What’s a 100-year flood? Do they really occur only once in 100 years?

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What’s a 100-year flood? Do they really occur only once in 100 years?
ENLARGE

Diagram: DNR.

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.

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.