Using Infiltration to Discharge Surface Water

November 20, 2023

Using Infiltration to Discharge Surface Water


What is infiltration?

Infiltration is the process of surface water flowing through the soil to the underlying geology.

Why is infiltration important?

Infiltration discharge of surface water is important in surface water drainage systems because it ensures no water enters sewers or watercourses, eliminating the proposition of watercourses or sewers becoming overwhelmed and causing downstream flooding. Infiltration discharge of surface water on development sites, mimics the way the natural environment disposes of surface water.

Infiltration discharge of surface water is top of the surface water drainage disposal hierarchy generally meaning it must be used on new developments unless its use can be satisfactory ruled out (typically by undertaking intrusive infiltration testing onsite)

How do we achieve infiltration in drainage systems?

Maximising the use of SuDS and green areas within new developments allows infiltration discharge to be maximised on new developments. Typical SuDS features encompassed within new development and which can support infiltration are shown below:

  • Geo-cellular storage tanks
  • Swales / Trenches
  • Basins
  • Bioretention systems/ Raingardens
  • Permeable Paving

 Barriers to the use of Infiltration

  • Poor infiltration rate

Poor infiltration rates of underlying strata is typically the largest barrier to the use of infiltration discharge. It is estimated around 60% of UK soil does not support infiltration.

The general type of underlying strata and its typical infiltration characteristics are provided below:

    • Sandy and gravel soils will typically have the highest infiltration capacity.
    • Cohesive soils such as clay will generally provide poor or no infiltration capacity.
    • Chalk generally provides variable infiltration capacity. Generally, infiltration is feasible in boulder / structureless chalk (it has been weathered to a more granular composition) and not so when it is more cohesive.
    • In rocky conditions, capacity will depend on the degree of discontinuity and the presence of pockets of granular material.

On many sites, the engineer can make a relatively accurate prediction on the chance of infiltration disposal being viable on a given site via review of British Geology Survey desktop mapping / historical borehole logs

Many local authorities within the UK however insist on intrusive infiltration testing (typically infiltration testing undertaken to BRE 365 specification) being carried out regardless of the likelihood of the strata not being suitable for infiltration potential based on desktop review.

Generally, the point at which the ground is considered too impermeable to utilise infiltration discharge is 1 x 10-6m/s. This is because it is around this value that obtaining half drain down times within 24 hours becomes increasingly difficult.

Allowing SuDS structures to drain down by half in 24 hours is a best practice requirement to prevent back-to-back storm events compromising the operation of the drainage system.

As of the SuDS Manual guidance, greater than 24-hour half drains down times are sometimes considered acceptable when designing for larger storm events (typically the 100 year plus 40% design event), however further measures are needed to be incorporated within their design. This is typically demonstrating a subsequent 1:10 year storm event can be accommodated within the storage within a 24-hour period without issue.

  • Downstream springs

On steep sites, infiltration discharge should sometimes be avoided as it may give rise to the presence of springs appearing at lower elevations causing flood risk.

  • High water table

It is a mandatory stipulation that the formation level of infiltration structures should be an absolute minimum of 1.00m from groundwater (ideally 1.20m). This is to prevent contamination to groundwater and ensure some level of saturation zone exists in which to adequately discharge surface water to the ground.

It should be noted that whilst trial pits and borehole logs can give a good indication of the likely groundwater level, they only give a single snapshot in time. Ground water levels fluctuate seasonally and therefore they are not conclusive.

Where groundwater elevation is within 1.5 – 2m below the formation level of infiltration structures, we would suggest groundwater monitoring is carried out over a minimum of the wettest 6 months (ideally 12-months) to verify high groundwater levels will not compromise infiltration discharge.

  • Risk of pollution

Where existing ground pollution exists, infiltration discharge may need to be ruled out as it can cause leaching of existing contaminants around and away from the development site and into groundwater.

Where it is believed existing contamination exists, we would suggest a contamination survey is undertaken by an appropriate consultant before the suitability of infiltration discharge can be established.

  •  Pollution mitigation

When designing any types of SuDS system (disposing of infiltration or otherwise), it is necessary to identify the pollution level of surface water runoff entering the SuDS and the likely pollution mitigation that the SuDS will provide. This is especially important when using infiltration discharge as the consequences on pollution to existing groundwater is amplified.

The most common assessment that can be made in this regard is the Simple Index Approach (SIA) as included in CIRIA 752, The SuDS Manual.

In summary, different catchment areas are assigned differing pollution hazard indexes depending on their use. Runoff from industrial roads for example is assigned a higher pollution categorisation than that from residential roofs.

It is then ensured as part of the design, the total pollutant removal capacity of the SuDS elements proposed is equal or higher than the assigned pollution index to ensure the quality of the infiltrating water.

Heavy polluting scenarios or more vulnerable soils might require a more detailed risk assessment and input from a specialist as well as proprietary treatment systems (such as il interceptors, washdown separators and the like).

  • Inadequate clearance to habitable buildings and road

Building Regulation guidance requires a minimum separation from soakaway structures to habitable buildings of 5m. This often means for densely developed or smaller sites, infiltration discharge is not possible.

Many local highway authorities also request a 5m clearance from soakaways causing a similar constraint.


Infiltration discharge of surface water is top of the surface water drainage disposal hierarchy generally meaning it must be used on new developments unless its use can be satisfactory ruled out.

SuDS features incorporated within new developments provide ample opportunity to utilise infiltration however there are many reasons why infiltration should not be used as identified within this article.

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