I was going to do a bigger post on this, but I've sort of let it slide, but I didn't want to completely forget about it.
From here: Managing Risk to Humans and to Floodplain Resources (PDF). The chart shows chance of a levee breach on the vertical axis and number of fatalities on the horizontal axis.
When this chart was put up at the Tulane Engineering Forum, the audience took a second to process it (note the log-log scale), but when they did, there was a gasp. It's not every day you see a graph of fatalities on a logarithmic scale. Each line is 10 times worse than the line preceding it. You don't want to see the New Orleans levees in the upper right quadrant of this chart.
The big point I was going to make in the long post I never wrote was we need MUCH more than 100-year level levees.
Related: Residual Risk / YRHT. Very good quote from Dr. Bea.
Risk-based design of flood defense systems (PDF).
Remember, hurricane season is less than 3 weeks away.
UPDATE- Some minor edits.
UPDATE 2 - Presentations Online