As Allen said above, you can't manage a system based on an extreme event that might happen once in a hundred years. They could have had 30' of space that year and we still would have flooded. We will never contain mother nature.
I would consider one flood in 65 years to be pretty good "flood control", it would have been so much worse without the dam. Its not "if" the river will flood again its "when", it might be next year, it might be 100 years. If you cant accept that uncertainty, then its time to move to higher ground.