The levee breaches along two major canals that flooded New Orleans after Hurricane Katrina resulted from massive soil failures under concrete storm walls, not from hurricane surges that sent water over the tops of the walls as Army officials initially said, according to teams of investigators who have examined evidence in the last week.
The findings appear to chip away at the simple story that the storm surge was much larger and higher than the walls were designed to handle, though investigators caution that it is too early to blame design or construction.
“No question there was soil failures,” said Peter G. Nicholson, a University of Hawaii engineering professor who is leading an investigation for the American Society of Civil Engineers. “But we can’t speculate whether it was a construction, material or design flaw.”
If soil problems are widespread in New Orleans’ 350 miles of protective levees, then upgrading the system to protect against hurricanes more powerful than Katrina could require a major investment.
Immediately after the storm, the Army Corps of Engineers thought that a surge from Lake Pontchartrain had moved up drainage canals in the city and overflowed concrete storm walls, eroding foundations and leading to the breaches.
Investigators have found no evidence of such overflow and foundational scouring at the breaches in the London Avenue and 17th Street canals, two main failures behind the central New Orleans flooding. In fact, in one case, water marks are a full 2 1/2 feet below the tops of the walls.
Instead, investigators have found strong evidence that the soil structure was too weak for the pressure of the water, wind and waves.
Soil failure is like pushing on a chocolate cake that is sitting on a plate: At first the cake sticks to the plate, but if you push hard enough the gloppy structure eventually moves, said Raymond B. Seed, an engineering professor at UC Berkeley who is leading a separate National Science Foundation investigation of the levee breaches.
So far, Seed thinks the failure of the soil structure initiated the breaches at London Avenue and 17th Street. The levees were constructed on “particularly unfavorable” foundations of organic peat, which is both compressible and weak, he said.
The teams from the National Science Foundation and the American Society of Civil Engineers are expected to produce preliminary reports in weeks; the Army Corps of Engineers, which designed and built the levees, expects to issue its own formal report next year. The three groups are conducting separate investigations but have shared fact-finding and technical data gathering over the last week.
All three are examining a range of other failures over miles of levees in and around New Orleans. The teams have identified at least 25 breaches, most of which happened when storm surges washed over the levee tops and badly eroded the earthen structures.
Those failures are not getting as much attention, because they occurred in less-populated areas that were largely evacuated before the storm. So many kinds of levees exist in so many locations that the investigations are probably going to find many causes for levee failure.
“There aren’t a lot of simple answers,” Nicholson said. “That is the truth of it.”
But central New Orleans was flooded by breaches along the two canals. In those cases, masses of earth shifted, said Paul Mlakar, a scientist at the Army Corps of Engineers who is leading the corps’ investigation. Along the London Avenue canal, he found that a 100-foot-long block of soil, about 15 feet deep, was pushed back 35 feet.
As the earth berm shifted, the concrete storm wall on top collapsed into the hole left by the moving soil and disappeared into the water, Mlakar said. In some cases, evidence of the levee breaches has been buried under the emergency repairs made immediately after the hurricane.
Only one canal, the Inner Harbor Navigation Canal, showed signs that water levels rose over walls, and it is not clear whether that caused the levee to fail, Mlakar said. A shipping barge did break loose from its mooring and may have rammed the wall, though investigators say it is too soon to know whether that caused the failure.
Engineering experts have offered numerous theories about the failures, saying the walls were improperly designed, the soil strength was miscalculated, and the levees were improperly maintained under the authority of the local levee district.
But Seed, Mlakar and Nicholson said they had not completed an analysis of whether Katrina exceeded the design strength of the levees. Seed noted that hurricanes, like earthquakes, could impose sharply different loads from one location to another. Levee loads were affected by the water depth, but also by wind and waves against the storm walls.