With the sea rising around it, New Orleans could be headed for new lows.

Credit: Falk Amelung/University of Miami

In the midst of preparing for the arrival of another intense hurricane season, the city of New Orleans is continuing to rebuild from the decimation caused by Hurricane Katrina less than a year ago. 

Urban planners in the Big Easy have their work cut out for them. But in addition to protecting the city from future storms, scientists are recommending that it be rebuilt as a sinking city. 

According to researchers at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science, New Orleans is sinking—in some areas up to 20mm per year. This phenomenon, combined with the expected worldwide rise in sea levels resulting from melting glaciers, means that, over the course of 100 years, the city could fall further under sea level, with levees falling several meters below the level where they were originally built. The group says this subsiding should be kept in mind with future levee designs.

“They are happy to fix those levees, but they haven’t had time to do a new design,” said Falk Amelung, a professor of geophysics at Miami and coauthor of the study. “When they really do a new design and want to build levees that can handle a category 5 hurricane, they must take this subsiding into account.” 

In its research, published in the June 1st issue of Nature, researchers used a Canadian satellite equipped with radar technology to obtain map-images of New Orleans. The images were taken at different times from the same spot in the satellite’s orbit. Using “exploits points,” or man-made structures on the ground that strongly reflect radar, scientists were able to calculate the rate at which parts of the city are sinking. 

This drop may have contributed to their inability to protect New Orleans from Katrina last year.

“In the Mississippi River-Gulf Outlet Canal, where we have a high subsidence rate, the levees were completely overtopped and St. Bernard Parish was heavily flooded because of it,” said Amelung

According to the team’s estimates, some of the current levees may now stand three feet shorter than when they were built 40 years ago. By 2106, for example, they believe the ground will be nearly three feet lower on average than it is now.

“Its very important that the levees are updated frequently to account for the sinking,” said Amelung. “That means every few years: make them higher. Or from the very beginning, making a high levee that is up to the cause.”

While engineers have a tough task ahead, so do researchers, who are still in search of just what is causing the city to subside. Some believe the city is slumping into the gulf due to tectonic shifts while others point fingers at a large drainage system that is at work at the Mississippi Delta.

“[The rate of subsiding] may be a combination of both, at this point we don’t know,” said Amelung. “If it has deep roots, then it will continue to sink as we see it now, and that is bad news for New Orleans.” 

New Orleans isn’t the only city slowly crawling into the nearest body of water; Venice has been falling into the Adriatic Sea for years. City engineers devised a controversial system of gates to protect the lagoon from rising tides, said John Keahey, author of Venice Against the Sea. New Orleans’ fragile delta location makes it even more difficult to save from rising tide levels. 

“Gates will not be the solution for New Orleans, they need a whole new concept of neighborhoods and rebuilding,” said Keahey. “They have to ask if homes should be allowed to be reconstructed if they were built in swamplands below sea level.”

Originally published June 1, 2006

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