Good question.

Panel: Catastrophic storm not assessed:

"The final report on why the New Orleans area levees failed during Hurricane Katrina should address why the system was built without adequately assessing the risk of a catastrophic hurricane hitting the city, an American Society of Civil Engineers panel said Monday.

'It would be helpful to point out to readers that this effort is the first attempt to conduct a comprehensive risk assessment for an entire levee system on this scale,' the engineers from the external review panel said in a letter to Lt. Gen. Carl Strock, chief of the Army Corps of Engineers.

'One question for the (corps) is: Why was the risk not formally assessed prior to Katrina?' the letter said."