Two Years Later, New Orleans Still Sinking

Aug. 29, 2007 — Two years after Katrina, the planet's message to southern Louisiana is loud and clear: The land is sinking there and nothing can stop it.

Or that's the message from geologists, anyway. The latest science on the sinking comes from a study in which researchers modeled how the Earth's rigid crust, or lithosphere, there is responding to the weight of Mississippi River sediments.

"Every geologist knows if you put a load on top of the Earth, it's going to cause the lithosphere to bend," said geologist Roy Dokka of Louisiana State University. Dokka, along with Caltech's Erik Ivins and Ronald Blom, published their results in the August issue of Geophysical Research Letters.

Their models were designed to see whether the sinkage, apparent in actual survey and extensive GPS data, can be explained by deep geological processes, rather than short-term, local causes — such as groundwater pumping, soil compaction or vertical movements along relatively shallow local faults.

"The GPS measurements are in reasonably good agreement with our model predictions, both in magnitude and spatial pattern," reported Dokka and his colleagues. Both data sets show the coast is sinking into the Gulf — a process known as subsidence — at a rate of about 1 to 8 millimeters (1/25 to 1/3 of an inch) per year.

The subsidence of the Mississippi Delta has never been a surprise to most geologists. It is, in fact, the traditional way large river deltas are thought to behave all over the world. It's the reason why, for instance, there are ancient sunken ruins of cities found in the Nile Delta region today.

But figuring out exactly what's causing the subsidence is critical in light of plans to rebuild New Orleans — a city which has always been sinking, though gradually, along with its levees. Even before Katrina, it was long understood by officials and scientists that it was only a matter of time before a storm would lead to widespread flooding in the city, which lies below sea level.

Now, in the wake of that disaster, it remains highly controversial whether the subsidence is caused mostly by shallow, human-induced factors like groundwater pumping, or deeper "tectonic" changes beyond our control. Dokka and many other geologists argue the latter, and say that rebuilding many parts of New Orleans is folly in light of the geologically doomed location.

The physics of the situation are a "no-brainer," Dokka told Discovery News. The mouth of a large river is where giga-tons of muddy sediment drop to the bottom and pile up.

The pile-ups can even temporarily build land into the sea, as is the case in southern Louisiana. As this happens the river periodically switches the route of its main channel to get around the muddy dams it creates.

At the same time, the lithosphere is steadily feeling the weight and bending downward. It always lags in time behind the mud load, because the viscous rock underneath needs time to be pushed away to accommodate the bending.

"It's like walking on thin ice with molasses underneath," explained geophysicist Norm Sleep of Stanford University, considered by many to be the world's leading authority on the lithosphere. You walk on it and it flexes slowly as the molasses moves out of the way. "This is the classic way of representing the lithosphere."

The bad news, of course, is that this sort of subsidence is beyond any human engineering solution.

"Developing in some of these areas of southern Louisiana — you need to have your head examined," said Sleep.

"It cannot in any shape or form be mitigated," said Dokka. "No matter what you do, it will go on for another thousand years into the future."

So far, the Army Corps of Engineers is not looking at the larger geological picture, said Dokka. Instead, they are designing more and higher levees which will also fail to protect the city when another Katrina comes along — which is a mathematical certainty.

"They are basically getting ready for the next disaster," Dokka said.

"The bigger story here is what does it take to get the [Army] Corps of Engineers to wake up to changes in science," he added. Running alongside that question is a lot of money and politics, he said. After all, if the science says the region is hopelessly subsiding and everyone agrees on it, "What's going to happen to the funding to rebuild New Orleans?"

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