Why are the properties of the ground important for SuDS and infiltration?

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

Will the ground accept water?

Schematic cartoon of the subsurface beneath a soakaway system

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.


What is the potential for ground stability or water quality problems associated with soaking water into the ground?

An example of ground stability hazards formed as a result of water infiltration from a burst water pipe within an area susceptible to dissolution and karst formation.

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:

  • voids and collapse features created by the dissolution of soluble rocks and minerals
  • down-slope waterlogging leading to increased potential of landslides
  • changes in subsurface water content leading to ground compression and subsidence
  • groundwater quality deterioration, caused by infiltration of surface water containing contaminants that originate from the surfaces over which it flowed prior to infiltration. Whilst pre-treatment (e.g. by incorporating swales or filter trenches) is necessary prior to infiltration of surface water, care is necessary in those aquifers that are used for public water abstractions and in those that have limited capacity to filter contaminants.

Contact

Contact Dr Rachel Dearden or SuDs for further information.