100-year Floodplain - Common term for a geographic area that has at least a 1% annual chance of flooding.

The term "100-year floodplain" can be misleading. It is commonly (but mistakenly) assumed to be an area that floods only once every 100 years or so. The "100-year floodplain" is more accurately defined as an area subject to flooding, where the ground elevation is below the Base Flood Elevation (BFE), which means the area is subject to a flooding event which has a frequency of 1% each year. Thus, a 100-year flood could potentially occur in the same place more than once in a relatively short period of time.

Despite the possibility of misinterpretation, the phrase "100-year floodplain" is still commonly used to describe the Special Flood Hazard Areas (SFHAs) found on Flood Insurance Rate Maps (FIRMs). In fact, SFHAs were once regularly defined by map legends on older FIRMs as an area subject to a "100-year flood". Today SFHAs are no longer commonly defined in FEMA map legends as an area of "100-year flood" but instead as an area subject to a "1% annual chance flood". This 1% standard is used by most Federal and State agencies and by the National Flood Insurance Program (NFIP). A structure in an area with a 1% annual chance of flooding has at least a 26 percent chance of experiencing a flood during the term of a typical 30-year mortgage.

On older FIRMs, SFHAs are darkly shaded; newer aerial FIRMS depict SFHAs with a blue dot-matrix. On both maps SFHAs are labeled with zone designations that begin with "A" or "V". For federally-regulated lenders, insurable structure(s) acting as collateral for a loan located in an SFHA on a current FIRM must have flood insurance as a condition for making, extending, increasing, or renewing the loan. This is referred to as the mandatory purchase of flood insurance requirement.