www.newscientist.com/article.ns?id=dn8038
Advertising Concrete flood walls that were supposed to protect New Orleans were not o verwhelmed by high waters during Hurricane Katrina as federal officials have claimed, but ruptured because they were structurally flawed, accord ing to Louisiana scientists. From the mud splattered on buildings still standing near to the flood wal ls and the results of a computer simulation of the storm - known as a hi ndcast - a team from the Hurricane Center at Louisiana State University in Baton Rouge claims it has pieced together how the walls, mounted on s mall earthen levees, must have broken. Either there was a design problem or a construction problem, says Paul Ke mp, an oceanographer at the centre. The US Army Corps of Engineers, responsible for building and maintaining the levees, still claims that the 4-metre walls were simply overtopped b y the storm surge - the wall of water that is dragged by a rotating hurr icane as it hits a coastline. We are working from the preliminary theory that the levees were overtopped, says spokesperson Paul Johnston in Was hington 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 prot ected by metal sheets, also mounted on low earthen levees. It is convinc ed that on the northeast side the water never reached the top of the con crete walls. This part of the city was blasted by floodwater that overfl owed from the London Avenue and 17th Street Canals, which link Lake Pont chartrain at the north to the Mississippi River. The first clue came when predictions from their computer simulation, whic h runs on the universitys supercomputer and can reconstruct weather even ts, indicated that the storm surge should not have been high enough to t op the levees. To investigate whether their simulation needed tweaking, the team went out to take measurements. The mud pattern splattered on nearby buildings showed that the waterline had reached a maximum of bet ween 3 and 35 metres, at least 50 centimetres shy of the height require d to top the levees. Kemp and his team also could tell from the levees that water had not flow ed over them. The way that they failed was not consistent with over-topp ing, says Kemp. If water goes over, it plunges down the backside and you get erosional scour. Walls breached The walls are made of large plates of concrete about 20-feet wide that ar e glued together with rubber to form a thin wall, a bit like the teeth i n a jaw. Kemp says that during the storm, plates had come apart from eac h 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 w alls stood firm as they were supposed to, he says. Elizabeth English, also at the Hurricane Center, says a lack of maintenan ce 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 leve es require constant checking to ensure they have not been weakened or da maged by shifting soil. She suspects that due to budget cuts, maintenance may have been overlooke d When funding is cut and money is short, maintenance isnt the issue th at screams the loudest, she says.
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