deflect, the concrete floodwall and its subsurface supporting steel sheet piling away from the canal and toward Lakeview.
-- The deflection created space between the wall and the levee on the canal side.
-- Such flexing is expected by designers, as is a small opening between the wall and the levee. But what happened in this case, and was not expected, was the separation extended the entire length of the sheetpile wall to 17.5 feet below sea level. Water rushed into this opening quickly, creating a channel separating the floodwall from the levee on the inside of the canal and allowing high water pressure to travel directly down to the soil layers beneath the wall.
-- The final blow came when a layer of clay about 15 feet below sea level that extended beyond the toe of the levee began slipping toward Lakeview, causing the levee to collapse and the wall with it.
The fatal flaw in the weak soils beneath the structure was not the now-notorious layer of peat widely cited by independent analysts for months, the task force said. In fact, the failure surface, as engineers call it, did not occur under the levee or canal, but at a level beneath the toe of the levee and in the yards of homes adjacent to the canal.
Link said task force tests showed the soil-strength estimates done by local firm Eustis Engineering when the walls were built proved to be more conservative than actual results. Further, he said there was no method of testing the plans for a combination of forces that caused the collapse -- called the "failure mechanism" by engineers.
"We've searched the literature and found nothing that resembles this," he said. "I'm not saying nothing exists, but so far we haven't found it."
There was disagreement on that point.
Bea said a 1986 corps study showed such separations could occur.
"That report was done by the Vicksburg (Miss.) research station for the New Orleans District, but there's no evidence it ever made its way to the (engineering) firms doing the work," said Bea, who added that a full discussion of the report would be in the National Science Foundation study to be published next month.
Corps officials acknowledged the report, titled "E-99 Sheet Pile Wall Field Load Test Report," but disputed Bea's interpretation.
Neither the interagency task force nor the corps dismissed the long-standing criticism that sheet pilings should have been driven at least to the bottom of the canals -- a standard engineering practice -- rather than stopped at 17 feet. While they agreed deeper pilings generally make stronger walls, they have yet to run simulations to determine whether deeper pilings would have prevented this collapse given the other conditions now known.
Soil subsidence levels in a region that was largely marsh and swamp fewer than 100 years ago is well known, but the rate of sinkage, which left many structures below the heights built to guard against storm surges, apparently took the panel by surprise. For example, the Industrial Canal floodwall that was built to 15 feet actually measured just above 12 feet when Katrina hit, a loss of 2.7 feet.
Task force teams "documented that many sections of the levees and floodwalls were substantially below their original design elevations, an effective loss in protection," the report said.
Corps officials said the Bush administration has budgeted almost $3 billion to repair and restore all levees and floodwalls in the region up to design heights during the next two years.
Louisiana State University professor Ivor van Heerden, a member of the state team investigating the failures, said he was not surprised by the report and generally agreed with its findings. He said the corps started using updated elevation data only five years ago, even though the state had been urging a change for years.
"So the fact that the corps have found some levees lower than they should be reflects local subsidence but also that they built them lower than they should be because they would not update their datum," van Heerden commented by e-mail. "It is cheaper to make a wall 12.8 feet tall rather than one 14 feet tall!"
Further, van Heerden wrote, "whether the fail plane (on the 17th Street Canal) was in peat or clay is really academic. The structure underwent catastrophic structure failure, the same for the two breaches on the London Avenue Canal."
Hassan Mashriqui, an engineer and storm modeler at the LSU Hurricane Center, said he would be cautious about estimates of wave forces inside the 17th Street Canal because his findings show that a huge pile of debris that stacked up against the Old Hammond Highway bridge across the canal probably blocked much of that force.
Bob Marshall can be reached at firstname.lastname@example.org or (504) 826-3539.