A flood occurs when water flows into a region faster than it can be stored in a lake or reservoir, absorbed into the soil, or removed by runoff into a drainage basin.
There are several conditions that can result in flooding: a long-lasting rainfall over a watershed, intense thunderstorms, or rainfall that causes rapid snow melt.
Because floods result under different circumstances and in different places, their impact varies. The term “100-year flood” allows us to place a particular flooding event in context with other floods.
It is wrong to think that a 100-year flood happens only once every 100 years. The phrase “100-year flood” describes the estimated probability of a flood event happening in any given year. A 100-year event has a 1 percent chance — or 1-in-a-100 chance — of occurring in any given year. While not likely, two 100-year flooding events can occur within a month of each other.
Scientists collect data on how frequently different sizes of floods occur and the time between these floods. They use this data to calculate the probability that a flood of a particular size will be equaled or exceeded during any year.
The term “100-year flood” is a statistical designation of an unlikely event. Statistically, a 100-year flood has approximately a 63 percent chance of occurring in any 100-year period, not a 100 percent chance of occurring.
Extreme and unlikely values are important for assessing the risk of unusual events. In the 1960s, the federal government decided to use the statistics of the 100-year event as the basis for the National Flood Insurance Program. These data continue to be used in determining flood insurance rates.