What caused the levee in New Orleans to break?
One of the cuases of the levees breaking or failing during Hurrican Katrina had to do with the actual design of the flood walls surrounding portions of the city of New Orleans. Designs called for pilings to be sunk to depths of up to seventeen feet in some places and investigation found that they were often only as deep as ten feet in many places.
Another problem with some of the levees was simply the incredible height of the water that went far higher than anyone had planned, simply overflowing the flood walls.
One other problem had to do with the surveys of the existing conditions on which levees were built, including soil and other factors that were not properly taken into account.