Sustainable Drainage Systems Design Guidance

This advice sets out the requirements that the LLFA has for drainage strategies and surface water management provisions associated with applications for development.

Infiltration

Infiltration structures (to ground) replicate natural greenfield conditions and benefit local groundwater recharge. They include soakaways, basins, swales and permeable paving.

Infiltration rates for soakage structures should be based on infiltration tests undertaken in the winter period at the location and depth of the proposed structures. The tests must be carried out in accordance with BRE Digest365, CIRIA R156 or a similar approved method.

If infiltration test results are not available to support a Full Planning Application, then full justification as to why these have not been provided should be included. Examples may include, issues with site access i.e. the site is a working brownfield site and as such it is not possible to dig trial pits. Such reasons will be reviewed on a site-by-site basis.

If infiltration is proposed and results are not available, the applicant should use a conservative infiltration rate such as 1 x 10-6 metres/second (or other appropriate rate with full justification) and provide an alternative drainage strategy should future test results show ground conditions are not suitable.

If infiltration is proposed, the drainage strategy should demonstrate:

  • that the base of the any infiltration feature is at least 1m above the peak groundwater level;
  • the half drain times (in hours) of the infiltration feature are at least within 24 hours from the end of the 1 in 30 (3.3%) including Climate Change rainfall event;
  • the factor of safety used in calculations. This should be at a minimum 2 or best practice figure;
  • the minimum distance of the infiltration feature from buildings is greater than 5m, unless specifically designed to be closer; and,
  • the measures proposed to prevent siltation of the infiltration device occurring.

Deep bore soakaways should only be considered after other forms of infiltration attenuation have been explored and in all cases, the applicant will be required to consult the Environment Agency (EA) hydrogeologist before their inclusion in a drainage strategy.

If infiltration has been discounted and infiltration tests are not available to disprove its suitability, intrusive ground investigations which conclusively show ground conditions are not suitable for infiltration are acceptable (e.g. due to clay, contamination, hazards etc).

Infiltration is not always appropriate, and the advice of the EA should be sought for developments in or close to source protection zones (SPZs), in areas with a known history of land contamination, or in areas with known high seasonal groundwater levels.