WASHINGTON – Everyone has known New Orleans is a sinking city. Now new research suggests parts of the city are sinking even faster than many scientists imagined — more than an inch a year.
That may explain some of the levee failures during Hurricane Katrina and it raises more worries about the future.
The research, reported 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 four or five times faster than the rest of the city. And that, experts 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 death traps. I don't think those areas should be rebuilt."
The blame for this phenomenon, called subsidence, includes overdevelopment, drainage and natural seismic shifts.
For years, scientists figured the city on average was sinking about one-fifth of an inch a year based on 100 measurements of the region, 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 ground in those areas sinks, 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, Dixon said, explaining why water poured over the levee and part of it failed.
"The people in St. 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 "is making the land more vulnerable; it's also screwed up our ability to figure out where the land is," Dokka said. And it means some evacuation roads, hospitals and shelters are further below sea level than emergency planners thought.
So when government officials talk of rebuilding levees to pre-Katrina levels, it may really still be several feet below what's needed, Dokka and others say.
"Levees that are subsiding at a high rate are prone to failure," 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 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.
Dixon and his co-author Dokka disagree on the major causes of New Orleans' not-so-slow fall into the Gulf of Mexico.
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 Dokka said much of the sinking is due to natural seismic shifts that have little to do with construction.
Dokka also thinks all is not completely lost. Smarter construction can buy New Orleans some time.
"We've made the pact with the devil by moving down here," he said. "If we do things right, we probably can get another 100-200-300 years out of this area."
The Army Corps of Engineers is adding extra height to earthen levees to compensate for sinking and is setting benchmark measurements of all levees for regular monitoring of how much they sink, corps spokesman Gene Pawlik said.
"It's something post-Katrina, we're much more focused on," Pawlik said Wednesday. "It's certainly an engineering challenge."