- The Washington Times - Thursday, June 1, 2006


Parts of New Orleans are sinking far more rapidly than scientists first thought, more than an inch a year, according to new research that may explain some levee failures during Hurricane Katrina and raises more worries about the future.

The research, being published today in the journal Nature, is based on new satellite radar data for the three years before Katrina struck in 2005. The data show that some areas are sinking — from overdevelopment, drainage and natural seismic shifts — four or five times faster than the rest of the city.

And that, specialists say, can be deadly.

“My concern is the very low-lying areas,” said lead author Tim Dixon, a University of Miami geophysicist. “I think those areas are deathtraps. I don’t think those areas should be rebuilt.”

For years, scientists figured New Orleans on average was sinking about one-fifth of an inch a year based on 100 measurements of the region, Mr. Dixon said. The new data from 150,000 measurements taken from space finds that about 10 percent to 20 percent of the region had yearly subsidence in the inch-a-year range, he said.

As the grounds in those rapidly sinking areas shift downward, the protection from levees also falls, scientists and engineers said.

For example, the Mississippi River Gulf Outlet, built more than three decades ago, has sunk by more than 3 feet since its construction, Mr. Dixon said. That, he added, explained why water poured over the levee and part of it failed.

“The people in Saint Bernard got wiped out because the levee was too low,” said co-author Roy Dokka, director of the Louisiana Spatial Center at Louisiana State University. “It’s as simple as that.”

The subsidence means some evacuation roads, hospitals and shelters are further below sea level than emergency planners thought, Mr. Dokka said.

So when government officials talk of rebuilding levees to pre-Katrina levels, it may really still be several feet below what’s needed, Mr. Dokka and others say.

“Levees that are subsiding at a high rate are prone to failure,” Mr. Dixon said.

The federal government, especially the Army Corps of Engineers, hasn’t taken the dramatic sinking into account in rebuilding plans, said University of Berkeley engineering professor Bob Bea, part of an independent National Academy of Sciences-Berkeley team that analyzed the levee failures during Katrina.

“You have to change how you provide short- and long-term protection,” said Mr. Bea, a former engineer in New Orleans. He said plans for concrete walls don’t make sense because they sink and can’t be easily added onto. In California, engineers are experimenting with lighter-weight, reinforced foam-middle levee walls, he said.

Mr. Dixon and his co-author, Mr. Dokka, disagree on the major causes of New Orleans’ not-so-slow falling into the Gulf of Mexico.

Mr. Dixon blames overdevelopment and drainage of marshlands, saying “all the problems are man-made; before people settled there in the 1700s, this area was at sea level.”

But Mr. Dokka said much of the sinking is because of natural seismic shifts that have little to do with construction.

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