Interception losses and the first 5mm of rainfall in SuDS

Interception losses are the part of a drainage design that deals with small, frequent rainfall rather than the extreme event. The criterion most lead local flood authorities apply is that runoff from the first 5mm of rainfall should not reach the receiving watercourse or sewer at all. It is a separate test from attenuation, it is assessed differently, and a strategy that sizes a tank for the 1 in 100 year event has said nothing about it.

The reason it exists is straightforward. Before development, a field produces no runoff at all from a light shower: the water wets the vegetation, soaks in and evaporates. After development, a roof and a driveway discharge almost all of it within minutes. Those small events happen dozens of times a year, they carry the dirtiest first flush off the surface, and they change the flow regime of a small watercourse far more often than a design storm ever does.

What interception is for

Two outcomes, both about frequency.

The first is water quality. Sediment, hydrocarbons, tyre and brake wear and animal waste accumulate on hard surfaces between events and wash off in the opening minutes of rain. Preventing that discharge is the most effective single measure in a treatment train.

The second is the flow regime downstream. A stream that receives a small pulse of runoff every few days behaves differently from one that does not: bed scour, bank erosion and loss of the low-flow habitat follow. Peak attenuation does nothing for this, because attenuation still discharges the water, just more slowly.

The 5mm criterion and where it comes from

The 5mm figure comes from industry guidance rather than from statute. The SuDS Manual, published by CIRIA as C753, sets out interception as a design objective and the 5mm rainfall depth is the value that has become standard practice in England. Defra's non-statutory technical standards for sustainable drainage deal with peak flow rates and volumes, and do not set an interception standard, which is why the requirement usually arrives through a lead local flood authority's own local guidance instead.

That matters for two reasons. It means the wording of the requirement varies between authorities, so read the local guidance rather than assuming. And it means the criterion is a design objective to be demonstrated rather than a numerical standard to be met exactly, so a reasoned case about what is achievable carries weight.

The volume involved is small, which is why designers should be able to answer it cleanly.

Those figures are illustrative, calculated as area multiplied by depth, and they ignore the losses that actually deliver interception.

Components that deliver it

Interception is achieved by losing water, not by storing it. The mechanisms are infiltration into the ground, evaporation and transpiration, and retention in a medium that dries out between events.

• Permeable paving, where the water enters the sub-base and either infiltrates or is held in the voids and lost slowly.
• Green roofs, where the substrate holds a depth of water that evaporates.
• Rain gardens and bioretention areas, which combine ponding, infiltration through an engineered soil and uptake by plants.
• Swales and filter strips, where shallow flow over vegetation is absorbed before it concentrates.
• Tree pits with structural soil, which are effective per unit area and often the only option on a tight urban frontage.
• Soakaways and infiltration trenches, where the ground allows.
• Rainwater harvesting, which counts only where the tank is reliably drawn down between events, and the credit is limited. The mechanics of that argument are set out in our note on rainwater harvesting and attenuation credit.

Note that a lined attenuation tank, an oversized pipe or a flow control chamber provide no interception at all. They delay the discharge and then release every drop.

Why winter is the harder test

Interception depends on losses that vary through the year, and the guidance distinguishes summer from winter performance for that reason.

In summer, evaporation and transpiration are high, soils are drier, green roof substrates have spare capacity and antecedent conditions are usually favourable. In winter, evaporation is close to nothing, soils sit near field capacity for weeks, and a green roof substrate can be saturated from the previous week's rain. The same design that comfortably intercepts 5mm in July may pass almost all of it in February.

So describe both conditions. Where a design relies heavily on evapotranspiration, say what happens when it is not available, and show that some infiltration or storage-with-drawdown remains. Designs that lean on infiltration into free-draining ground are the most reliable across the year, which is another argument for doing infiltration testing properly at the outset. A Drainage Strategy that treats interception as an annual average tends to attract a question from the lead local flood authority.

Showing it in a drainage strategy

Keep it short and make it checkable. A page is usually enough.

1. Divide the site into drainage catchments and state the impermeable area of each.
2. Give the interception volume for each catchment as area multiplied by the required rainfall depth.
3. Name the component providing interception in that catchment and the mechanism, being explicit about whether it is infiltration, evapotranspiration or drawdown storage.
4. Show that the component's available capacity or loss rate covers the volume, with the winter case stated.
5. Where the site is large or the case is marginal, present continuous simulation results over a long rainfall series rather than a single-event calculation, and state the percentage of events for which no runoff occurs.
6. Cross-reference the maintenance plan, because a blocked permeable surface intercepts nothing after year five.

Illustrative example: a 28-dwelling scheme with 3,400 m² of impermeable area. Roofs drain to rain gardens sized to hold 25mm over their own footprint, private drives are in permeable paving with a 350mm sub-base on sandy clay with a measured infiltration rate, and the adopted road drains to a swale. Interception volume required is 17 m³. The rain gardens and sub-base provide it in summer through infiltration and storage, and in winter the calculation is carried on infiltration alone, which still covers it with a margin.

When interception cannot be achieved

There are honest reasons why it sometimes cannot, and stating them is better than pretending otherwise.

Contaminated ground where infiltration would mobilise contaminants. A shallow water table, where there is no unsaturated zone to infiltrate into. Podium and basement decks with no route to ground. Very low permeability clay with measured rates that make infiltration impractical. Sites within a source protection zone where the Environment Agency resists infiltration.

Where that applies, the position to put to the authority is a partial case plus compensation: intercept what you can on the parts of the site that allow it, then add treatment stages to deal with water quality, and explain the residual. The argument is much stronger where the constraint is evidenced by test results and contamination findings rather than asserted, and where the surface water case sits alongside a coherent Flood Risk Assessment for the site as a whole.

Keeping it separate from attenuation

The two criteria answer different questions, and conflating them is the most common drafting fault.

Attenuation asks whether the peak discharge rate and the total volume leaving the site in a rare event match the pre-development position. Interception asks whether frequent small events produce any discharge at all. A scheme can pass one and fail the other in either direction. Give them separate headings, separate calculations and separate conclusions.

For a single house the same logic applies at a smaller scale, and the expected evidence is lighter, as covered in our note on drainage evidence for a single new dwelling.

Enter your postcode on PlanScope to draft the drainage documents your submission needs, with the interception case set out separately from attenuation.

This is general guidance, not legal advice; check the position with your lead local flood 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. For specialist technical reports, qualified professional input may be required.