Concrete flood walls that were supposed to protect New Orleans were not overwhelmed by high waters during Hurricane Katrina as federal officials have claimed, but ruptured because they were structurally flawed, according to Louisiana scientists.
From the mud splattered on buildings still standing near to the flood walls and the results of a computer simulation of the storm 鈥� known as a 鈥渉indcast鈥� 鈥� a team from the Hurricane Center at Louisiana State University in Baton Rouge claims it has pieced together how the walls, mounted on small earthen levees, must have broken.
鈥淓ither there was a design problem or a construction problem,鈥� says Paul Kemp, an oceanographer at the centre. 鈥淭hey were not supposed to break.鈥�
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The US Army Corps of Engineers, responsible for building and maintaining the levees, still claims that the 4-metre walls were simply overtopped by the storm surge 鈥� the wall of water that is dragged by a rotating hurricane as it hits a coastline. 鈥淲e are working from the preliminary theory that the levees were overtopped,鈥� says spokesperson Paul Johnston in Washington DC.
High-water mark
The team says this was likely to be true of the southeastern part of New Orleans, which borders the Mississippi River and was supposed to be protected by metal sheets, also mounted on low earthen levees. It is convinced that on the northeast side the water never reached the top of the concrete walls. This part of the city was blasted by floodwater that overflowed from the London Avenue and 17th Street Canals, which link Lake Pontchartrain at the north to the Mississippi River.
The first clue came when predictions from their computer simulation, which runs on the university鈥檚 supercomputer and can reconstruct weather events, indicated that the storm surge should not have been high enough to top the levees. To investigate whether their simulation needed tweaking, the team went out to take measurements.
They found that the simulation was correct. The mud pattern splattered on nearby buildings showed that the waterline had reached a maximum of between 3 and 3.5 metres, at least 50 centimetres shy of the height required to top the levees.
Kemp and his team also could tell from the levees that water had not flowed over them. 鈥淭he way that they failed was not consistent with over-topping,鈥� says Kemp. 鈥淚f water goes over, it plunges down the backside and you get erosional scour. We didn鈥檛 see that.鈥�
Walls breached
The walls are made of large plates of concrete about 20-feet wide that are glued together with rubber to form a thin wall, a bit like the teeth in a jaw. Kemp says that during the storm, plates had come apart from each other at the sides, letting water in through the gaps rather than over the top.
A large portion of the city that flooded would not have flooded had the walls stood firm as they were supposed to, he says.
Elizabeth English, also at the Hurricane Center, says a lack of maintenance as well as poor design or construction might have contributed to the failure of the walls.
Made of river sediment, the soil in New Orleans is very soft, so the levees require constant checking to ensure they have not been weakened or damaged by shifting soil.
She suspects that due to budget cuts, maintenance may have been overlooked. 鈥淲hen funding is cut and money is short, maintenance isn鈥檛 the issue that screams the loudest,鈥� she says.
The Army Corps of Engineers employed contractors to build the walls, but says it will not be able to make further comments on exactly what was to blame until it has investigated and submitted a report, due in June 2006.
Hurricane Katrina: The Aftermath 鈥� The most destructive US natural disaster in living memory. Keep up with the latest in our continually updated special report.
