alt Hacker NewsGrade school level Louisiana info (probably somewhat wrong): It's generally brackish marsh/swamp land. Basically think silt/dirt with a bunch of plants evolved for this specific environment.
Then you take a bunch of humans and you cut down all the really old cypress trees for lumber. To facilitate that process you cut channels through all the plants that are in the way so you can ship that lumber via new unobstructed waterways. You also do it for oil/gas exploitation.
So now you have a lot of infiltration of saltier or fresher water and less protection during storm surge and such. Lots of plants die and these plants were holding the dirt/soil in place (and the storm surge helps wash that away).
Also you re-engineer the original waterways that deposited silt/dirt and helped build these areas up so that they deposit silt/dirt in other places (also you harvest the silt/dirt for use in farming so it never even gets that far down the waterways in the first place).
In the Netherlands we rely on beach grass, a type of grass that can root well in loose sand, to keep dunes together. This forms the main sea barrier. The grass has historically been replanted at least partially to maintain the protection. This spans back at least a few hundred years I believe.
It's interesting that Lousisana works so differently. I always kind of assumed that dunes would always be needed to keep the sea out in low lying areas, but that's not the case at all.