SuDS treatment stages and the simple index approach

Attenuation deals with how much water leaves a site and how fast. Treatment deals with what is in it, and a drainage design that solves the first and ignores the second will come back from the lead local flood authority. The simple index approach in the SuDS Manual is the method most English authorities expect for demonstrating water quality, and it works by comparing a pollution hazard index for the surfaces draining to a point with the mitigation indices of the SuDS treatment stages you have put between that surface and the receiving water.

Get the logic right and the calculation takes an afternoon. Most of the submissions that fail do so because the surfaces were categorised too generously or because credit was claimed twice for components in series.

Why treatment sits alongside attenuation

Surface water carries sediment, metals from brake and tyre wear, hydrocarbons from engines and spills, nutrients from landscaped areas and whatever has been spilled in a yard. Discharge it untreated to a stream or into the ground and the harm is to the receiving water, not to the site.

The Defra non-statutory technical standards for sustainable drainage cover peak flow and volume in detail and treat water quality in general terms, which is why the published industry method fills the gap. The SuDS Manual, CIRIA C753, sets out the simple index approach for most development, with more detailed risk assessment reserved for high hazard sites and sensitive receptors.

National policy expects drainage to deliver multiple benefits including water quality, and the lead local flood authority is the statutory consultee on surface water drainage for major applications. Expect the water quality section of a Drainage Strategy to be read as carefully as the hydraulic calculations.

Pollution hazard levels by surface

The method starts by assigning a pollution hazard level to each contributing surface, then reading the published indices for three pollutant groups: total suspended solids, metals and hydrocarbons. Indices run from zero to one, and the published tables in C753 give the values, which should be taken from the current edition rather than from a secondary summary.

The hazard bands run roughly as follows, and the categorisation is where most errors start.

Two points of discipline. Work surface by surface rather than averaging a whole site, because a development with residential roofs, an estate road and a refuse vehicle turning area has three different hazard levels draining to the same basin. And where the use is uncertain, assume the higher band, because a reviewer will.

Mitigation indices and the treatment train

Each SuDS component carries its own mitigation indices for the same three pollutant groups, again published in the Manual. Filter strips, swales, bioretention, filter drains, permeable paving, detention basins, ponds and wetlands all score differently, and vegetated components with a filtering mechanism score better for metals and hydrocarbons than a lined storage tank, which scores nothing.

The test is simple: for each of the three pollutants, the total mitigation index of the treatment stages has to equal or exceed the pollution hazard index of the surface draining through them.

Where two or more components are arranged in series, the convention is that the first component takes its full index and later components are credited at half of theirs. That halving is the rule most often missed, and it is why a design with a permeable car park followed by a swale followed by a basin does not score three times.

Note that a proprietary treatment device can be used where the manufacturer's independently verified performance data supports the indices claimed. Without that data, assume nothing.

Interception is the other requirement to address in the same section. The Manual expects small rainfall events to be managed without run-off leaving the site, commonly expressed as handling the first five millimetres of rainfall, and the components that achieve it are source control features rather than end-of-pipe storage.

Building a train that balances all three indices

The common failure is a design that satisfies suspended solids and falls short on hydrocarbons, because sediment is easy to remove and dissolved and emulsified pollutants are not.

A workable sequence for a mixed residential scheme with an estate road and a parking court:

1. Roofs discharge to rain gardens or directly to a soakaway where ground conditions allow, taking the very low hazard surface out of the main train entirely.
2. The estate road drains to a filter drain or a roadside swale, giving source control close to where the pollutant is generated.
3. The parking court uses permeable paving with a sub-base reservoir, which performs well for hydrocarbons because the geotextile and sub-base support biodegradation.
4. The combined flow passes to a detention basin with a shallow forebay for sediment capture and a vegetated bed.
5. The outfall takes a flow control and a penstock for isolation in the event of a spill.

Keep the components in the landscape rather than beneath it wherever space allows. Vegetated features earn mitigation credit, carry biodiversity and amenity value, and they are the reason a Landscape Strategy and the drainage design should be drawn by people who have spoken to each other.

An illustrative case: a 40-dwelling scheme where the hydrocarbon index for the shared parking court could not be met by a basin alone, and was resolved by converting the court to permeable paving. The design change cost nothing in developable area and removed a consultee objection. That is indicative of the kind of fix available, not a standard solution.

Discharge to a sensitive receiving water

The simple index approach is not sufficient everywhere. Where the discharge is to a sensitive receptor, the Manual directs the designer to a more detailed assessment and the regulator's own requirements apply on top.

The receptors that change the answer are groundwater in a source protection zone or a principal aquifer, a site of special scientific interest or other designated habitat, a bathing or shellfish water, and a watercourse already failing its water body objectives. For infiltration to groundwater the Environment Agency's groundwater protection position statements govern what may be infiltrated and after how much treatment, and infiltration of run-off from a high hazard surface is generally not acceptable.

Discharge to a ditch or stream also needs land drainage consent for the outfall structure, which is a separate approval obtained as described in the note on land drainage consent for works to an ordinary watercourse. Keep the surface water and foul systems clearly distinguished throughout the documentation, for the reasons set out in the note on foul drainage with no mains sewer.

Where the site is in Flood Zone 2 or 3, or exceeds one hectare, the water quality section sits inside a Flood Risk Assessment rather than standing alone.

Reporting the calculation so a reviewer can check it

Present the water quality case as a table a consultee can audit without rebuilding it.

For each discharge point: the contributing surfaces with their areas, the hazard level and the three hazard indices, the treatment components in order with their indices, the halving applied to components in series, the resulting total for each pollutant, and a pass or fail against the hazard index. Add a plan with the drainage areas coloured by hazard level and the components numbered to match the table.

Then state the assumptions: the edition of the Manual used, the source of any proprietary device indices, the interception approach, and the maintenance frequency each component relies on.

What a reviewer looks for first

Six errors account for most second-round comments.

Categorising a commercial yard or a refuse vehicle route as low hazard. Averaging indices across a whole catchment instead of working surface by surface. Claiming full credit for every component in series. Relying on a lined attenuation tank as treatment, which it is not. Quoting a proprietary device's performance without the test data. And presenting a treatment train that cannot be maintained because there is no vehicle access to the forebay or the inlet.

One honest uncertainty: authorities vary in how strictly they apply the halving convention and how they treat combined components such as permeable paving over a geocellular base. Ask the lead local flood authority for its expectations at pre-application stage, and record the answer in the strategy.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required.

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