Saving New Orleans, a Sinking City

Katrina's damage to New Orleans is not surprising to geologists. The city was built on loose soil that compacts over time, causing it to sink every year, and the sea level is rising. Planners will have to consider this as they discuss how to rebuild the city.

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Politicians and officials debating what comes next for New Orleans will have to confront a simple and troubling fact of geology. The city is sinking rapidly into the Gulf of Mexico. Parts of town that survived flooding this time may not be so lucky the next time around, and there's no way to prop up the city. NPR's Richard Harris reports.

RICHARD HARRIS reporting:

The city of New Orleans is largely below sea level today but it wasn't always so. It's been sinking. Christopher Mathewson, a geologist at Texas A&M University, says the problem is the city was built on extremely loose soil, more than a thousand feet of sediment deposited by the Mississippi River over thousands of years.

Mr. CHRISTOPHER MATHEWSON (Geologist, Texas A&M University): The land surface is moving down as the soft sediments underneath are being compressed by the overlying sediment. That's subsidence.

HARRIS: And this subsidence occurs all throughout the Mississippi River delta, in fact, even down the coast into Texas.

Mr. MATHEWSON: The subsidence is really taking place in the lower portion of this big stack of sediment.

HARRIS: Before the city was built, the river would flood during the springtime and dump a new load of sediment on the surface. So the ground was being built up as fast as it was sinking away, but no more. These days, the sediment is kept in the Mississippi River by the levees and it simply washes out into the Gulf. As a result, New Orleans is dropping further and further below sea level.

Ms. VIRGINIA BURKETT (US Geological Survey): The present surface that the city is built upon at one time was at or near sea level.

HARRIS: Virginia Burkett from the US Geological Survey says the deep bowl that the city now sits in has taken shape over the past few hundred years and regional subsidence is not the only reason the city is sinking.

Ms. BURKETT: You've got groundwater withdrawals. You've got oil and gas withdrawals.

HARRIS: Those also help lower the ground locally as these underground fluids are sucked to the surface. And not only is the land sinking, the ocean is rising. Global sea level has risen by six to eight inches in the past century.

Ms. BURKETT: There are multiple factors at play here.

HARRIS: Burkett and colleagues have been measuring the rate at which New Orleans has been sinking since the 1950s.

Ms. BURKETT: We analyzed the changes in elevation at roughly 300 benchmarks within the leveed area of the city of New Orleans, and the average subsidence rate for the past 50 years was about a half centimeter per year.

HARRIS: That's two inches a decade or almost two feet per century. And in places with especially soft soil, like near Lake Pontchartrain, the rate has been twice as fast. Burkett says incidentally she expects that the city will sink more than unusual this year. All that water sitting on the city right now will likely compact the soil down below. If you project the long-term subsidence rates into the future along with forecasts of future sea level rise, you get a scary number.

Ms. BURKETT: We're looking at the city sinking one meter over the next 100 years.

HARRIS: One yard or one meter subsidence would presumably put the French Quarter underwater if the city were to flood again.

Ms. BURKETT: Yes, that's correct.

HARRIS: Wow.

Ms. BURKETT: So within the next century, the areas that did not flood this time will be likely to flood under a similar hurricane situation.

HARRIS: And there's not much to be done about the sinking, says Christopher Mathewson from Texas A&M.

Mr. MATHEWSON: You could look at the areas that are really subsided and import sediment and fill it, but that's going to be very, very expensive; in other words, raise New Orleans back up again by filling it. But it's always going to keep subsiding, so it's only a temporary fix.

HARRIS: Of course, the levees could be strengthened and raised yet again, and more long-term solutions would involve shoring up the Barrier Islands and the vast areas of wetlands between the coast and the city. That at least would absorb some of the punch from a future hurricane, but the city itself will forever be below sea level and it will continue to sink year by year.

Richard Harris, NPR News.

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