What happened in New Orleans [during Hurricane Katrina], while a terrible “natural” disaster, was also the cumulative result of excessive and inappropriate management of the Mississippi River and delta, inadequate emergency preparation, a failure to act in time on plans to restore the wetlands and storm protection levees, and the expansion of the city into increasingly vulnerable areas [Costanza, Mitsch & Day 2006: 467].
Mismanagement here refers to damming, leveeing and canal dredging of the Mississippi River Delta, resulting in a significant loss of wetlands and the erosion of barrier islands over the past 100-plus years. Coastal marshes and barrier islands depend on regular inputs of sediments deposited by the river, which has been isolated from the delta plain and unable to thus nourish it. Two thirds of the river empties directly into the depths of the Gulf of Mexico, while one third empties into shallow waters, where it nourishes wetlands via the Atchafalaya, the river’s single remaining distributary (other distributaries having been closed off).
Damage from Hurricane Katrina was exacerbated by its prior loss of wetlands. Expansive coastal wetlands protect coastal communities from hurricanes by “decreasing the area of open water (fetch) for wind to form waves, increasing drag on water motion and hence the amplitude of a storm surge, reducing direct wind effect on the water surface, and directly absorbing wave energy” [Costanza, Mitsch & Day 2006: 468].
For the rebuilding of New Orleans after the hurricane, the authors recommended several core principles aimed at social and ecological resilience. Among their recommendations, they advise that areas of the city currently below sea level (by as much as 5 meters in some parts) not be rebuilt, but, rather, be restored to wetland. This would allow for temporary water storage within the city, water filtration, and biodiversity protection. They also suggest the reopening of distributaries and the controlled breaching of certain levees to allow the river to resume its ancient task of distributing sediment over a greater expanse of coastal marshes, allowing these marshes to gradually rise in step with sea level rise.
Costanza, Robert, William J. Mitsch & John W. Day, Jr., 2006, A new vision for New Orleans and the Mississippi delta: applying ecological economics and ecological engineering, Front Ecol Environ 4(9), https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/1540-9295(2006)4[465:ANVFNO]2.0.CO;2.