Both Katrina and Ivan were storms that produced more substantial storm surge and damage than would normally be expected based on their landfall intensity. Both of those storms were much stronger at one time over the Gulf than they were at landfall. Some of the damage may be due to having once been stronger before landfall with respect to building up a storm surge, though it also makes a difference that both Katrina and Ivan were very large storms in size and both came in at basically a right-angle to the coastline. I don't remember the specifics of Isabel, though I believe it was also large in size. The size of the storm increases the duration that a single point will experience strong winds and increases the potential damage.