Yeah, I see what you mean. I was just recalling those talking heads saying that they were supposed to be 17 feet high, but that there had been settlement and they were no longer that high.
You would know better than I how long it takes for a seawall to "settle" more than 10 feet.
You know just as well as I do how long it took the levees to settle ten feet. They were built in 1965 when the MRGO was first dredged, and they were as low as 6.6 feet in 1999 when the shuttle took their picture.
The mechanism is also clear. Imagery from shortly after the MRGO was built shows it to be about a third as wide as it is now. See the pictures here:
http://www.louisianasportsman.com/stories/2003/paradise-lost/future-of-mrgo.htm
The combined effects of daily tides, the wakes from passing freighters, saltwater incursion which killed the freshwater plants growing in the area and lesser storms caved in the banks of the waterway, allowing the marshy land under the levees to slump. In the last image from the lik above, you can visibly see the low spots in the levee just by following the top of it with your eyes. There is a low spot dead center in the frame, and another halfway from there to the lower left. Further up and to the right, you can see a straight line which bisects the levee's profile, giving you a clear idea of what a cross section of the levee looks like.
Well off in the distance, you can see where the levee starts to regain its original heigth. Now traverse back down the shoreline to the low spot centered in the image.
See the "missing" gouge out of the coastline? Had the levee held its shape when that gouge peeled off and slid down into the channel, the gouge would have been even deeper, but the levee slumped and the soil assumed it's natural angle of repose, two different angles actually, one above and one below the waterline.
You don't build permanent structures out of sludge, because it tends to flow and flatten when it gets wet. Note also that this article was posted in 2003.
Also, this thread here at FR:
http://www.freerepublic.com/focus/news/1484878/posts?page=1
contains an article from the WSJ which admits the subsidence, and that nothing was done about it.
"Joseph Suhayda, a retired Louisiana State University coastal oceanographer, who told the Journal he suspects the levees aren't actually that tall, partly due to sinking of the land beneath them. Mr. Suhayda
now consults for a maker of flood-protection barriers. If he's right, that would mean the levees weren't high enough to handle even a Category 2 or 3 hurricane. Katrina was nearly a Category 5.
The Corps of Engineers concedes some of its levees in the area "have settled and need to be raised to provide" the level of protection for which they were designed, according to a fact sheet on the Corps's
Web site dated May 23, 2005. But federal budget shortfalls in fiscal 2005 and 2006 "will prevent the Corps from addressing these pressing needs." Even had sufficient funds been available the work could not
have been completed in time to prevent the Katrina floods."
Note the attempted spin there, as if the subsidence had just been detected and the Federal cuts for 2005 and 2006 determined the outcome of an issue that's been a problem for at least 6 years now.
The real beauty of all this is that the good stuff, the 1 meter per pixel datasets, are already in the hands of the Feds, if they have the will to use them.