The U.S. Gulf coast was devastated by hurricanes Katrina and Rita in the late summer of 2005, killing more than a thousand people and causing billions of dollars in property damage. One of the areas that suffered the greatest loss of life was New Orleans, a city that rests partly below sea level.

While the public and scientific community has responded with outpourings of donations and support, it is the duty of the scientific community to assess the causes of this disaster and to try to prevent such losses from occurring again.

The tragic losses from Hurricane Katrina in New Orleans and the Gulf Coast have led many to inquire about why the Gulf coast is sinking, or subsiding, and what can be done to protect coastal residents.

Most geologists agree that 1) much of New Orleans is below sea level and subsiding rapidly; 2) the coast line is retreating toward the mainland as wetlands sink below sea level making urban New Orleans more vulnerable to storm surges; 3) sea level is rising and hurricanes are entering a more active and intense cycle; 4) the current system of coastal defenses and levees are inadequate to protect lives and property in New Orleans. There is considerable disagreement and healthy debate about how best to manage these risks, and whether or not to rebuild the parts of New Orleans that are furthest below sea level, and that suffered the greatest damage.

The cause of the subsidence can be best understood as follows. As the weight of deltaic and other sediments is added to the shelf/passive margin on the Gulf coast, it presses down and compacts underlying sediments. The weight also causes the entire thickness of the crust and lithosphere to sink (tectonic subsidence), much like dirt added to an iceberg would cause it to float lower in the water. As some sediments at great depth, particularly salts and muds, may be able to flow, they tend to move out toward the open Gulf at deep levels, where they can ooze out on the continental slope and form pressure ridges and fold belts. Another mechanism by which the weight of the sediments causes subsidence is that gravity and the weight triggers motion on long-lived curved or listric growth faults. These faults curve toward the open gulf, and sink slowly downward and outward as material is deposited on them, causing the surface above the faults to sink. Together, these processes all allow the delta and shelf to gradually grow outward and the surface to slowly sink. If sediments are not regularly added to the surface layer, then the surface will subside below sea level. This sediment deprivation can be natural (as when a river switches course) or human-induced (as when a river has levees built on its bank not allowing sediment to overflow its banks).

The links, papers, news reports, and discussions on this page highlight some of the scientific data on these issues, and how the nation might best respond to protecting lives and property in future storm events on subsiding coasts.