The Need for Interception
Interception storage should be provided for the first 5mm of rainfall as much as possible in order to closely mimic greenfield scenarios.
Interception is the capture and retention for the first 5mm of the rainfall resulting in no runoff. Interception and evaporation can account for 15-50% of yearly precipitation. Interception should be utilised to closely reflect the greenfield runoff behaviour, and to decrease the risk of pollution downstream further. Where possible infiltration or other techniques, such as rainwater reuse, should be used to try to achieve no runoff to either surface water sewers or watercourses for the first 5mm of rainfall.
Interception is however not guaranteed due to the variability of soil moisture and in evapotranspiration, therefore as stated in Chapter 24.8 of the CIRIA SuDS Manual C753 compliance for 80% of rainfall events should be required during summer months and 50% compliance during winter months. The CIRIA SuDS Manual C753 box 24.3 highlights some simple approaches which can be undertaken for interception delivery and compliance assessment, whilst table 24.6 shows some mechanisms for interception.
Page updated: 17/02/2020