Infiltration is the natural process by which water on the ground surface enters the soil. In sustainable drainage design, infiltration-based techniques are considered the most desirable form of surface water management because they replicate the natural water cycle, recharge groundwater, and eliminate the need for a piped connection to a watercourse or public sewer.

Common infiltration SuDS features include:

Soakaways — excavated chambers or crate systems filled with aggregate or modular storage units
Infiltration basins — shallow depressions that allow ponded water to soak into the ground
Infiltration trenches — linear trenches filled with permeable aggregate
Permeable paving over permeable sub-base, where the pavement and sub-base act as a combined storage and infiltration system

The feasibility of infiltration depends on the site’s ground conditions. BRE Digest 365 testing (soakage testing) must be carried out to determine the soil’s infiltration rate. Key constraints include:

Soil permeability: Clay soils and made ground typically have insufficient permeability for infiltration
Groundwater levels: A minimum clearance (usually 1 metre) is required between the base of the infiltration device and the peak groundwater table
Contaminated land: Infiltration is generally not appropriate on contaminated sites, as it risks mobilising pollutants into groundwater
Proximity to buildings: Soakaways must be located at least 5 metres from building foundations to avoid structural damage

Where infiltration is viable, the LLFA and planning authority strongly prefer it over discharge to watercourses or sewers. Aegaea undertakes infiltration feasibility assessments as part of our drainage design service.