Interception storage and the first few millimetres of rain

Interception storage in a SuDS design is about the small rain, not the storm. On a greenfield field, the first few millimetres of a rainfall event never reach the ditch at all. They wet the vegetation, fill the hollows, soak into the top few centimetres of soil and evaporate. Cover the same ground with a roof and a driveway and every one of those millimetres runs straight off, carrying with it the oil, the brake dust and the sediment that has accumulated since the last shower.

Interception is the design requirement that puts that behaviour back. The usual criterion, taken from the industry SuDS guidance most lead local flood authorities follow, is that there should be no runoff from the site for the first 5 millimetres of rainfall in a summer event.

What interception seeks to achieve

Two things, and they are easy to separate.

The first is hydrological. Small events are frequent events, and a site that discharges every time it rains alters the flow regime of the receiving watercourse far more than the occasional big storm does. Restoring the initial loss restores something close to the natural pattern.

The second is water quality, and it matters more. Most of the pollutant load washed off a hard surface comes off in the first few minutes of rainfall, in what engineers call the first flush. Retaining and treating that volume near the surface removes the bulk of the contamination before it gets anywhere near a pipe.

So interception is not a small version of attenuation. Attenuation holds back the peak of a large event and releases it slowly. Interception stops a small event leaving at all.

The small rainfall depths that matter

Five millimetres over a roof of 80 square metres is 400 litres. Over a 300 square metre plot with driveway and patio, it is a cubic metre and a half. Those are volumes that ordinary landscape features handle without any engineering.

That is the encouraging arithmetic behind the requirement. On a single dwelling, interception is usually satisfied by things you would do anyway: a permeable driveway, a soakaway taking roof water, a rain garden at the corner of the terrace.

It becomes harder where the ground will not accept water, where the whole plot is hard surfaced, or where the design drains everything to a single piped outfall because that is what the contractor is used to.

Features that deliver it on a small site

The reliable performers are the ones that put water into the ground or into a soil-filled feature.

1. Permeable paving with a sub-base reservoir, which intercepts the rain that falls on it directly.
2. Soakaways and infiltration trenches taking roof water, sized to the drainage guidance and tested by infiltration measurement rather than assumption.
3. Rain gardens and filter strips receiving downpipe or surface flow, with a shallow ponded depth and a free-draining soil profile.
4. Swales along a boundary, which intercept along their length as well as conveying.
5. Green roofs, which retain a meaningful depth of small rainfall in the growing medium.
6. Tree pits with structural soil, which work well on constrained frontages.

Water butts are the one people reach for first and the one that counts for least. A butt only intercepts if it is empty when the rain arrives, and a full butt intercepts nothing. Councils will rarely accept butts alone as the interception measure.

Note the ordering of the drainage hierarchy. Infiltration first, then discharge to a watercourse, then to a surface water sewer, and only then to a combined sewer. Interception features tend to sit at the top of that hierarchy anyway.

Demonstrating it in the calculations

How much proof is needed scales with the size of the scheme.

On a single dwelling or a small infill, a drawing showing where each surface drains, with a note of the feature receiving it and its retention depth, is normally enough. Say plainly that all impermeable areas drain to a feature capable of retaining the first 5 millimetres, and show it.

On a larger scheme, the demonstration is done in the hydraulic model, using an initial loss or depression storage value consistent with the features proposed, and reported as part of the Drainage Strategy. Reviewers check that the model's assumed losses match the features actually drawn, which is where inconsistencies show up.

Infiltration rates need testing, not assuming. A soil infiltration test to the recognised method, carried out at the depth and location of the proposed feature, is a small cost that prevents a redesign. Test in winter conditions if you can, because summer results flatter.

Water quality benefit and treatment stages

Interception is the first stage of what the guidance calls a treatment train, where water passes through a sequence of features, each doing part of the job.

The approach used in the industry guidance scores the pollution hazard of the surface being drained against the mitigation the proposed features provide. A domestic roof is a low hazard. A driveway where cars are parked and washed is higher. A small commercial yard higher again.

For a self-build, that usually means roof water can go more or less straight to a soakaway, while driveway and hardstanding water is better routed through permeable paving or a filter strip first. Where the site drains towards a watercourse, being able to say the first flush is intercepted and filtered removes most of the objection an environmental officer might raise. Our note on foul drainage where there is no mains sewer covers the separate question of foul water, which must never be combined with these features.

Where it is waived and why

Interception is not always achievable, and a strategy that pretends otherwise is worse than one that explains the constraint.

The recognised reasons for a departure are ground conditions that will not accept infiltration, groundwater close to the surface, known ground contamination where infiltration could mobilise it, unstable ground or a steep slope, and the presence of a basement or other structure that cannot tolerate water near it. Where contamination is a possibility, investigation by a suitably qualified and experienced person is required before any infiltration is designed, and that is work where professional input is legally expected rather than optional.

The guidance also acknowledges that the criterion is framed around summer conditions. Winter interception is harder because soils are wetter and evapotranspiration is negligible, so demonstrating it for summer events is the normal expectation.

Where interception cannot be achieved, say which of those reasons applies, evidence it, and compensate with additional treatment stages further down the train. Residual surface water risk then belongs in the Flood Risk Assessment, and our note on reading the surface water flood risk map without overstating it covers how to present that honestly.

What to include in the submission

Keep it short and specific. A drainage submission that covers interception properly shows:

The impermeable area schedule, surface by surface, with the feature receiving each one. The retention depth or volume each feature provides, and the calculation linking it to 5 millimetres over the contributing area. The infiltration test results with their date, location and depth. A cross section through each feature type. And the maintenance regime, with intervals and responsibility.

That last item is the one councils chase. Permeable paving that is never swept clogs, and a rain garden that is turfed over by a later owner stops working. Put the regime in the strategy and, where there is a management company, in its documents.

Illustrative example: a single self-build on a 420 square metre plot with 95 square metres of roof, a 60 square metre permeable drive and a 25 square metre patio. Roof water goes to a soakaway sized on a measured infiltration rate. The drive intercepts directly into its sub-base. The patio falls to a rain garden holding 150 millimetres of ponded depth over 4 square metres. The whole interception case is one drawing, one table and a paragraph. The figures here are illustrative and any real design needs site-specific calculation.

This is general guidance, not legal advice; check the position with your local planning authority or lead local flood authority before relying on it. For specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.

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