From Science:
Causing the largest natural disaster in U.S. history, Katrina slammed into the Gulf Coast on 29 August with its eye hitting about 55 km east of the city. Although the storm initially brought more destruction to other areas along the Mississippi and Louisiana coast, several levees protecting New Orleans failed the following the day, and the city, about 80% of which is below sea level, filled with water. The floods may have killed thousands, stranded many more, and triggered a massive relief and evacuation effort.
As Katrina traveled through the Gulf of Mexico, unusually warm waters strengthened it into a monster hurricane. According to these models, Katrina’s storm surge should not have submerged the city. Instead of overtopping, the catastrophic collapse of several levees–ones that had been upgraded with a thick concrete wall– apparently sealed the city’s fate.
More here.