In fact the levees were designed to survive a Katrina-sized storm, but of course never had to prove they could, until they failed.
They were not, and the fact was well-known in advance. Their limit was a category 3 hurricane and there had been years of passing the buck about upgrading the levees to give the city a shot.
Then Katrina happened and everyone who had passed the buck claimed that it wasn't their fault.
Katrina was actually weakened to category 3 when it hit new Orleans. They should have lucked out (yet again) if the levees performed as they were supposed to.
They were not, and the fact was well-known in advance. Their limit was a category 3 hurricane and there had been years of passing the buck about upgrading the levees to give the city a shot.
Then Katrina happened and everyone who had passed the buck claimed that it wasn't their fault.