But all the predictions and simulations before Katrina predicted nothing but water going above the levees, not breaking them.
One simulation was called "Hurricane Pam":
"Hurricane Pam brought sustained winds of 120 mph, up to 20 inches of rain in parts of southeast Louisiana and storm surge that topped levees in the New Orleans area. More than one million residents evacuated and Hurricane Pam destroyed 500,000-600,000 buildings. Emergency officials from 50 parish, state, federal and volunteer organizations faced this scenario during a five-day exercise held this week at the State Emergency Operations Center in Baton Rouge.
A USA Today article says the "Army Corps of Engineers, which built most of the flood-protection levees in the region, pulled its personnel to a safe distance, expecting rising water from the storm would top the levees."
Brainster has many more examples in this well researched post.