I also did a bit of research and thought others might be interested in what I found.
Here's a general view of what a 100-year flood means.
http://ga.water.usgs.gov/edu/100yearflood.html
This is an interesting, recent report on flooding in NH that includes data from the May 2006 storms that I mentioned earlier. One of the more interesting extracts,
"in mid-May 2006, 8 to12 inches of continuous heavy rain over a three-day period caused the worst flooding in New Hampshire since the hurricane of 1938 (NOAA National Climatic Data Center). USGS reported the highest ever flows recorded on 12 rivers in central and southern New Hampshire; six were higher than the predicted 100-year flood. A reported 600 roads were closed, hundreds of families had to evacuate, and dozens of homes were lost."
The full report here, http://des.nh.gov/organization/divis...t_chpt_2.7.pdf
Finally, another recent and detailed report on flood plains in NH.
http://pubs.usgs.gov/sir/2008/5206/pdf/sir2008-5206.pdf
My argument to the bank would be if it didn't flood in 2006, it won't now!