Note that the Superdome was established as a Last-Ditch Refuge, not a shelter. This means it was designed to handle last minute walk-ins and those who couldn't evacuate and that those people were to be evacuated from the last-ditch refuge to a shelter once the hurricane passed. By their definition a last-ditch refuge only has supplies to last through the hurricane.
If they were expecting a Cat 5 that would flood the entire city, what was their plan to evacuate people from the Last-ditch refuge into a shelter? How - by boat? What shelter? What was their plan to shelter thousands of people for months?
The Dome was to be a staging Area, only for those who could not be evacuated.
From the beginning, they treated it as a shelter. And only 200 Guard Troops.