Sustainable drainage

Frequently asked questions

What is the difference between attenuation and infiltration?

Where the water ends up, and they satisfy different requirements. INFILTRATION puts the water into the ground, so it leaves the drainage system entirely — a soakaway, a permeable pavement over free-draining ground, an infiltration basin. ATTENUATION holds the water temporarily and releases it slowly into a sewer or watercourse, so the same volume still leaves, just not all at once — a tank, a pond, an oversized pipe with a flow control. The distinction matters because approvals usually impose two separate limits: a peak DISCHARGE RATE, often pegged to what the site would have shed before it was built on, and sometimes a total VOLUME. Attenuation meets a rate limit and does nothing for a volume limit; infiltration meets both but only works if the ground will actually take water. That is why the infiltration test comes before the design rather than after it: on clay the answer is attenuation whatever the planning statement aspires to.

Why is a percolation test done in a pit rather than a borehole?

Because the number wanted is how fast the ground will accept water from a full soakaway, not how permeable a sample is. The British method digs a trial pit of realistic dimensions, fills it, lets it drain, and measures the time to fall between set fractions of the effective depth — repeated three times, with the SLOWEST result used, because the first fill runs into dry ground and flatters the answer. Doing it in a narrow borehole changes the ratio of wetted side area to stored volume, which is exactly the geometry the result depends on. Two further conditions decide whether the design is honest: the test must reach the depth the soakaway will sit at, since a permeable topsoil over clay gives a fine result and a useless soakaway; and it should not be done after a dry spell, when shrinkage cracks in clay let water away at a rate that closes up as soon as the ground swells.

Is permeable paving a drainage system or a surface?

Both, and treating it as a surface is how it fails. The blocks or the porous asphalt only get water through the top; the working part is the sub-base beneath, which is an open-graded stone with a large void ratio acting as a tank, plus whatever outlet the design gives it — infiltration into the ground, a controlled pipe, or both. So the design questions are the storage volume in that stone, the rate the ground beneath will accept, and what happens in the storm that exceeds both. The failures follow from ignoring the layer that matters: laying it over a compacted clay formation that cannot infiltrate, bedding it on ordinary sand which migrates into the voids and fills them, allowing silt-laden runoff from an adjacent area onto it so the joints clog, or never sweeping it. A permeable pavement is a piece of drainage infrastructure with a maintenance regime, and the one that gets neither is impermeable within a few years.