The simple index approach to surface water quality

The simple index approach surface water method is how most drainage strategies demonstrate that runoff will be clean enough to discharge. It comes from the industry SuDS manual, and it works by scoring the pollution hazard of each surface, scoring the treatment capability of each drainage component, and showing that the treatment adds up to at least the hazard. Three pollutant groups are scored separately: suspended solids, metals and hydrocarbons.

It is not the same exercise as attenuation. A scheme can pass every discharge rate and storage volume test and still be refused on water quality, because the drainage train has no treatment stage in it. Lead local flood authorities now ask for the comparison explicitly.

Why quality is assessed separately from quantity

Quantity controls protect people and property from flooding. Quality controls protect the receiving water, whether that is a watercourse, a surface water sewer discharging to one, or groundwater.

The two are assessed against different sources. Discharge rates and volumes come from national technical standards for sustainable drainage and from local policy. Water quality comes from national planning policy on pollution and the water environment, and in practice from the method set out in the SuDS manual, which authorities and statutory consultees treat as the reference approach.

That split has a design consequence. Attenuation in a sealed tank underground satisfies the volume requirement and provides no treatment at all. If your strategy is a tank and a flow control, you have not addressed quality, and the answer is to put a treatment stage upstream or to change the approach.

Hazard indices for different surfaces

The method starts by classifying each contributing surface by pollution hazard level, from very low to high, and assigning an index for each of the three pollutant groups.

The pattern is intuitive once you see it. Residential roofs sit at the bottom: little in the way of solids, metals or oils. Metal-clad roofs score higher for metals because that is what washes off them. Driveways, small residential car parks and lightly trafficked roads sit in the low band. Commercial yards, delivery areas, heavily used car parks and busier roads sit in the medium band. Haulage yards, industrial sites handling oils and chemicals, and waste facilities sit at the top, where the manual expects a detailed assessment rather than the simple method.

Use the tables in the manual for the actual index values rather than a figure remembered from a previous job, and cite the edition. And split the site by catchment rather than averaging. A scheme with a roof, a car park and a service yard has three hazard classifications, and the treatment each needs is different.

Mitigation indices for SuDS components

Every drainage component is then given mitigation indices for the same three pollutant groups. Again the pattern makes sense: components with soil, vegetation and a filtration path score well, and components that only hold water score modestly.

Bioretention areas, rain gardens and constructed wetlands score highest, because runoff passes through a medium that traps solids and binds metals. Permeable paving scores well for the same reason. Swales and filter strips sit in the middle. Detention basins and ponds contribute mainly through settlement, so they help with solids more than with hydrocarbons. Sealed tanks contribute nothing.

Proprietary treatment devices can be used where their performance is evidenced by independent testing, and the evidence has to accompany the strategy rather than being asserted by the supplier's brochure.

Matching mitigation to hazard

The comparison is made per pollutant group, and the total mitigation must equal or exceed the hazard index for each. Where components are arranged in series, which is what a management train is, the manual discounts the contribution of the second and subsequent components rather than allowing them to be added at full value.

An illustrative example, and the figures are illustrative rather than taken from a real scheme. A medium hazard car park needs treatment for all three groups. A single swale might reach the required index for solids and metals but fall short on hydrocarbons. Adding permeable paving upstream, with the swale second and discounted, takes the total above the requirement for all three. The design answer was not a bigger swale. It was two components in series.

That is the practical lesson. When the comparison fails, the usual fix is a second treatment stage in the train, not an enlarged single feature.

Discharges to groundwater and the extra step

Infiltration brings a second layer of assessment, because the receptor is groundwater rather than a watercourse. The method in the manual for infiltration considers the hazard of the surface alongside the protection offered by the soil and the unsaturated zone above the water table.

On top of that sits the regulator's position on groundwater protection. Infiltration of runoff from trafficked or contaminated surfaces in a source protection zone, or over a principal aquifer, is treated with much more caution, and in the innermost protection zones it may not be acceptable at all without treatment and sometimes not even then. Check the groundwater position before designing an infiltration-based strategy, because discovering it late means redesigning the whole drainage layout.

Two further points on infiltration. Infiltration into made ground or contaminated soils can mobilise contaminants, so the contamination assessment and the drainage strategy need to be read together. And infiltration testing has to be done properly, with the method and the results reported, not assumed from a soil map.

Where a detailed assessment is needed instead

The simple method is a screening tool for ordinary situations. Expect to go further where the site has a high pollution hazard, where the discharge reaches a sensitive receptor such as a designated site, a shellfish water or a drinking water abstraction, where the receiving water body is already failing its objectives, or where the comparison cannot be satisfied by any reasonable arrangement of components.

A detailed assessment means quantifying pollutant loads and concentrations and demonstrating the effect on the receiving water, which is specialist work. Where you are in that territory, involve the statutory consultee early, because their view on method will shape the whole study.

Documenting the comparison

Put the water quality case in its own section of the Drainage Strategy, not in an appendix. It should contain the catchment plan, the hazard classification with the source cited, the proposed management train shown in order, the mitigation index comparison for each pollutant group, and the maintenance schedule that keeps the treatment working.

Two things reviewers look for. Whether the treatment is upstream of the attenuation, because treating water after it has sat in a tank achieves less. And whether the maintenance regime is realistic for whoever will actually own the feature, because an unmaintained filter medium stops treating within a few years.

Where the site also has a surface water flood risk issue, the quality case belongs alongside the Flood Risk Assessment rather than duplicating it. And where rainwater harvesting forms part of the strategy, the interaction with attenuation credit is a separate question, covered in our note on rainwater harvesting and attenuation credit.

One honest uncertainty: authorities differ in how strictly they apply the discount for components in series, and some accept proprietary devices more readily than others. Ask the lead local flood authority for its position at pre-application stage, and record the answer in the strategy.

Where a scheme proposes to affect an existing watercourse, the quality assessment is only part of the argument, as our piece on culverting a stream explains.

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This is general guidance, not legal advice; check the position with your local planning 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.