Question

If a flood is called a &#x27;50-year flood,&#x27; does that mean it will only happen once every 50 years?

Accepted Answer

No, a &#x27;50-year flood&#x27; does not mean it will only happen once every 50 years. This term refers to the flood&#x27;s recurrence interval, which is a statistical measure of probability, not a fixed schedule or a guarantee of timing. A 50-year flood is a flood event that has a 1 in 50 chance, or a 2% Annual Exceedance Probability (AEP), of occurring in any given year. The Annual Exceedance Probability (AEP) is the likelihood that a flood of a specific magnitude, or greater, will occur in a particular year. The &#x27;magnitude&#x27; of a flood describes its size or intensity, often measured by the peak discharge, which is the maximum volume of water flowing past a point per unit time. This probability remains constant and independent for every single year, regardless of when the last flood of that magnitude occurred. For example, similar to rolling a six-sided die, there is always a 1 in 6 chance of rolling a specific number on each roll, even if that number was just rolled. Therefore, a 50-year flood could theoretically happen two years in a row, or multiple times within a much shorter period than 50 years. Conversely, it could also not happen for 100 years or even longer. The &#x27;50-year&#x27; designation means that, over a very long period, a flood of that particular magnitude is expected to be equaled or exceeded on average once every 50 years.

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If a flood is called a '50-year flood,' does that mean it will only happen once every 50 years?