Actually scratch my last post....Found this in another post here:
They don't bury the dead in New Orleans. The highest point in the city is only 6 ft. above sea level, which makes for watery graves. Fearful that rotting corpses caused epidemics, the city limited ground burials in 1830. Mausoleums built on soggy cemetery grounds became the final resting place for generations. Beyond providing a macabre tourist attraction, these "cities of the dead" serve as a reminder of the Big Easy's vulnerability to flooding. The reason water rushes into graves is because New Orleans sits atop a delta made of unconsolidated material that has washed down the Mississippi River.
I've seen those crypt cemetaries, but I'm curious how they fared given the pressure that water creates. They could be dislodged. Many are old and water under speed and pressure could undermine their weight and solidity. It may be days before they know the effect upon those above-ground tombs.