How would they get the people out of the dome - I think it would be better to move them to buildings where helicopters can air lift them out from the roofs if needed. You cannot do that from the dome I don't think - but I could be wrong.
There appears to be a big deck surrounding the Superdome, a good ways above street level.
Those people won't be leaving the Dome .. it's possible they may be bringing more people to the Dome for all flooding that's going to be worsening as the minutes tick by.