Last edited 16 Feb 2021

Critical duration event

The SuDS Manual published by CIRIA in 2015 defines a critical duration event as:

‘The duration of rainfall event likely to cause the highest peak flows or levels at a particular location, for a specified return period event.’

Where a rainfall event is: ‘A single occurrence of rainfall before and after which there is a dry period that is sufficient to allow its effect on the drainage system to be defined.’

And a return period is: ‘An estimate of the likelihood of a particular event occurring. A 100-year storm refers to the storm that occurs on average once every hundred years. In other words, its annual probability of exceedance is 1% (1:100).’

A design event is: 'A synthetic rainfall event of a given duration and return period that has been derived by statistical analysis.'

Rainfall intensity is the: ‘amount of rainfall occurring in a unit of time, expressed in mm/hr.’

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