100-year flood

A one-hundred year flood is calculated to be the maximum level of flood water expected to occur in a 100-year period. Sometimes, 500-year or 10,000-year floods are calculated (especially in low lying countries, such as the Netherlands). The 100-year flood is sometimes referred to as the 1% flood, since there is a 1% chance of it occurring in any year. Based on the expected water level, an expected area of inundation may be mapped out according to elevation above sea level. This area figures very importantly in building permits, environmental regulations, and flood insurance.

10-year floods have a 10% chance of occurring in any given year; 50-year flood, 2% chance; 100-year, 1%; 500-year, 0.2%; and 1000-year, 0.1%. The percent chance of an x-year flood can be calculated by dividing 100 by x.

The mathematical field of extreme value theory was created to model rare events such as 100-year floods for the purposes of civil engineering.

100-year floods caused by over a week of continuous heavy rains ravaged Europe in 2002.