The 100-year floodplain is land that is predicted to flood during a 100-year storm, which has a 1 percent chance of occurring in any given year according to the U.S. Geological Survey. A 100-year storm is defined by the USGS as rainfall measuring 15,000 cubic feet per second.
According to the conservation organization Friends of the River, floodplains are low, flat, periodically flooded lands near rivers, lakes and oceans. Statistically, a 100-year storm can occur more often than once every 100 years. Over the course of 1 million years, these events would be expected to occur 10,000 times. But within those 1 million years, the 10,000 occurrences can take place consecutively or sporadically depending on environmental conditions, development and other factors. Risk levels can also change over time as these factors change and as more data is collected to build accurate statistical risk models.
The Federal Emergency Management Agency says the term "100-year flood" has been misleading since it was introduced by the National Flood Insurance Program in 1973. To help prevent confusion, FEMA uses the term "1-percent-annual chance" to reflect the 1 percent chance of such an event taking place in any given year. Most state and federal agencies use this term.