There's a BIG difference between living somewhere that is likely to be subject to disaster on a geologic time scale, and living where daily measures have to be taken to keep you safe!

The various flood plains are well-known. Some places flood every 100 years or so...others every 5-10. It ain't so hard to set rules about what should be built in those areas. If you are likely to have 500 years between disasters, go on, build higher. If you are dodging several close calls every year or two, re-think what goes in places like that.
   New Orleans...(and San Francisco) were rated as high-risk, and in the last couple years studies were done showing almost exactly what happened this week. Johnstown, PA is another place that simply WILL be hit again.

My 'hometown' of Wichita, Kans., sits right in tornado alley, but has had only one small twister touch in the city limits. It 'may' get a hit someday, but not 2-3 close calls every year, and does not present the recovery problems that New Orleans does....

I don't like terms like "cost/benefit analysis", but in these decisions, that's the kind of analysis that is needed. Seems to me like a billion dollars a year (if we can even do it for that)just for maintenance and repair, is a bit high.