Study: Sediment Makes New Orleans Sink

NEW ORLEANS (AP) -- Heavy sediment deposited in the Mississippi River delta in the last ice age has caused New Orleans to sink and will continue to drag down coastal Louisiana bit by bit for hundreds of years, according to a new study by NASA and Louisiana State University scientists.

The study, published recently in Geophysical Research Letters, adds an important perspective to the puzzle of natural and human factors complicating the effort to save New Orleans.

The weight of glacial period sediments has caused coastal Louisiana to sink between .04 inches and 0.3 inches a year and will continue to do so for hundreds of years, the study said. New Orleans, it said, will sink about 0.17 inches a year, or nearly three feet over the next 200 years.

Parts of the city are 5-10 feet below sea level now.

''It's sort of one of these processes that you can't stop,'' said Erik Ivins, a scientist with NASA's Jet Propulsion Laboratory in Pasadena, Calif., and the study's lead researcher.

The study was done by comparing a 60-year-old catalog of elevation measurements in coastal Louisiana to a model of the Earth's crust over the past 750,000 years that calculated the rate of sinking by both the weight of sea level rise and the flow of sediments into the Gulf of Mexico. The mathematical study corroborates a theory that the Mississippi's sediment load has contributed to sinking in coastal Louisiana.

The heavy load of ancient sediment is found here because coastal Louisiana was the drainage point for the entire North American ice sheet during much of the last glacial period, about 22,000 years ago, Ivins said.

The massive sediment slug was pushed downriver by advancing glaciers and it is still working its way down the Mississippi each year, researchers said.

Scientists have known for a long time that New Orleans is sinking, but recent advances using Global Positioning System technology and updated geodetic data have deepened the understanding of the geophysical forces at play.

The challenge will be to take what science has to offer to help save New Orleans, said Roy Dokka, executive director of LSU's Center for GeoInformatics and one of the study's researchers.

''We have to build smart. We also have to make sure that we understand what's happening exactly,'' Dokka said. ''If we get the science and engineering right, we can save New Orleans for hundreds of years.''

Measuring how much the land may sink is critical for the Army Corps of Engineers. It is embarking on a massive effort to build up levees and flood defenses around New Orleans and the surrounding region of swamps and marshes that are home to fishermen, Cajun culture and such critical infrastructure as ports and oil refineries.

Factors clouding New Orleans' future are formidable: The sea may rise by 3 feet over the next century because of global warming; hurricanes have destroyed important bulwark-like wetlands and barrier islands; ongoing human activities such as oil extraction are causing land loss; and the building of levees actually speed up subsidence.

Scientists are busy trying to evaluate the risks. That work was helped by Hurricane Katrina, which acted as a catalyst for scientific inquiries.

''I wouldn't say anybody was wandering through the dark before, but there is a better body of knowledge, both big picture and detailed,'' said Ed Link, a University of Maryland engineer who's led corps' efforts to study and improve levee building in the storm's wake.