“100-year flood” sounds like something that happens once a century. It isn’t. It’s a flood with a 1% chance of happening in any given year, and FEMA’s flood maps are built around it. The name describes an average over a very long time, not a schedule.
The math
If a flood has probability p in any year, the chance of seeing it at least once in n years is:
For a 30-year mortgage, that’s about 26%. Over longer horizons it climbs quickly.
It can happen twice
Each year is a fresh roll of the dice. A 1% flood this year doesn’t make one less likely next year. Communities sometimes see two “100-year” floods within a few years and assume something is wrong with the math. Usually the math is fine and the name is misleading.
What it means for you
- Inside the special flood hazard area, floodplain permits and elevation rules usually apply, and lenders require flood insurance on federally backed mortgages.
- Outside it, the risk is lower, not zero. The 0.2% annual chance flood (the “500-year”) still has about a 6% chance over 30 years.
- Maps are snapshots. Rainfall standards change, and NOAA Atlas 15 is on the way.
Start with a flood zone check, then talk to an engineer if the site is in or near the floodplain.
Draft written for Creel’s review before publishing. General information, not engineering advice for a specific site.