Here's a link to an explanation of "Hurricane Categories" by the "National Weather Service's Hurricane Center".

Katrina was the top Category, Category Five - and the point is, Category Five Hurricanes had stuck the US mainland no fewer than three times since records began, in 1935, 1969 and 1992, with another near miss in 1988.

This was predictable. The failure to prepare for it, to put the work into maintaining and improving the flood defences, in line with the recommendations of the professional experts, and the failure to have a proper evacuation plan in place, are nothing short of criminal. Literally so.