Reportedly, they are both- raised earth berms capped with concrete. It's also reported that they were built to withstand cat-3 hurricane. And ... what's in Ponchartrain? How about all the crap pumped out of the sewers of New Orleans?
The real poor design was to locate a city on alluvial soils so incredibly deep, with so little freeboard, knowing full well that the surface level would continually drop. This was not done up front, but allowed to continue/expand. Just like allowing residential construction, etc. along the coast to the east. Somebody profited.
J