Last edited 29 Sep 2021

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BRE Group Researcher Website

Surface water run-off

Surface water run-off is: 'Water flow over the surface of roofs and ground surface to a drainage system. Ground surface run-off occurs if the ground is impermeable, is saturated or if the rainfall is particularly intense.'

Volume of run-off is: 'The volume of run-off that is generated by rainfall occurring on the site. This is typically measured in cubic metres. Additional predicted volume of run-off is the difference between the volumes of run-off; pre-development, greenfield and post development.'

Ref Home Quality Mark One, Technical Manual SD239, England, Scotland & Wales, published by BRE in 2018.

--BRE Group

NB Run-off coefficient is a measure of the amount of rainfall that is converted to run off.

A recurrence interval is: ‘The average time between runoff events that have a certain flow rate, for example, a flow of 2 m/s might have a recurrence interval of two years.’ Ref The SuDS Manual (C753) published by CIRIA in 2015.

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