Why is New Orleans Vulnerable?
New Orleans is a city more vulnerable than most when it comes to storm surges. There are two main reasons for this. The first reason is New Orleans’ low elevation in relation to sea level, the second reason is the lack of nature’s best defense against a storm surge; wetlands and barrier islands.
New Orleans’ Low Elevation
The site of the city was originally very low in relation to sea level, but human interference has caused the city to sink even lower. When New Orleans was being constructed they ran out of good land. To make more room, engineers drained swamplands around the area so they could continue expansion. This drainage led to subsidence. Subsidence is sinking or settling to a lower level, in this case it was the earth’s surface sinking lower in relation to sea level. This sinking effect has led to present day New Orleans being, on average, six feet below sea level.
Elevation map of New Orleans showing high elevation areas in orange and red, lower elevations in greens and blues.
Further compounding this problem is the construction of levees. New Orleans is situated between the levees along the Mississippi River, and those around Lake Pontchartrain. This situation leaves New Orleans with a “bowl” effect. Due to this “bowl” effect, once water gets into the city, it is very difficult to get it out.
This map shows the "bowl" effect New Orleans experiences due to it's low elevation and levee systems.
New Orleans’ Disappearing Wetlands and Barrier Islands
The second major factor in New Orleans’ susceptibility to storm surges is their lack of nature’s best natural defenses against them; wetlands and barrier islands. For every mile of continuous wetlands a storm surge can be reduced by three to eight inches. Although it doesn’t seem like much, those valuable inches can be the difference between a city under water and a city completely dry.
These two photos show the same area of wetlands in the New Orleans area. The top photo is on July 17, 2001, the bottom one is from August 31, 2005. The extreme wetland loss is easily seen.
In an undisturbed area wetlands are naturally replenished every year by sediment from a flooding river, in this case the Mississippi. However, human interference has caused the wetlands and barrier islands off the coast of New Orleans to disappear at an incredible rate. Dams upriver from the city have caused the amount of sediment in the river to be reduced by up to 67%! Along with that, the levees built around New Orleans to protect it now divert the river’s flow much further out into the Gulf of Mexico, meaning that much of the remaining sediment is washed out to sea, and not deposited in either the wetlands or the barrier islands. These factors are denying nature’s best defenses their replenishment, and causing them to disappear. Over the past 50 years the wetlands have been disappearing at a rate of about 60 square kilometers per year.
An example of one of the upriver dams denying the wetlands their naturally replenishing sediment.
Not only are these areas not being naturally replenished, but they are also being eroded away. Canals built in and around New Orleans for shipping purposes are inadvertently destroying wetlands. The canals allow saltwater from the ocean to intrude much further inland than would naturally happen. The saltwater is infiltrating freshwater marshes and wetlands. The additional salinity in the water is killing off the native plants, and when they die their roots no longer hold the soil together. This leads to even more destruction of the wetlands.
Example of a canal that allows saltwater intrusion into freshwater wetlands.
The combination of low elevation and lack of natural defenses in New Orleans has proven to be a combination that leads to extreme vulnerability when faced with a large storm surge.