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.
Wouldn't that be nice. Sadly, levees actually contribute to the problem due to several factors (IIRC 2 factors were the blocking of silt deposition and the lowering of ground water levels). The ground water levels are something that can be managed, the silt deposition is something you need to learn to live without.
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.
Yes the Netherlands is sinking. Hilariously enough its because of draught. The entire country is basically man made and a natural disaster. Luckily there is virtually unlimited budget and good engineering to keep things going.
