gee, M.Ted, I hadn't intended to simply trade clichés, but "You don't know what's possible until you try" calls for "Throwing good money after bad" or possibly even "spitting in the eye of a hurricane" ;>)

But I think the best one might be "calculated risk", with 95% of the emphasis on the "calculated". It seems to me that many calculations were ignored around the Mississippi delta in the last 40 years. (I see in this morning's paper that, quite apart from whether they can 'rebuild', there is doubt that the water supply can be made safe for several years). There are a lot more factors than good levees in the decision about the future of the city.

(and it's possible I failed to convey that the picture I linked to was of the plaque on City Hall showing the height of the 1977 flood...8'6". *THAT* flood was not from a failed dam...and the 2nd picture was the partially cement ditch which was inadequate.)