Relative frequency of once-in-five-year precipitation events falling over a 2-day period, averaged over the continental U.S, 1901-2012. Green bars in recent decades show these events have become more frequent, when averaged across much of the country. SOURCE National Climate Assessment (U.S. Global Change Research Program, 2014)

The Thing about 100-Year Floods


A 100-Year Flood, of course, is an extreme flooding event that happens, on average, only once every 100 years. A hydrologist would describe it as a “recurrence interval” or talk about an “annual exceedance probability (AEP).” The recurrence interval can change. If the stream basin is altered, say by urbanization or wildfire, then less rain…