It's neither. Erosion and sea level changes are not at all the problem here. The problem is that keeping the lowest point dry causes subsurface water to be rem

It's neither. Erosion and sea level changes are not at all the problem here. The problem is that keeping the lowest point dry causes subsurface water to be removed from the area. Also, that water doesn't get replenished. Things tend to shrink when they dry out, and the land under New Orleans is no exception. It's becoming like a shriveled up raisin or sponge. The more you pump the place dry, the more the place sinks lower.