September 04, 2005

• This is an article I've seen linked a few times now, but I think it's good to get it out there as much as possible. I actually first heard about the problem of the Mississippi levees, and the subsidence of the delta (sinking into the crust beneath the weight of the deposited sediments) back in a Geology class 25 years ago. Geologists, meteorologists, levee engineers, and so on have known about the danger New Orleans faces for decades. We screwed with nature by containing the Mississippi in levees, and now nature has screwed us. The sad fact is, we can dry out New Orleans, and rebuild now, but in some 1,2,3 generations or more in the future, it will be swallowed by the Gulf. There is virtually no way to stop that from eventually happening.
• Well, one way to stop it would be to let the Mississippi flood, and the silt to restore the marshes.
• Letting the Mississippi flood the delta will help restore the marshes and that will help provide a barrier, but the city is still sinking. The city itself can't really be allowed to flood periodically. So the more it sinks, the greater the danger you are in. Just imagine if it was 5 feet lower than it is now, all those folks who were waiting on their roofs to be rescued would be dead. It may take a few centuries to sink New Orleans, but then again, the rise of sea levels is accelerating that danger too. We do have time to dry the city out now and enjoy it for a few more generations. And I think we should. But politicians are going to have to heed the engineers and scientists because some time down the road, we will have to abandon it. New Orleans has been irrevocably changed by Katrina. I have no idea what they plan to do with the immense number of houses that will have be condemned. It's possible they will be able to raise parts of the city, as they'll essentially have a clean slate after they knock down all that waterlogged housing. But that's an enormous, expensive project.
• I haven't done that much geology/geography, so please correct me - but it doesn't make sense that the delta would be subsiding into the crust from the weight of the silt. Rocks (like the Canadian shield) would be much heavier, and they aren't subsiding. Similarly, silt produced marshes in Eastern England are actually building land out into the North Sea. I have heard of different parts of plates subsiding or coming up - apparently places like Newfoundland with lots of fjords are places where the whole plate has been subsiding, whereas the eastern coast of the US (where sand bars form and extend the coast) are actually raising very slowly. I have heard of marshes shrinking from being drained - this is a very common problem in peat marshes (one of the reasons certain inland parts of the Netherlands and now the English Fenlands are under sealeval), but I didn't know if silt would work the same way. If it's the first, there is nothing to be done but to rebuild inland. If it's the second, it may be possible to build up with fill (which presumably wouldn't subside more). BUT the wetlands would still need to be restored as a protection - the article suggests some ways (too technical for me to summarise). I could see rebuilding, but not the city just as it was. The French Quarter and much of the downtown is actually above sealevel. If it is possible, they could build areas up. The Dutch have been working on new levee technology, especially since their own deadly sea surge in 1953 - recent floods in 1993 and 1995 had no fatalities.
• As more earth is deposited on top of it, the crust is forced downward in a process geologists call downwarping. Yes, if you put weight on top of the crust, it sinks into the mantle. Continents float like corks in the plastic mantle. When the Northern hemisphere was covered by ice, it sank under the weight, when the ice melted, it popped back up. When mountains are uplifted, they start to sink back down into the crust. When large deposits of silt are laid down (the Mississippi delta is absolutely massive), they cause the crust to sink. Of course, the sediments also are compacting. And the sea levels are rising. In the long term (and it may be centuries, I really don't know), the outlook is grim.
• Ah - there are two forces at work here. Along with the crust subsiding, the soil itself is compacting. I would still be surprised if it's the weight of the delta (or else similar deposits in England would be sinking, not raising), but part of larger platectonic movements. They have their own patterns. I found this quote from someone studying it: "It's below sea level because of the levees. The levees stop the river from flooding and the river's what built the whole coast of Louisiana through 7,000 years of alluvial soil deposits. And if you stop that flooding, the other second natural phenomena in any delta region in the world is subsidence. That alluvial soil is fine, it compacts, it shrinks."*
• And even with crust subsidence, with non-packing fill you could build the land to be above sea level. Or else we wouldn't have any land above sealevel on the Gulf Coast.
• Rereading - I see I'm wrong about the weight/crust issue. But still, weird. And I don't know how much is crust, and how much marsh drainage and soil compacting.
• I was talking with a friend from the area yesterday and we were wondering... What about all the cajuns in those swamps? What happened to them? After this is it possible there are no cajuns left?