100-Year Flood

100-Year Flood Defined
"Does a 100-year flood occur once every 100-years?" is a question that we hear frequently when people discuss floodplains.

By definition, a 100-year flood, also known as the "base flood," is a flood that has a 1% chance of occurring in any given year. The area susceptible to flooding during the base flood is called the Special Flood Hazard Area, or floodplain. The elevation of the water for a flood event of a given probability is determined by engineers studying rainfall intensities, stream and river cross-sections, and topography.

Period of Time 10-Year Flood 25-Year Flood 50-Year Flood 100-Year Flood
1 Year 10% 4% 2% 1%
10 Years 65% 34% 18% 10%
20 Years 88% 56% 33% 18%
30 Years 96% 71% 45% 26%
50 Years 99% 87% 64% 39%

Statistic Chances

Studies suggests that during the 30-year life of a typical mortgage, a structure located within the 100-year floodplain has a 26% chance of experiencing flooding but 4% chance of experiencing a fire. This statistic is the reason why flood insurance coverage is mandatory for structures whose mortgage is backed by the federal government. Remember, your general homeowners’ insurance policy does not cover losses from floods. Therefore, even if you do not live in the Special Flood Hazard Area, you should consider purchasing flood insurance.

FEMA Resources