What is Interception?
When rainfall takes place on greenfield sites there is, for the majority of rainfall events during the year (which are small), no discernible surface water runoff to receiving water bodies. The rainwater normally evapotranspires, and in winter it can also result in groundwater recharge. However, impermeable surfaces generate runoff from virtually all rainfall events, and this can have a negative impact on the morphology and ecology of receiving water bodies. Interception is aimed at trying to replicate greenfield runoff conditions. In most urban locations in the UK more than 50% of all rainfall events are less than 5mm in depth, therefore preventing runoff from these events is very significant in terms of both hydrological and pollution impact reduction on receiving waterbodies.
The total annual pollution load from runoff is closely correlated to the total volume of runoff. Therefore prevention of runoff from the majority of all small rainfall events and also reducing runoff volume from larger events contributes very effectively to reducing the pollution load passed to receiving waterbodies. This is particularly important during the summer months when dilution flows in receiving watercourses is normally low.
The Interception criterion is rarely applied, and this is due to the twin effects of lack of awareness of its benefits in spite of the evidence from research, and the knowledge needed in being able to design for achieving it. In trying to meet the Interception criterion it is necessary to apply it in a probabilistic way. No runoff is not expected to be achieved during particularly wet periods when permeable surfaces and soils are saturated, so compliance requirements are set on a probabilistic basis (i.e. Interception should be delivered for a proportion of all events, either on a seasonal or an annual basis). A suggested target is that 80% compliance to no runoff from the first 5mm of a rainfall event should be achieved for summer rainfall events, but only 50% in winter.
A high level of Interception provided for some parts of the site is not to be considered as adequate compensation for a low degree of interception provision for other locations. Compliance is required for the whole site, or at least road paved areas, for it to be considered effective.
Interception mechanisms are based on runoff retention. This can be achieved using rainwater harvesting, or using soil storage and evaporation. Either infiltration or transpiration rates can dispose of the runoff from minor events to enable the next event to be captured. Infiltration rates of soils can be very low and still be effective at providing interception.
This criterion reinforces the importance of using vegetative and soil based SuDS components.
The use of continuous rainfall series with detailed simulation models can be used to demonstrate effective interception compliance of a site. However this can be avoided by the use of various design rules of thumb - see the SuDS Manual (2015).