What’s a 100-year flood? Do they really occur only once in 100 years?
The name is misleading: a “100-year flood” does not happen only once every 100 years. The name describes the estimated probability of that particular flood happening in any given year. A 100-year event has a 1 percent chance of occurring in any given year. It’s not likely, but two 100-year floods can occur just a month apart!
Scientists collect data on when and how often different sizes of floods occur, then use this data to calculate the probability that a particular size of flood will occur (or be exceeded) during any year.
Statistically, a 100-year flood has approximately a 63 percent chance of occurring in any 100-year period, not 100 percent!
Climate can be defined as the average state of the atmosphere for a given place over a specified period. Extreme events, such as 100-year floods, are part of a region’s climate. These statistics are important for assessing the risk of unusual events and more specifically for determining flood insurance risks and premiums.