How many levels of SuDS treatment does the site actually need?

Clients often ask me: "How many levels of SuDS treatment do we need here?"

The honest answer is: it depends.

It depends on the pollution hazard from the area the runoff comes from. A roof, a car park, an industrial yard, they all generate different levels of pollution. And traffic adds another layer, a residential road is not the same as a high-trafficked one.

This is the logic behind what's known as the Simple Index Approach, or SIA. It's the method most commonly used in the UK to work out how much treatment a SuDS scheme needs to provide. The idea is straightforward: every source of runoff carries a pollution hazard, and every SuDS component provides a certain amount of treatment. The job of the designer is to match the two, so the treatment provided is enough to bring the pollution risk down to an acceptable level before water reaches a watercourse or soakaway.

It also depends on the type of SuDS you use.

Vegetated systems like swales, basins or raingardens give you more treatment. The water slows down and is pushed through vegetation and soil, giving sediment time to settle and pollutants time to break down.

A filter drain, by contrast, provides some treatment, but it's limited, water moves through the aggregates too quickly for much to happen.

This is also why SuDS rarely work in isolation. A single component is often not enough to bring a high hazard site down to an acceptable level, so designers build a treatment train instead, several SuDS components placed in sequence, each one doing part of the job. For example; a dry swale, a swale with filter drain underneath, can handle the first level of treatment, and by the time the water reaches a basin, most of the pollution has already been dealt with.

So there's no single answer.

Sometimes one SuDS component is enough. Sometimes you need to combine two or three to bring the pollution risk down to an acceptable level. That's the engineer's job, making it work within the space, the levels and the geometry of the site.