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New Orleans Sinking Faster Than Thought, Satellites Find

Brian Handwerk
for National Geographic News
June 1, 2006
 
New Orleans may be sinking into the Gulf of Mexico even faster than
scientists realized.

Satellite images reveal that some areas of the city have been sinking at the rate of 1 inch (2.5 centimeters) a year.

The findings, published in the current issue of the journal Nature, may shed new light on the failure of the city's levee system and the flooding that followed Hurricane Katrina last August.

(See "Hurricane Katrina: Complete Coverage.")

A scientific team used one of Canada's RADARSAT satellites to map New Orleans and found that most of the city sank about a quarter inch (0.06 centimeter) annually in the three years leading up to Katrina's landfall.

But many areas, including some of the levees designed to hold back the waters of the Gulf of Mexico, sank at four or five times that rate.

"What we found is that some of the levee failures in New Orleans were [in] places where subsidence was highest," University of Miami geophysicist Tim Dixon told the Reuters news service.

The data suggest that some levees could be 3 feet (0.9 meter) lower than when they were built some 40 years ago.

The Mississippi River Gulf Outlet—the levee-protected canal notorious for its role in the flooding that followed Hurricane Katrina—has sunk more than three feet, the team's study reveals.

"That levee certainly was much lower than what it was originally designed for," said Louisiana State University geologist Roy Dokka, who took part in the research.

Rapidly sinking ground under a levee could cause structural weaknesses in the levee system, he explains.

But sinking levees aren't the only cause for concern raised by the research.

Key infrastructure like roads and hospitals may also be far lower than previously believed, putting them at greater risk of future flooding, Dokka said.

What's Causing New Orleans to Sink?

While scientists agree that parts of New Orleans are sinking, they don't agree on the rates or the root causes of the subsidence.

"There is a raging debate on rates of subsidence, how you measure it, and what's really happening," said University of Texas at Austin geologist Charles Groat, former director of the U.S. Geological Survey.

Groat explains that understanding this process is critical, but answers have proven elusive.

Subsidence has been attributed to geological processes, the weight of aboveground development, and the withdrawal of oil, gas, and water from shallow underground reservoirs.

(See a 2004 National Geographic magazine feature on New Orleans' vanishing coastline.)

"Members of the scientific community have squared off against each other in polite and not-so-polite terms," Groat said.

Areas built on reclaimed marshland, like much of New Orleans, may be particularly susceptible to continued subsidence, he explained.

"All [scientists] are in agreement that the subsidence issue is a huge one. Combined with sea level rise, it's a supreme challenge in low-lying coastal areas and a key part of any future plans for the [Gulf] Coast."

The study's authors hope that their findings will cast more light on a critical issue.

"There's an interagency group studying why the levee system failed, and a [National Academy of Sciences] report that came out, but neither of these really dealt with subsidence in any meaningful way," Dokka said.

"Engineers need to understand more about geology. Maybe this will help them."

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