'100 year' flood
This video addresses the term “100 year flood”, to help you understand what that actually means.
The term can '100-year flood' can be confusing. It could be mistaken to mean a flood height that occurs every 100 years. It in fact refers to a flood height that has a long-term likelihood of occurring once in every 100 years. Another way of describing this is a flood height that has a long-term average that has a 1% chance of occurring in any year. This is calculated based on historical data about rainfall and previous flooding.
It’s possible to have two ‘1-in-100-year floods’ in 100 years, or even in the same year, because these rates are probabilities not certainties. Climate change is increasing the frequency and intensity of floods in some places, so a 1-in-100-year flood height might become more common and change to, for example, a 1-in-50-year flood height.
