Last edited 02 Feb 2021

100-year event


A 100-year event (or 100-year return period event) is an event that occurs (or is exceeded) on average once in every one hundred years (such as a storm, flood or rainfall event). This can also be expressed as 1 in 100 or 1:100.

For example, a 100-year storm is a storm that is likely to occur once every hundred years.

A ‘return period’ is an estimate of the likelihood of a particular event occurring. The return period of a rainfall event is the average time between events of at least a given magnitude.

Events can also be described by their ‘probability of occurrence’ (or frequency of occurrence) which is expressed as a percentage, or as 1 divided by the return period. The probability of occurrence of a 100-year event is 1%.

The ‘critical duration’ is the duration of the event for a specified return period event.

Ref CIRIA. 2015. C753. The SuDS Manual.

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