In the case of New Orleans, the Federal Government was the main author of the disaster.
Responsibility for the Inner Harbor Navigation Canal is Federal. This is the canal that caused major flooding in the Ninth Ward and Jefferson and St. Bernard Parishes. It is part of the Navigable Waters of the United States. For years, there have been calls for upgrades. Rather than do that, Congress authorized a new lock to be built at a cost of 748 million, and a new bridge over the canal (separate project; more millions), although traffic was decreasing. NO money was voted for flood control.
The lock is (was?) a porkbarrel project of the Corps of Engineers and the Louisiana federal congressional delegation and their congressional buddies. These congressmen, of course, bear partial responsibility, but there were no objections from the Executive. City and State government officials had no effective say in the matter.
The responsibility of the Federal government for navigable waterways is spelled out under Title 33, Navigation and Navigable Waters. Three Federal agencies are involved:
Department of Transportation
Army Corps of Engineers
Coast Guard, Dept. of Homeland Security.
Also involved is Port of New Orleans (don't know who is ultimately responsible here, but I am sure the City has little or no power here).

Finally, there is NO way for the local and State governments to be ready for a disaster of that magnitude in the short time involved, and where they have little effective input on protective measures for the federally-controlled canal whose levees and walls were overtopped.

Now the Corps of Engineers is trying to restore Level 3 protection by mid-summer, and Level 5 protection is only on paper.

All of this has been posted before (thread 84801 and others).