Return Period

This describes the likelihood of the design storm Rainfall Rate being exceeded. As it is impossible to design a drainage system to cover all possible storm events, the designer must decide the frequency at which the design Rainfall Rate will be exceeded and the drainage system overloaded. A Return Period of 5 years accepts that the Rainfall Rate will be exceeded on average once every 5 years.

Typical values are given below;

Typical Return Periods
Simple Methods / No Surcharging	Years
Rural Areas	1
Residential Areas	2
City Centres	5
Industrial/Commercial Areas	5
Underground Railway	10
Underpasses	10

Complex Methods / Surcharging Allowed	Years
Rural Areas	10
Residential Areas	20
City Centres	30
Industrial/Commercial Areas	30
Underground Railway	50
Underpasses	50

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