Proving infiltration is not feasible in the drainage hierarchy
Moving down the surface water hierarchy takes site evidence, not a geology map. What to test, what to record and how to set out the justification a flood authority accepts.
A lead local flood authority will not accept "the site is clay" as a reason to skip infiltration. The drainage hierarchy puts discharge to ground first, and moving down it requires evidence that infiltration was tested and found unworkable on this site, at this depth, in these ground conditions. Get that evidence in the right order and the drainage hierarchy infiltration argument takes two pages. Get it wrong and you collect a holding objection that sits on the file for weeks.
Where the hierarchy comes from
Two sources matter. Planning Practice Guidance on flood risk and coastal change sets the preference order for surface water disposal: into the ground, then to a surface water body, then to a surface water sewer or highway drain, and only then to a combined sewer. Requirement H3 of the Building Regulations sets a similar order for rainwater drainage, and the Defra non-statutory technical standards for sustainable drainage systems sit alongside both.
The order is not a formality. Each step down increases the volume the public network or a watercourse has to carry, so the authority checking your Drainage Strategy is entitled to ask what happened to the step above. Note that the hierarchy applies to the whole site, not to the scheme as a single unit: partial infiltration for the car park and attenuated discharge for the roofs is often the right answer and is rarely offered.
Testing before ruling infiltration out
The test everyone recognises is the soakage trial pit described in BRE Digest 365. You excavate a pit at the proposed soakaway location and depth, fill it, and record the time for the water level to fall from 75 per cent to 25 per cent of the effective storage depth. Three consecutive fills per pit, with the slowest result used for design, and at least one pit per drainage catchment on the site.
Three points decide whether the results survive scrutiny.
- Test at the depth the soakaway will sit at, not at 1 m because the digger was available. Permeability changes sharply through made ground into natural strata.
- Record groundwater. The base of an infiltration device needs clearance above the highest expected groundwater level, so a single dry-season dip tells you very little. Winter monitoring is the honest answer where the margin looks tight.
- Report the infiltration rate in metres per second with the raw drain-down readings attached. A rate quoted without the readings invites a request for the readings.
Where results straddle the viability line, the useful move is a sensitivity run: model the attenuation volume at the measured rate and at half of it, and show what changes.
Ground conditions that defeat a soakaway
Low permeability is the obvious one, and firm clays routinely test at rates that cannot drain a device within the required period. But several other constraints rule out infiltration even where the soil drains well.
| Constraint | Why it blocks infiltration | Evidence to supply |
|---|---|---|
| High groundwater | No unsaturated zone beneath the device | Monitoring dips across a wet period |
| Made ground or contamination | Creates a pathway to the aquifer | Phase 1 desk study and, where needed, intrusive results |
| Source Protection Zone 1 | Environment Agency objects to infiltration of anything but clean roof water | Position statement reference and zone plan |
| Dissolution features | Soluble chalk, gypsum or limestone risks collapse | Geotechnical appraisal and BGS records |
| Shallow bedrock or steep ground | Insufficient depth, or lateral seepage to neighbouring land | Trial pit logs and levels |
| Proximity to foundations | Saturation of bearing strata and cellars nearby | Layout showing the separation achieved |
Contamination is the constraint most often missed on brownfield land. Where a pathway to a receptor exists, infiltration of anything other than clean roof water is usually unacceptable, and the Environment Agency's groundwater position statements set out the reasoning. That reasoning is also why the drainage engineer and the land contamination consultant should be talking to each other in the first month rather than the last.
Discharging to a watercourse
The second rung needs three things: a physical outfall, a legal right and a consent. Riparian ownership gives you the right to discharge at a natural rate from your own land, and nothing more. If the outfall crosses third-party land you need an easement, and "we assumed an easement would be granted" is the single most common reason a drainage strategy unravels after permission.
Works in, over or under an ordinary watercourse need land drainage consent from the lead local flood authority under section 23 of the Land Drainage Act 1991. On a main river the route is different, as explained in our note on flood risk activity permits for works near a main river. Either way the discharge rate is normally restricted towards the greenfield equivalent, with the volume controlled as well as the peak.
The public sewer as the last option
Sewerage undertakers apply the same hierarchy you do, and they will ask what happened to infiltration and to the watercourse before they will discuss a surface water connection. A pre-development enquiry is the mechanism, and it takes weeks rather than days, so raise it early.
Two things surprise developers. A surface water connection is not a right in the way a foul connection under section 106 of the Water Industry Act 1991 is, so the undertaker can refuse or impose a rate. And discharge to a combined sewer, the bottom rung, usually attracts a requirement to reduce existing flows from the site, sometimes substantially, because you are adding to a network that spills at capacity.
Writing the justification an authority will accept
The document that works is short and sequential. For each rung of the hierarchy, state the option, the test applied, the result and the conclusion, then attach the evidence. What we hand to a lead local flood authority normally contains the trial pit locations and logs, the drain-down readings, groundwater monitoring, a contamination summary, the outfall ownership position, the undertaker's pre-development response and the modelling for the option chosen.
Our guide to what a lead local flood authority checks in a drainage strategy sets out the wider checklist, and the question of who maintains the system afterwards is covered in SuDS adoption and long-term maintenance. Both get asked at the same time as the hierarchy question.
What a weak justification looks like
Case officers and flood authority engineers see the same four failures. A statement that the site is underlain by clay, with a geological map extract and no site testing. Trial pits at 1.2 m for a device designed to sit at 2.5 m. A single infiltration rate with no drain-down data. And an outfall drawn across a neighbour's field with no comment on rights at all.
The consequence is rarely refusal. It is a request for further information, a holding objection, and a determination period that slides past its 13 weeks while the committee cycle moves on. On a site where the layout depends on where the attenuation basin goes, that delay can be worse than the cost of doing the ground investigation properly at the outset. Rules of thumb here are general guidance, not legal advice, and the position on any particular site should be checked with the lead local flood authority and a qualified drainage engineer.
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Frequently asked questions
What infiltration rate is too low for a soakaway?
There is no single national cut-off. The test is whether the device can drain its design volume within the period the guidance sets while keeping clearance above groundwater, so a rate that works for a small roof area may fail for a car park. Report the measured rate with the drain-down readings and let the design demonstrate the answer.
How many soakage tests does a flood authority expect?
At least one trial pit per drainage catchment, with three consecutive fills in each pit and the slowest result taken for design. Tests must be at the proposed device depth and location. On sites with variable made ground, more pits are usually needed, and a single pit covering a hectare will attract a request for further work.
Can I infiltrate surface water on contaminated land?
Rarely, and only where no pathway to a receptor exists. Infiltrating through made ground can mobilise contaminants towards groundwater, which is why the Environment Agency's groundwater position statements resist it. Clean roof water is sometimes accepted where the design keeps it away from affected soils, but this needs contamination evidence rather than assertion.
Do I need consent to discharge to a ditch?
Yes. A ditch is normally an ordinary watercourse, so works in, over or under it need land drainage consent from the lead local flood authority under section 23 of the Land Drainage Act 1991. You also need riparian rights or an easement if the outfall crosses land you do not own, and the discharge rate will usually be restricted.
Is a surface water sewer connection a right?
No. Foul drainage connection rights under section 106 of the Water Industry Act 1991 do not extend to surface water in the same way, so the undertaker can refuse or impose a restricted rate. Make a pre-development enquiry early, because the response often sets the attenuation volume the layout has to accommodate.
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.