The term can '100-year flood' can be confusing.
It could be mistaken to mean a large flood that only occurs once every 100 years.
In fact, it is a flood where the likelyhood of its ccurrence is statistically 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.
