A floodplain is a geographic area subject to flooding and/or land adjacent to a waterway necessary to contain a flood. Floodplains can be associated with rivers, lakes, streams, channels, or even small creeks that are normally dry most of the year. There are two types of floodplains we generally deal with:
1. 25 year floodplain: an area subject to flooding as a result of the occurrence of a 25 year storm event - a storm that has a 4% chance of occurring in any given year.
2. 100 year floodplain: an area subject to flooding as a result of the occurrence of a 100 year storm event - a storm that has a 1% chance of occurring in any given year.
A 100-year flood is not a flood that occurs every hundred years. In fact, the 100-year flood has a 26 percent chance of occurring during a 30-year period, the length of many mortgages. The 100-year flood is a regulatory standard used by Federal agencies and most states, to administer floodplain management programs. The 100-year flood is used by the National Flood Insurance Program (NFIP) as the basis for insurance requirements nationwide.
See also the Federal Emergency Management Agency (FEMA.gov) web site for more information on floodplains and floodplain management.