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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