New Orleans is one of America's great historic cities, and our emotional response to the disaster is to rebuild it grander and greater than before. However this may not be the most rational or scientifically sound response and could lead to even greater human catastrophe and financial loss in the future.
New Orleans is located on a coastal delta basin up to 10 feet below sea level and still sinking as much as one inch per year. Much of the city could be 18 feet below sea level by the end of the century, or even more if sea level rise becomes significant.
The city has other problems of location. To protect communities along the Mississippi River, the Army Corps of Engineers built a 2,000 mile long system of levees that help prevent river flood waters surging from the channel and inundating low lying areas. However, the levees also channel sediments that normally get deposited on the flood plain and delta far out into the Gulf of Mexico, causing the land surface of the delta south of New Orleans to sink below sea level at an alarming rate. A total land area the size of Manhattan is disappearing every year, meaning that New Orleans will be right on the Gulf Coast by the end of the century.
The projected setting of the city in 2100 is in a hole up to 18 feet below sea level directly on the hurricane-prone coast. The city will look like a fish tank battered by coastal waves, surrounded by 50- to 100-foot-high seawalls that are barely able to protect it from hurricanes that are only as strong as Katrina. Such a city is untenable, and we as a nation need to face this reality.
The levees have an additional collateral effect that may doom the future of New Orleans. A river confined by levees builds its base higher than without levees. Catastrophic floods occur when the river base rises tens of feet above the flood plains, then breaks through the levees. The Chinese know this from their history of flooding along the Yellow River, known as the River of Sorrow after the millions of people who have died there, more than from any other natural disaster in the world. As we consider rebuilding New Orleans we need to remember China's experience.
The Mississippi has over geological time altered its course, with its mouth migrating east and west by hundreds of miles. Each abandoned delta subsides below sea level after the river jumps to another location, as buried muds compact and the river no longer replenishes the delta with sediment. The lower Mississippi now follows a long and circuitous course from the Atchafalaya River junction, through New Orleans, to its mouth near Venice. The river is ready to switch its course to follow the Atchafalaya, offering it a shorter route to the gulf. When this occurs, perhaps triggered by catastrophic flooding and a drenching hurricane, it will be devastating to the lower delta, which will quickly subside. New Orleans will be rapidly inundated by waves and storms from the Gulf of Mexico. To mitigate this hazard the Army Corps maintains an extensive system of diversions, levees, and dams at the Mississippi/Atchafalaya junction, with the aim of keeping the Mississippi in its channel.
New Orleans is sinking further below sea level every year, and the shoreline is rapidly approaching the city. The river is rising, and more hurricanes and floods are certain to strike the region in the next 100 years. The decision whether to rebuild or relocate an historic city is a difficult one. Moving the bulk of the city would be more costly, at least at this stage before sinking increases and another disaster strikes. The costs of either decision will be enormous, but relocating makes more sense and will eventually be inevitable. Whether we cut our losses now and move or wait until a super-hurricane makes a direct hit and kills hundreds of thousands of people must be carefully considered.
One option would be to begin building newer, higher, stronger seawalls around the business and historic parts of the city, and declare other parts a national monument, in tribute to those who lost their lives to Katrina. The process of moving could be gradual, relocating refugees, destroyed businesses, port facilities, and other infrastructure to a new New Orleans.
Katrina (even before Rita) was a warning: New Orleans is sinking unbearably below sea level, and it's time to move to higher ground.
Timothy M. Kusky is a professor of natural sciences at Saint Louis University.