What is a 100-year flood?

A ‘1 in 100-year flood’ refers to a flood or inundation height that has a long term likelihood of occurring once every 100 years or has a 1% chance of happening in any given year.  It is also known as the 100 year recurrence interval.

However, this does not mean a flood or inundation of this type and magnitude will only occur once every hundred year as 100-year floods can happen two years in a row. 

With changes in the intensity and frequency of severe weather events and the observed trend towards more intense tropical storms and cyclones the chances of 1 in 100-year flood events occurring is increasing.

In terms of coastal adaptation planning, where the probability of an event occurring is higher than 50% within the lifespan of the asset, you should consider actions to prepare for such an event. 

More information on the impacts floods can have here

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