Best Practice Guidance for Infiltration
Best Practice Guidance for Infiltration (Percolation) Testing in Accordance with an Approved Methodology
1. Why should testing be done?
Infiltration testing is required to establish whether infiltration is a viable method of surface water management and to inform the design of soakaways, infiltration basins, trenches and other infiltration-based sustainable drainage features. In line with the drainage hierarchy set out within the National Planning Policy Framework, Planning Practice Guidance, the National Standards for SuDS and the North Lincolnshire Council SuDS Guidance, infiltration must be fully explored before alternative methods of discharge are considered. As such, infiltration testing should be undertaken for all sites to determine the viability of this method of discharge.
2. When should testing be done?
Testing must be undertaken at any planning application stage to ascertain the viability and is fundamental to the deliverability of a development and not assessed at the discharge of conditions stage as viability should have been proven before that point. Testing is required regardless of site size, land use or perceived ground conditions, as desk-based information alone is not sufficient to demonstrate infiltration performance and pocket geology within sites can and does occur.
3. Things to consider
Whilst infiltration may be successful there are times when this simply will not be an appropriate method of discharge, some of these are covered below, but if you believe this is the case then robust reasoning should be presented with evidence to support the claim. A point of note is that in almost all cases where a development is more than a single dwelling, the size of the site will likely not be an appropriate reason to discard this method of discharge.
3.1 Minimum Rates
Infiltration is not suitable for all sites and if the minimum calculated rate is below the percolation value of 1 x 10-6m/s then this would be considered unsuccessful. This however does not immediately mean infiltration cannot be used, only that the rate is too low to be used as a sole mechanism for the discharge of surface water.
3.2 Pollution
If a site is shown to be at risk of pollution such as having previous industrial uses, located in sensitive areas or is a development which poses environmental risk, infiltration methods may not be suitable, this will ned o be assessed through geoenvironmental reporting.
3.3 Groundwater
If the site has a history of groundwater flooding or is evidence to have a high groundwater table when the trial pits are dug, then this will not be an appropriate method of discharge and the hierarchy should follow the next logical step (watercourse).
4. How should testing be done?
Trial pits must be excavated to at least 1m below proposed base level of the infiltration device. Each pit should be of sufficient size to allow accurate observation and measurement and must be stable and safe throughout the testing process. The dimensions of each pit, including length, width and depth, must be recorded. A description of the soil profile and strata encountered should be provided, together with observations of any groundwater, seepage or perched water. Photographic evidence must be supplied showing the pit empty, the pit filled with water, and the pit during drawdown.
Testing should follow and approved methodology. The method used for the purposes of this document will be set out in line with BRE Digest 365. Following excavation, the pit should be pre-soaked to saturate the surrounding ground and remove the effects of soil disturbance. The pit should then be filled with water to a known depth and the drawdown of water measured over time. The time taken for the water level to fall by 75%, 50% and 25% of the initial water depth must be recorded, together with the time taken for the water level to drain specifically from 75% to 50%. All drawdown times must be recorded in minutes.
5. How should testing be calculated
The infiltration rate must be calculated using and approved methodology such as the BRE Digest 365 approach and reported in metres per second (m/s) as shown below. The standard calculation is based on the volume of water infiltrated divided by the product of the infiltration area and the time taken for infiltration. The infiltration area should include the base area and sidewall area of the pit unless a clear justification is provided otherwise. Where multiple drawdown stages are recorded, infiltration rates should be calculated for each stage and the lowest rate adopted for design purposes.
5.1 Infiltration Rate Calculation (BRE Digest 365)
Where:
- f= infiltration rate (m/s)
- V_(p75-25)= volume of water discharged from the trial pit between 75% and 25% of the effective storage depth (m³)
- A_p50= internal wetted surface area of the trial pit at 50% of the effective storage depth, including the base area (m²)
- t_(p75-25)= time taken for the water level to fall from 75% to 25% of the effective storage depth (s)
The volume V_(p75-25)is calculated from the plan area of the trial pit multiplied by the change in water depth between the 75% and 25% levels.
The wetted area A_p50is calculated using the internal dimensions of the trial pit to the midpoint depth (50%), including both sidewalls and the base.
All time measurements must be converted to seconds prior to calculation. The resulting infiltration rate must be reported in metres per second (m/s) and should be the worst-case rate derived from repeat tests at each location.
5.2 What Should be Included Within Results
Groundwater conditions must be clearly identified through testing. This includes confirming whether groundwater is present, the depth at which it is encountered, and whether any perched water tables exist. Testing during winter months is encouraged, as groundwater levels are typically closer to their seasonal maximum and provide a more conservative assessment. Infiltration devices must be designed with appropriate separation distances above the maximum recorded groundwater level and with regard to any low-permeability strata identified during testing.
The infiltration testing report submitted to the Local Planning Authority must include a site location plan showing test location, full trial pit dimensions, soil descriptions, recorded drawdown times, calculated infiltration rates expressed in m/s, photographic evidence, groundwater observations and confirmation of any factor of safety applied (typically 2). Assumed, derived or back-calculated infiltration rates are not acceptable and will not be supported.
The results of infiltration testing must be used directly within the drainage design and hydraulic calculations to demonstrate that proposed infiltration features have adequate capacity, drain within acceptable timescales and do not result in surface water flooding or groundwater emergence. Where infiltration is shown to be unviable, this must be clearly evidenced through testing and alternative drainage solutions considered in accordance with the drainage hierarchy.
Failure to provide infiltration testing in accordance with BRE Digest 365 or another approved methodology, failure to report results in m/s, omission of photographic evidence, or reliance on borehole logs or assumed values will result in the drainage strategy being considered inadequate.
Infiltration testing undertaken in accordance with this guidance is essential to demonstrate compliance with national and local policy, ensure the long-term performance of surface water drainage systems and protect people, property and the environment from flood risk.
Once testing has been completed to the above standards, it is a requirement for infiltration sizing to be completed using an appropriate hydraulic modelling software taking into account a range of storm intensities and return periods, more details of these parameters can be found within NLC SuDS Guidance document.
Version Control
| Version | Date | Details | Revised by | Checked and approved by |
|---|---|---|---|---|
| V.1.0 | 21 May 2025 | Draft | Mike Smith, BSc (Hons), GradCIWEM, AMIEnvSc | Mike Kitching, MCIWEM, C.WEM |
| V.2.0 | 28 July 2025 | Draft | Mike Smith, BSc (Hons), GradCIWEM, AMIEnvSc | Mike Kitching, MCIWEM, C.WEM |
| V.3.0 | 10 June 2026 | Issued | Mike Smith, BSc (Hons), GradCIWEM, AMIEnvSc | Mike Kitching, MCIWEM, C.WEM |