Are shifts in Earth's crust causing New Orleans to sink?

[n2doc]

Are shifts in Earth's crust causing New Orleans to sink?
By Peter N. Spotts | Staff writer of The Christian Science Monitor

Hurricane Katrina's devastating strike on New Orleans last fall highlighted shortcomings in the city's levee system. It also focused attention another long-term problem: The city and the region around it are sinking.
New research suggests, however, that at least for nearby Michoud, La., the dominant driver pulling the region under may not be among the usual suspects: oil extraction, pumping groundwater to the surface, or diverting the Mississippi for navigation.

Instead, the King of Slump may be a deep fault that cuts across southeastern Louisiana and under Michoud. During the 1970s, the fault appears to have contributed from 50 to 73 percent of the subsidence in this section of Orleans parish, depending on the time period measured. If sustained over a century, that would equate to as much as a six-foot sea-level rise, independent of any increase tied to global warming.

"Something dynamic is going on down there," says Roy Dokka, who heads the Center for GeoInformatics at Louisiana State University in Baton Rouge, La. "It doesn't occur everywhere," but it certainly appears to be affecting Orleans parish, he adds.

If these results hold up, they would imply that to build new levees properly, engineers will have to take into account the effects of further slumping along the fault - data hard to come by because the fault is so deep and difficult to study. The work is controversial. It builds on a study Dr. Dokka and Kurt Shinkle of the National Oceanic and Atmospheric Administration's National Geodetic Survey (NGS) completed in 2004 for NOAA. That study drew on some 2,700 measuring points around southern Louisiana to measure subsidence rates. It yielded far higher sinking rates than other scientists had calculated.
More:
http://www.csmonitor.com/2006/0331/p02s01-sten.html

[dipsydoodle]

1. I recall this subject

from a documentary on plate tectonics. Increase in sea level is a red herring - no joke intended. If the continental shelf is tilted upwards by pressure from below as a result of movement under it then the effect is to lower the land inland.

The effect was first noticed somewhere up the Canadian coast, I think by the guy who developed plate theory, where there was a line of mussel shells fifty feet so so up a cliff face. The intial erroneos assumption was that sea level had fallen. It was subsequently found that a mile or so inland the land had sunk by an almost equal amount.

That's the sum total of my knowledge on this subject. Really needs a geologist to be more precise.

[China_cat]

2. That may be happening

but a good part of the problems in New Orleans has been caused by people trying to make the Mississippi River do what THEY want it to do instead of what it would do naturally.

You don't mess with Mother Nature and not expect her to come back and bite you in the ass.

[sutz12]

3. I thought it was the weight of the buildings...

coupled with the levees that channel the annual floods out to the far end of the delta.

[Igel]

4. In part.

The delta's always sinking; it's basically a huge mound of sludge that the river deposited: organic material, saturated clay and dirt. Like any other mound of wet sludge, it tends to flatten; as water squishes or is pumped out, it subsides. It survived for millennia because regular flooding of the swamps and marshes deposited silt and sludge, and kept the gumbo fairly well hydrated (gumbos a kind of clay; it's nasty stuff, and expands when wet ... Oregon's Willamette Valley and SE Texas also have gumbo). The river would meander, and that would help flooding and silt deposition. Now it's all channeled, and apart from Katrina, no silt's been deposited for a long time.

There's also this fault: it was reported in the news early last September that part of the levee system was subsiding faster than expected, and they blamed a fault. The report was quickly drowned out by the blame-assigning crew, discussion of underfunding of levee improvements, dynamiting of levees, etc., etc., etc. The results were known to the ACoE, but we can't hold them all that guilty for not acting on them. As with all such initial reports, they need to send somebody out to confirm it; then they need to evaluate the subsidence rate, if they can stop the subsidence, if they need to increase the levee protection, and if that is even possible; then they need to design a program to implement their solution, get funding for it, bid it, and have it built. There wasn't really not enough time for the first step, much less the entire shebang. If true, however, this poses a real problem: it means that they have to check lots of parts of the levee system for subsidence, and that might greatly complicate flood-plain map certification.

[seriousstan]

5. How dare you bring ratonal evaluation to the table. ROVE ROVE ROVE