The concept of infiltration is aimed at trying to prevent any runoff taking place from sites when there are small rainfall events. The aim is to minimise the discharge of polluted runoff from entering streams and rivers, particularly in summer periods when they have low flows and the water is warm; conditions which are already stressful for flora and fauna. The emphasis is on achieving no runoff for small rainfall events which are less than 5mm. A small study was carried out by HR Wallingford on Interception (Supermarket representation for SuDS Guidance, Interception storage analysis (2014) using time series rainfall analysis to show how SuDS with little or no infiltration capability could deliver Interception. The three key points to note are:
- Evapotranspiration in summer is a key mechanism for preventing runoff using soil storage;
- Very low rates of infiltration can be effective in significantly enhancing prevention of runoff;
- An Interception criterion of 5mm of rainfall does not mean that rainfall from every event up to 5mm in depth will be prevented from runoff as antecedent conditions for some events will be particularly wet. A probabilistic approach is therefore needed for defining and assessing compliance.
Detailed time series runoff analysis need not be carried out if this is considered to be too complex, and simple rules of thumb can be used. These are based on showing that losses based on using evapotranspiration and marginal infiltration are greater than average daily rainfall (annual rainfall / 365) using summer conditions. Guidance on this is provided in the SuDS Manual.
Rainwater harvesting is another mechanism by which Interception can be delivered. See “How can Rainwater harvesting be used for surface water management?”
Infiltration should be used wherever possible, subject to protection of groundwater against polluted runoff. As all runoff is to some degree contaminated, protection is provided by ensuring that some treatment is provided by SuDS systems and that the base of all infiltration units should be at least 1.0m above the highest expected groundwater level. This is often difficult to establish, so groundwater levels should be measured through at least one winter period and a judgement made as to how much higher it might get in a really wet winter.
Where there are important aquifers, or the runoff is particularly contaminated, there may be a need to prevent the use of infiltration, even where the infiltration rate is low. The SuDS Manual and other guidance should be followed where this situation applies.