Update on New Orleans floodwalls
From the L. A. Times.
Remember, these are floodwalls, not levees. They look more like the soundwalls on freeways than anything else, and I assume they are used where there isn't enough land for levees. As far as I know, there were no levee failures.
I have no idea how much high-value land would have to be appropriated to replace floodwalls with levees.