I understand that. But New Orleans is subsiding. It's sinking every year. It's easier to maintain levees if the land beneath them isn't sinking. I don't know why my comment is getting downvoted.
Thing is, if the land wasn't sinking to begin with, exactly why were you considering levees in the first place? ;-)
Whatever the case may be, if you're building levees, you need to factor in the sink rate of the land around you, among the many factors. And you need to factor in the cost to maintain them indefinitely.
It's an expensive proposition; but by the simple fact that you're alive, standing there, and considering it already, chances are the ROI is worth it.