Pardon my ignorance, but wouldn’t it be safer to simply bleed the levies along both sides of the Mississippi, Missouri and Ohio rivers? Some large pipes installed at five kilometer intervals near the top and slanting down the slope could do the job. I’m no engineer, but that’s really the first thought that occurred to me, after seeing that this wasn’t another crazed Mennonite caught by Homeland Security.
Seems like a fair question. The only reason I can figure is that the way the some levees are constructed, flooding on the 'dry' side can weaken them by an unknown amount, requiring that they be dismantled and rebuilt. Arguably, dynamiting them and letting the water carry them away would reduce the amount of work required to dismantle them. Plausible theory, except that dismantling levies and keeping the material on hand to rebuild would seem easier than rebuilding the levies with material from elsewhere.
Even if the sole loss caused by the flooding of farmland is the inability to plant and harvest one year's crop, that would be millions of dollars. How much cash money would one have to offer every inhabitant of Cairo for them to decide moving wasn't so bad?