Attenuation and infiltration: two ways to hold water
Infiltration puts water into the ground, attenuation holds it back and releases it slowly. Ground investigation decides which, and the discharge hierarchy decides the order.
Infiltration puts rainwater into the ground. Attenuation holds it in a tank or a basin and lets it out slowly to a pipe or a watercourse. Every drainage strategy for a development site is built from one of these, or from both, and the choice is settled by ground investigation rather than by preference.
Get the order right and the rest of the design follows. Get it wrong and you find out at technical design, when the soakaway fails its test and there is no space left for a tank.
The discharge hierarchy in order
Surface water should be dealt with in the following order of preference, which appears in Planning Practice Guidance and in Approved Document H, and which every lead local flood authority applies.
- Into the ground, by infiltration.
- To a surface water body, meaning a watercourse, lake or pond.
- To a surface water sewer, a highway drain or another drainage system.
- To a combined sewer.
The hierarchy is not a preference list to be skimmed. Each step down has to be justified by showing why the step above is not reasonably practicable, with evidence. A drainage strategy that opens at option three with the sentence "infiltration is not viable on this site" and no test results will come back from the drainage officer.
Discharging to a combined sewer is the last resort, and sewerage undertakers increasingly refuse it outright. Where the site drains that way today, the redevelopment is normally expected to do better.
When infiltration works and when the ground says no
Infiltration needs three things to be true at once.
Permeable ground within reach of the excavation. Sands and gravels drain well, chalk usually does, and firm clay does not, which is why the same design fails on one side of a geological boundary and works on the other.
Enough unsaturated depth beneath the base of the soakaway. Where groundwater sits close to the surface, the soakaway has nowhere to discharge to, and the usual expectation is at least a metre of unsaturated ground between the base of the structure and the highest recorded groundwater level.
And ground that it is acceptable to put water into. That is the constraint people forget.
Two further siting rules bite on small plots. Soakaways should be kept a sensible distance from buildings, with five metres a common minimum in Approved Document H, to avoid saturating foundations. And a soakaway upslope of a neighbouring property can push water into their land, which is a civil problem as well as a planning one.
Infiltration testing and what the results tell you
The standard method for a soakaway is set out in BRE Digest 365. A trial pit of representative size is filled with water and the time taken for the level to fall from 75 per cent to 25 per cent of the effective storage depth is recorded, and the test is repeated three times so that the slowest result can be used.
Three practical points about the testing.
Test at the depth and in the position where the soakaway will actually go. A test in topsoil tells you about topsoil.
Test in the wettest reasonable conditions. A result taken in August on a clay site can look encouraging and be meaningless in February.
Use the third and slowest repeat. Successive fillings wet up the surrounding soil and the rate falls, which is the point of repeating the test.
The output is a soil infiltration rate, and it feeds directly into the storage volume calculation. Where the rate is very low, the design is not a soakaway with a bigger hole. It is attenuation.
A geotechnical investigation on the same visit is efficient, because the trial pits, the groundwater observations and the soil logs are useful for foundations as well as for drainage. A Geotechnical Assessment prepared to Eurocode 7 principles will normally cover both.
How attenuation storage is sized and controlled
Attenuation holds the difference between what falls on the site and what is allowed to leave it.
The permitted discharge rate is agreed first, usually the calculated greenfield rate for a greenfield site or a betterment figure on a previously developed one. Then the storage volume is calculated for a range of storm durations and return periods, up to the 1 in 100 year event with a climate change allowance, and the critical duration is the one that needs the most storage. That is often a long, moderate storm rather than a short intense one, which surprises people who size for the cloudburst.
The flow leaving the storage is limited by a flow control device, a vortex control or an orifice plate, sized to the permitted rate. Very low rates are a problem in themselves, because a small orifice blocks, and manufacturers set practical minimum sizes.
| Feature | Infiltration | Attenuation |
|---|---|---|
| Where water goes | Into the ground | To a sewer or watercourse at a controlled rate |
| Governing test | BRE Digest 365 infiltration rate | Agreed discharge rate and storm modelling |
| Main constraint | Permeability, groundwater level, contamination | Available volume and the rate the receiving system accepts |
| Typical forms | Soakaway, infiltration basin, permeable paving with open sub-base | Tank, geocellular crate, basin, oversized pipe, blue roof |
| Maintenance burden | Silt management at the inlet, long-term clogging | Flow control access, silt trap, inspection chambers |
Half-drain time matters for both. A system that has not emptied before the next storm provides no storage, and 24 hours is the usual expectation for a soakaway to half empty.
Groundwater and contamination constraints on soakaways
Infiltration through ground that carries contamination can mobilise it into groundwater, which is why the Environment Agency takes an interest in soakaways in Source Protection Zones and on any site with a history of industrial use.
Where a Phase 1 desk study identifies a potentially contaminative former use, infiltration should not be assumed until the ground has been characterised. On some sites the answer is that infiltration is acceptable in one part of the site and not in another. On others it is a straight no, and the strategy moves to attenuation.
Land contamination work is one of the areas where the local planning authority expects reports prepared in accordance with BS 10175 by suitably qualified and experienced persons, and a Contaminated Land Assessment draft is a starting point for that professional rather than a substitute for them.
Made ground is a separate problem. A soakaway in uncontrolled fill can wash out fines and cause settlement, whatever the infiltration test says.
What the drainage section should show either way
Whichever route the site takes, the Drainage Strategy should contain the same skeleton so an officer can check it quickly.
An existing conditions plan with impermeable areas measured. The discharge hierarchy worked through in order, with the evidence for rejecting each option. The infiltration test results, or a statement of why testing was not possible and what will be done at detailed design. The permitted discharge rate and how it was derived. The storage volume with the critical duration identified. A drainage layout with levels, pipe sizes and the flow control position. Exceedance routing for events beyond the design standard. And a maintenance schedule with the responsible party named.
An illustrative example of the arithmetic, not a target. A site with 900 m² of impermeable area discharging at a permitted 2 litres per second will accumulate storage during any storm where the inflow exceeds that rate. Run each duration from 15 minutes to 24 hours, find the one where the difference between inflow volume and outflow volume peaks, and that difference is the storage you build. The method is public and the modelling is routine, but the permitted rate has to be agreed first, so ask the lead local flood authority early.
PlanScope drafts a Drainage Strategy and a Flood Risk Assessment structured to UK planning policy, with a "to be confirmed" flag wherever infiltration testing or modelling outputs are missing. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
What is the SuDS discharge hierarchy?
Surface water should go into the ground first, then to a surface water body, then to a surface water sewer or highway drain, and only as a last resort to a combined sewer. Each step down needs evidence that the option above is not reasonably practicable. A strategy that skips to option three without infiltration test results will be sent back.
How is soakaway infiltration testing done?
BRE Digest 365 sets the standard method. A trial pit is filled with water and the time for the level to fall from 75 per cent to 25 per cent of the effective storage depth is recorded, repeated three times, with the slowest result used. Test at the depth and position of the proposed soakaway, and in wet rather than dry conditions.
How far should a soakaway be from a building?
Approved Document H gives five metres as a common minimum, to avoid saturating foundations. Position also matters relative to neighbours, because a soakaway upslope of an adjoining property can push water into their land, which creates a civil dispute alongside the planning issue. Check the local authority's own guidance as well.
When can infiltration not be used?
Where permeability is too low, where groundwater sits within about a metre of the base of the structure, or where putting water into the ground would mobilise contamination. Source Protection Zones and sites with an industrial history attract Environment Agency interest. Made ground is a separate problem because infiltration can wash out fines and cause settlement.
How is attenuation storage volume calculated?
Agree the permitted discharge rate first, then model storm durations and return periods up to the 1 in 100 year event with a climate change allowance. The critical duration is the one requiring the most storage, and it is often a long moderate storm rather than a short intense one. The stored volume is the difference between inflow and permitted outflow.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.