New studies that used satellite radar data from the three years before Katrina struck show that New Orleans is sinking by more than 1 inch per year. Some areas, particularly those developed on reclaimed marshlands, are sinking 4 to 5 times faster than the rest of the city.
This phenomenon is called subsidence and is caused mainly by overdevelopment, drainage and natural seismic shifts. As the ground sinks, protection by levees in those areas also falls.
For example, the Mississippi River Gulf Outlet, built more than three decades ago, has sunk by more than three feet since it was built. That’s why water simply came right over the levees there. They were three feet below their initial height!
Now, when planning for the rebuilding of New Orleans is taking place, the government, and in particular the Army Corps of Engineers, need to take this new data into consideration, and to be safe, no one should rebuild in those areas that are sinking faster.