SuDS that infiltrate to the ground rely on the subsurface to accept and store surface water. The properties of the ground are important in deciding where infiltration-based SuDS can be located.
Most importantly, the physical and hydrogeological properties determine how easily water will soak into the ground and whether or not it will have an effect on ground stability and water quality
For infiltration-based SuDS to drain effectively, the topsoil and the underlying geology need to be free-draining.
Sands and gravels for example, are generally more permeable than silts and clays. Superficial or bedrock deposits that are free-draining generally have higher porosities and hence more space to provide storage capacity.
Percolating water from SuDS schemes can cause a temporary rise in groundwater level, as pore space fills with water. The unsaturated zone (the zone above the groundwater table) must be thick enough to accommodate this groundwater level rise.
Release of limited quantities of water into the ground is generally an effective mechanism for surface water disposal.
Depending on the nature of the underlying geology, release of water into the ground may increase the susceptibility to ground stability hazards. It may also impact on water quality.
Ground stability hazards and water quality impacts include: