In a city partially below sea level, a lot of people were not surprised that most of the city flooded under the effects of the massive hurricane Katrina. However, few people expected that 75% of the city would be flooded when levees failed, one by one.
Prior to the hurricane, there was some consolation in the fact that levees were in place, even though there were concerns, and it was a well known fact that these old safe guards were in desperate need of being replaced or bolstered up. It took a major disaster to discover just how unstable and unsafe they were. The levees that were built, initially to protect the city had been built to withstand hurricane forces, but no one was totally aware of just how badly restructuring was needed.
In a great many cases, the levees failed simply because of overtopping. Storm surges simply went over the existing levees. Severe overtopping caused breaches in the levees along the eastern areas of the protected zones and south along the Mississippi River. Even in areas where the overtopping did not completely breach the levees, minor overtopping and splashing over of water was significant enough to contribute to the flooding.
Investigators that examined the levees after the flood waters receded, found that many of the breaches were caused by the difference between different sections of the levees. Not all levees were built to the same specifications, height, or materials. Breaches often occurred where various sections met.
While most of the flooding and breaches were due to overtopping, a great many of the breaches were caused by soil erosion beneath the levees. The soft soil and sand beneath the barricades eroded away from the surge, allowing water to flow beyond the levee without actually breaching the wall. In many cases, water boiled and bubbled up on the other side of the levees where levees were not anchored sufficiently deep enough to block the flow of water.
Levee foundations failed, partly due to age, inadequate materials, and the sheer force of the storm. Not only were they damaged during the actual hurricane and the storm surge, they were furthered compromised from the receding water.
Teams of researchers have been evaluating these levees and others around the country since the disaster in New Orleans to determine how these barriers can be improved, and work is under way to strengthen and make all levees more standard in their materials, height, and durability.
http://soundwaves.usgs.gov/2006/01/index.html
