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.
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.
Climate Change 2021, The Physical Science Basis, published by the Intergovernmental Panel on Climate Change in 2021, defines runoff as: ‘The flow of water over the surface or through the subsurface, which typically originates from the part of liquid precipitation and/or snow/ice melt that does not evaporate, transpire or refreeze, and returns to water bodies.’
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