PBS interviewed John Rennie, the editor of Scientific American magazine, and got a somewhat better answer, if an obvious one.
In this case, unfortunately, what happened was that the surge of water associated with Hurricane Katrina moved through the lake, struck the levees and opened up these holes in a few places allowing the waters from the lake to then start to flow down into the city…
New Orleans has been hit by a lot of hurricanes over the years and the levees really are constructed so that they can withstand a lot of the sorts of pressures and strains associated with typical hurricanes.
The fact is that even if Katrina had really hit New Orleans dead on and if we’d seen the kind of 25-foot surge that was associated with other parts in the worst part of the storm, if that had hit New Orleans, even if the levees had held up, an enormous amount of water would have still spilled over them and flooded the city.
So really nothing is built to withstand something the strength of Katrina. But the fact is that in practice, New Orleans didn’t have to experience that extraordinary level of force. It was still enough though to break open these areas of the levees.
The interview pointed me to this article detailing how New Orleans got into this mess, and how a $14 billion marsh-restoration project might have started getting it out:
Restoring coastal Louisiana would protect the country’s seafood and shipping industries and its oil and natural-gas supply. It would also save America’s largest wetlands, a bold environmental stroke. And without action, the million people outside New Orleans would have to relocate. The other million inside the bowl would live at the bottom of a sinking crater, surrounded by ever higher walls, trapped in a terminally ill city dependent on nonstop pumping to keep it alive.
"Terminally ill" sounds about right.
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