About Water and the Water Table...
(Katrina Note: As everyone knows, the city flooded very badly during Katrina and not one inch of New Orleans went unaffected, including our cemeteries. Lafayette Cemetery No. 1 was one of very few that escaped even partial inundation [No. 2 flooded]. Regardless, the information below is valid. As I was there, I can verify that the scope of flooding and destruction we encountered would have affected the vast majority of cemeteries around the world in much the same way.)
It has been a longstanding misconception about New Orleans' cemeteries that the reason burials in our cemeteries are predominantly in above-ground interment vehicles (ie. tombs, copings, mausoleums, etc.) is because of problems related to flooding and the city's close proximity to sea-level have indicated a practical impossibility in the burying of people in below-ground plots. Indeed, it is so deeply ingrained into cultural reflections, replete with (completely verifiable) stories of new burials, literally, bobbing out of the ground or even floating down the city's streets, that it has only been in the last few years that anyone has given the matter any real consideration.
But there are two major factors which need to be completely understood...
The first is that the burial traditions which predominate in our oldest cemeteries, namely our tombs, are all simply traditions which were introduced to the region by the French and Spanish colonists and are, in fact, very common in other areas settled by these peoples, even regions in higher elevations. But because the (historical) Latin influence is limited in North America and the United States, Americans and other northern visitors have incorrectly associated the two.
The second is that ground interment has always been practiced in the region for either cultural or economic reasons. There are these influences which pointedly recognize solely soil interment, and, of course, because of the fact that cremation wasn't practiced at all, during the numerous epidemics the masses of poor immigrants who were succumbing to a variety of illnesses were necessarily placed into in-ground vehicles of some sort.
You have to admit, as an historian reflecting what actually happened, that anyone fascinated by our history, as well as anyone relating it, can certainly get a lot of mileage from the story. Regardless of the fact that water is not the specific reason for above-ground interment, you can see that our cemeteries were quite a mess back then.