The city of New Orleans sits largely below sea level in a bowl bounded by the Mississippi River and Lake Pontchartrain. It relies on more than 500 kilometers of levees, a system of drainage canals and floodgates, and a network of pumping stations to keep it from filling with water. The arrangement has worked fairly well for more than 40 years. What happened this time? And what can be done to ensure it doesn't happen again?
Levees were first used more than 5,000 years ago in ancient Mesopotamia to control the destructive seasonal flooding of cities along the Tigris and Euphrates Rivers. According to Rafael Bras, Professor of Civil and Environmental Engineering at the Massachusetts Institute of Technology, today's levees serve the same purpose. "When you have a situation like in New Orleans," he explains, "where large portions of the land are below sea level or subject to tidal or storm flooding, then the only way ultimately to prevent disaster and flooding is to separate the large body of water from whatever the inner part (is) - lagoons, land, etc. When properly operating, and well-designed and maintained, levees in many situations are the only way to deal with the issue."
The levees that protect New Orleans today date from the 1960s. They were built in response to earlier floods that had severely damaged the city, and were considered state of the art at the time. Journalist John McQuaid says the engineers who designed that system of levees did so without the benefit of today's advanced technology. "They didn't really know, since they didn't have computers up and running that could model storm surges and the like, exactly what level of protection it afforded, in terms of how likely it was to be over-topped, but they were pretty proud of it and thought it would last a long time."
Once they got computers, he says, they did model the effect of a hurricane, and establish a rating for exactly the strength of storm the levees could withstand. "It was a fast-moving Category 3 storm. Anything stronger than that, the levee system could not be guaranteed to protect the city."
And the levees were no match for Katrina -- which came off the Gulf of Mexico as a much stronger, category 5 storm -- one of the most powerful hurricanes to hit the United States in years. It was heading straight for New Orleans but veered off at the last minute. The city was spared a direct hit. But a storm surge in its wake pushed water from Lake Pontchartrain over the floodwalls and levees, eating at their foundations until large sections collapsed.
John McQuaid co-authored a series of articles published in 2002 in New Orleans' main newspaper, , which described just that vulnerability. "This issue is something that every public official was aware of," he says. "We published our series, which splashed it all over town, and the state, and most people who lived in New Orleans were aware that this was a risk. Most people, I think, hoped and prayed that it was a relatively remote risk. But in part, New Orleans always had this fatalistic undercurrent to its character from the very beginning, and so I think some people thought, well, we'll let the good times roll and we'll deal with it when it happens."
The reporter puts more of the blame on the government agencies and bureaucracies that evaluate risk and decide how much money to spend to counter that risk.
But it's not just a question of money, says Neil Grigg, a professor of civil engineering at Colorado State University. Levees - like roads and bridges - need constant, too. "Once the levee's built and it's in there, and people forget about it, as they will do, things happen to make its condition deteriorate." He enumerates some of those things: animals can burrow into it, weeds and trees can grow on the slopes, water can weaken it. "It needs a lot of maintenance and a lot of attention, continuously, if it's going to be something you can rely on. It's like these other infrastructure problems, it's not something you can just put in place and forget about it, it requires a lot of attention in the future."