It looks like this Monday may be a bad day for New Orleans residents because of the impending hurricane. My heartfelt best wishes to anyone living there or nearby.
Because of the impending hurricane, there’ve been numerous stories this evening highlighting that N.O. is particularly vulnerable to hurricanes because parts of the city are 10 feet below sea level. I was curious if the New Orleans had always been below sea, or if ground water pumping or other issues had caused the general area to subside.
I googled and found several cites of this sort: Here
but nothing very explanatory.
Question: has New Orleans always (well, since it was built) been below sea level, or has it subsided, either through groundwater pumping or another mechanism?
They have drained wetlands and diverted natural river paths to create areas fit for habitation. The floodwalls and other safety devices were created to forestall the city’s inevitable destruction.
Is it hard to imagine a worse place to build a city, outside of the slope of an active volcano or on top of a major faultline.
OK. Has this made things worse over time for the sea level of the city?
Yep, probably. But, since the city dates back almost to the 17th century, I was wondering if this riskiness dates back historically, or has gotten worse over time.
Its gotten worse since the invention of the Army Corps of Engineers. My point was that the wetlands (below sea level) were buffers between land and sea. Once these were gone, and development put there, they erected barriers to keep high waters out. These barriers allowed them to build deeper below sea level.
David Brin (physics Ph.D.) riffed extensively on the foolhardiness of (a) draining wetlands and (b) attempting to maintain New Orleans in his monumental 1990 novel Earth.
Of course, people everywhere are stubborn about giving up cities to the sea. It’s human nature. OTOH, as the melting glaciers begin to produce noticeable increases in ocean levels, there will come a point when there’s no choice in the matter - leave, or construct an underwater city.
New Orleans is hardly the only - nor even the first - such city that will be affected. Indeed, it’s not even the only southern U.S. city that will have a big problem. I mean, it’s not going to be a question of oceanfront property in Arizona, or anything like that, but there will be some really serious effects.
I do feel sorry for the people in NuAwlens, though. And in the coastal cities of Florida and Texas, when their times come. Which I do think it plausible I’ll live to see.
[Aside]
An amazing novel, isn’t it? We’re about one-third of the way through the half-century between when he wrote the book, and the time he set the novel in. And it looks more visionary with each passing year.
About 90-100 pages in, he has a passing mention of the North African desert jumping the Straits of Gibraltar and taking over a good deal of the Iberian Peninsula. I was reminded of that about a month back when I came across an article talking about the desertification of Spain, which they expect to be about 1/3 desert by 2050.
[/aside]
Like cheap hamburger on the grill, the soil of a swamp shrinks when you dry it out. So if the swamp was at sea level or just above when saturated, it will end up a bit lower when you get it dried out enough to build on.
How much shrinkage you get depends on the soil. Pea gravel won’t shrink at all. peat will shrink a bunch.
Yes indeed! And the part about the ghost of Elvis driving through old, abandoned gas stations is truly classic!
Back to the OP: the trick about low-lying wetlands is that the flood waters deposit a lot of the sediments that maintain the level of the land. You build barriers to keep out the flood waters, you lose sediment deposition. You haven’t stopped runoff, though, because it still rains on your city. You also pump out water for drinking, etc. Eventually your city winds up lower than the water.
when N.O. was founded, in the early 1700s, it was built on the high land. The old part of the city was built on the natural river levee. Dry, but not very big. So people almost immediately started building levees and drainage canals. That allowed them to move into the swamps. Things have been going downhill (literally) ever since.
John McPhee’s The Control of Nature has an excellent history and detail of the Army Corps of Engineers, New Orleans District Mississippi River
and Tributaries Project, including the development of the flood control levee system and the “Old River” control efforts, which have kept the Mississippi running through New Orleans rather than shifting to the west as it wants to do. (Other essays in the book discuss lava “control” in the Hawaiian Islands and flood/mudslide control of the San Gabriel Watershed north of Los Angeles.)
New Orleans is a disaster waiting to happen. But then again, so is Venice, Italy and a good portion of the Netherlands. And the city of St. Louis is just begging for a three-way smackdown.
Flood (though the older parts of the city are built on elevated land), sinkhole (the entire area is undermined with porous limestone bedrock that has become saturated and softened), and earthquake (the nearby New Madrid fault, the largest fault east of California).
Whatever the storm damage Uncle Sugar, that’s you and me kid, as US Tax Payers will foot the bill for the folly of building and/or staying in a city below sea level.
If N.O. had to foot the bill they would move of find better ways to deal with the problem.
You can count of the Army Corps of Engineers to do more dredging and more levee building at our expense for the benefit of the few.