It is the Mississippi levee and what started the evacuation of everyone. That is according to an expert that WWL had on a short time ago.
There is some breeching downriver from the main part of New Orleans. Plaquemines has multiple breeches; the guys working down Belle Chase HWY were talking about as they went further down river, they couldn't see any land. there's the Industrial Canal breech, Algiers across the river has a breech I think. Wouldn't be surprised if there were breeches in St. Bernard parish. There is none by the French quarter part of New Orleans - yet. But all this water makes stress on the levees.