Current mood:

sad

the scoop on southern Louisiana hydrogeology

There's a lot of rumors and speculation and simple lack of knowledge about the history and current conditions of southern Lousiana with regard to the River, storms, wetlands loss, sinking land, levees, floods, etc. Since I worked for the US Geological Survey's Water Resources Division for 4 1/2 years there, I've become intimately familiar with the hows and whys of the aforementioned issues. So I'm going to try to give a concise explanation that'll cover what's going on in NOLA.

1. Basics of river deltas: River deltas are built by centuries of yearly overbank flooding that deposits sediments. These sediments are loose, contain much organic matter, and plenty of water in interstitial spaces. Over time these sediments compact as water is squeezed out and flat clay particles line up together. In order to maintain this newly-formed land at a minimum elevation (i.e. just above sea level) these sediments MUST be regularly replenished, this includes naturally-formed levees which have coarser sediment and are therefore a bit higher.
2. Louisiana has a long (300 years) history of "starving" its back bayous, swamps, and marshes of river-borne sediment through the creation of levees down the entire length of the Mississippi which causes all that wonderful sediment to be dumped way out in the Gulf of Mexico.
3. Over the past century huge amounts of marshes and swamps in the delta have disappeared as their sediments compact and allow brackish water in. This kills the swamp and many marsh plants that would otherwise help stabilize the land. This shallow open water is prone to severe erosion in storms. This is the "natural hurricane protection" that southern Louisiana has lost.
4. New Orleans was founded on the levee of the Mississippi, and grew to cover the lower backswamp area between the levee, the levee of Bayou Chef Menteur, and "Lake" Pontchartrain (semi-brackish area we easterners know as a "sound"). As it grew, the Mississippi levee was built up, the Pontchartrain levee was created, and canals (and eventually an underground stormsewer system with pump stations) were dug to drain the backswamp areas and maintain mostly dry land. The Mississippi levee was built to be quite strong after many Mississippi floods, some of which were catastrophic, some were near-misses.
5. Since the land was no longer flooded by the Mississippi, the sediments were not replenished. The existing sediments compacted, and because they were dry, their organic matter decomposed, causing even more subsidence. The levees on the Mississippi and Pontchartrain sides began to grow to handle the increasing hydrostatic pressure of having the city below the average water line of these two waterbodies.
6. Unfortunately, this has doomed the city to an eventual catastrophic inundation. The Army Corps was quite proactive in making sure people understood the potential for complete inundation after a strong hurricane in just the right spot. It was fully explored in the NOLA Times-Picayune. Sadly, without somehow raising NOLA up entirely, this remained a constant threat, whether or not levees were strengthened and raised. Yeah, I suppose you could build a huge seawall similar to the one the Dutch built. But no one wants to spend the money for that.

I'm not entirely sure what really could have been done to avoid this. What truly mystifies me is why houses in southern Louisiana are not built up on stilts like those across the rest of the coastal South. I believe it is incredibly lax state building codes and insufficient pressure from FEMA to improve them. Make no mistake, I'm very sad, but surprised only that it would happen so soon.