You have stated one of the elementary reasons for subsidence in SE Louisiana.
No doubt that draining swamplands have aided to the compaction of soils.
But there are many other reasons and most are natural reasons.
Soil types and the effect of natural energy upon them. I know that when you pick out that full bag of potato chips at the store you become bewildered that you open a bag that is now half full when you get home. If you handled those chips like a new born baby, you may have had a full bag when you got them home.
Energy vibrations caused by water movement such as the Mississippi River causes a continuous compaction. The river is a mighty force to be reckoned with .
Then there is tectonic plate movement. The late Dr. Roy Dokka of LSU showed that the deposit of soils by deltaic action has caused a sinking off the coast of Louisiana that is bringing everything down, This delta plate is truly a downer so to speak.
Lowest elevations that I have dealt with a in the New Orleans area have been (- 9) to (-14 ft) in eastern Orleans parish. I did full recovery of NGS monuments for the multi-purpose cadastre map project in the late 80's.
When I did this work there was only one part of the city that I found stayed "stable".
I don't know why . Mist have had a sand lens of soil close to the surface.
Blame Wikipedia
I have been to (-14) and (-9) in New Orleans east during my survey dazes in the past.
I have known a few who have sunk lower in Louisiana but that has nothing to do with elevations or surveying.
Very nice list with lots of fascinating details.
I have flown in a private airplane a few feet above the Salton Sea in southern California and got a kick out of seeing the altimeter drop below zero.
Also, here is a photo I took in 2002 of the highest point in West Virginia. (easy drive)
Thanks & cheers! John
the lowest part is in central La. I think it is a gravel pit or similar.