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, 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. They 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 eventually got computers, and they did a rating for exactly the strength of storm that the levees could withstand, and 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.
John McQuaid co-authored a series of articles published in 2002 in New Orleans' main newspaper, the Times-Picayune, which described the city's vulnerability.
"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 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 attention, 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. Animals can burrow into it, weeds and trees can grow, water can weaken it, so it needs a lot of maintenance and a lot of attention, continuously, if it's going to be something you can rely on."
Civil engineers agree that the future of New Orleans must include a rigorous and regular levee maintenance program, and a more robust pumping system… and money to pay for it all.
In spite of all efforts to protect New Orleans, civil engineering professor Neil Grigg cites a 1993 study that concluded it was futile to try to stand against one of nature's most powerful events.
"What we need to do is to learn to adjust to those (events), and not to live in vulnerable areas, to be ready to evacuate, to have warning systems, and to use these sort of non-structural approaches rather than to build levees higher and higher and stronger and stronger. That just doesn't work."
Despite the inherent peril of living below sea level, officials so far are pledging to rebuild the city. Engineers stress that a new New Orleans will need better protection from the surrounding water… and better emergency plans in case efforts to keep out the water fail.