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How is Interception depth calculated?

Interception storage is required in order to ensure that no run-off passes directly to the river for the majority of rainfall events with depths of 5mm or less. This is aimed at trying to replicate greenfield runoff response when no runoff is likely to take place for most small events. This type of storage is principally aimed at river water quality protection - polluted water is prevented from entering the water course for all small rainfall events. A 5mm rainfall threshold which, if effectively applied, will reduce the number of events with runoff into a receiving water body by around 50% and reduce total runoff volumes from the site by a significantly higher proportion.

A probabilistic approach to compliance is required (for instance 80% compliance to no runoff from the first 5mm of a rainfall event should be achieved during the summer and 50% in winter). The use of continuous rainfall series with detailed simulation models can be used to demonstrate effective interception compliance of a site. However where this is considered to be too difficult it can be avoided by the use of various design rules of thumb - see the SuDS Manual (2015).  




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