Anyone who watched media reports from August 2005, when Hurricane Katrina made landfall in New Orleans, will remember the images of water flooding over the levees which were designed to protect the city. A number of factors caused the levees surrounding New Orleans to fail, ranging from poor design to the sheer ferocity of the storm, and these factors were considered when rebuilding the levee system in the wake of the storm. Other low-lying American cities also considered the failure of the levees in New Orleans when evaluating their own preparedness for storms.
New Orleans is a city in a rather unique position, because it is entirely below sea level. Residents must contend with the surrounding Mississippi River, Lake Pontchartrain, and Gulf of Mexico with a series of levees which are designed to keep floodwaters out of the city. When circumstances caused the levees to break in the wake of Hurricane Katrina, the effect was akin to slopping tea into a saucer; the water pooled with nowhere to go.
Within the first 24 hours of the storm, 28 levees had failed, and the total level of broken or failed levees rose to over 50 within a week. The United States Army Corps of Engineers (USACE), which had built the levees, issued an official explanation within days of the storm. According to USACE, the levees were only designed to protect New Orleans from a Category Three storm, and the storm surge generated by Hurricane Katrina was simply too massive for the levees to handle, which caused the levees to break.
USACE officials justified the inadequate protection by explaining that the funding for the levee project had been too restrictive for additional safety measures. The funding decision was made on the basis of careful risk analysis, which weighed the potential for storms above Category Three against the cost of installing levees, and the potential cost of coping with the after-effects of a major disaster. Risk assessment is often a gamble, and in the case of Hurricane Katrina, it would appear that the house won.
However, within weeks of the hurricane, additional information about the levee failures emerged, and professional engineering organizations posited several other scenarios which could have caused the levees to break. One of the most significant pieces of information in these investigations was the debris line, which was below the level of the top of the levee in many cases. This means that the floodwaters could not have topped these levees, and therefore they must have failed in some other way.
Engineers who criticized the levee failure pointed out that many of the levees were poorly reinforced, or built on substrata with a low shear strength, which meant that when floodwaters pushed against the levees, they simply gave way. In addition, the levee sections were often not interlocked, which would have increased their strength, and some of the levees were built over dirt or peat levees which were severely eroded by the floodwaters, causing those levees to break.
Independent studies concluded that low-quality construction and poor design caused the levees to break. While USACE officials initially resisted this conclusion, they ultimately carried out their own investigations, and admitted culpability in a series of Senate hearings held to discuss Hurricane Katrina. Oddly enough, despite this lesson, early versions of the plans to replace the levees were of even lower quality than the original levees.