SuDS Water Quality Calculator
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SuDS Water Quality Calculator
Check whether a SuDS treatment train gives enough pollution mitigation for its land use, using the Simple Index Approach from The SuDS Manual (CIRIA C753). It compares the site's pollution hazard indices against the mitigation indices of your chosen components.
Simple Index Approach check
Step 1: pick the land use (sets the pollution hazard). Step 2: build the SuDS train. The total mitigation index must equal or exceed the hazard index for all three pollutant groups.
Pollution hazard & mitigation indices
From The SuDS Manual (C753). Indices run 0 (no hazard / no mitigation) to ~1 (high). All values are indicative; agree adaptations with the drainage approving body.
| Land use | TSS | Metals | Hydrocarbons |
|---|---|---|---|
| Residential roofs (very low) | 0.2 | 0.2 | 0.05 |
| Other / commercial roofs (low) | 0.3 | 0.2* | 0.05 |
| Drives, resi car parks, low-traffic roads (low) | 0.5 | 0.4 | 0.4 |
| Commercial yards, busy parking, most roads (medium) | 0.7 | 0.6 | 0.7 |
| Heavy / industrial, trunk roads & motorways (high) | 0.8 | 0.8 | 0.9 |
*Up to 0.8 where there is potential for metals to leach from the roof. Trunk roads/motorways also follow Highways guidance; high-hazard sites need regulator pre-consultation.
| Component | TSS | Metals | Hydrocarbons |
|---|---|---|---|
| Filter strip | 0.4 | 0.4 | 0.5 |
| Filter drain | 0.4 | 0.4 | 0.4 |
| Swale | 0.5 | 0.6 | 0.6 |
| Bioretention system | 0.8 | 0.8 | 0.8 |
| Permeable pavement | 0.7 | 0.6 | 0.7 |
| Detention basin | 0.5 | 0.5 | 0.6 |
| Pond | 0.7 | 0.7 | 0.5 |
| Wetland | 0.8 | 0.8 | 0.8 |
| Overlying treatment | TSS | Metals | Hydrocarbons |
|---|---|---|---|
| Dense vegetation + good attenuating soil ≥300 mm | 0.6 | 0.5 | 0.6 |
| Good attenuating soil ≥300 mm | 0.4 | 0.3 | 0.3 |
| Infiltration trench w/ graded filter gravel + soil ≥300 mm | 0.4 | 0.4 | 0.4 |
| Permeable pavement w/ filter layer + geotextile + soil ≥300 mm | 0.7 | 0.6 | 0.7 |
| Bioretention + good attenuating soil ≥300 mm | 0.8 | 0.8 | 0.8 |
Method & notes
How the Simple Index Approach works and where it does and doesn't apply.
The two steps (C753 Box 26.2). Step 1: allocate pollution hazard indices for the land use (Table 26.2), for three indicators — total suspended solids, metals and hydrocarbons. Step 2: choose SuDS whose total mitigation index equals or exceeds the hazard index for each of the three.
Treatment train rule. Where one component isn't enough, components in series are combined as: Total = index₁ + 0.5 × (index₂) + 0.5 × (index₃)… The 0.5 factor reflects the reduced performance of secondary/tertiary stages treating already-cleaner flows. This tool takes the strongest component (per pollutant) as index₁.
Destination. For surface-water discharge use Table 26.3. For infiltration to groundwater use Table 26.4, and provide at least 1 m of unsaturated material above the maximum groundwater level (C753 Chapter 25). High-hazard land uses and groundwater Source Protection Zones need the more detailed risk-assessment method and regulator consultation.
Where it fits. The Simple Index Approach is the routine water-quality method for most developments in England, Wales and Northern Ireland and supports the DEFRA 2025 National Standards / Welsh statutory SuDS standards. Scotland uses an equivalent approach. Proprietary treatment systems must demonstrate performance separately (C753 Chapter 14) and aren't auto-scored here.
This SuDS water quality calculator checks whether a SuDS treatment train gives enough pollution mitigation for its land use, using the Simple Index Approach from The SuDS Manual (CIRIA C753). It compares the site's pollution hazard indices against the mitigation indices of your chosen components. Treating runoff before discharge is part of the national SuDS standards.
How the Simple Index Approach works
First pick the land use, which sets the pollution hazard for three pollutant groups (total suspended solids, metals and hydrocarbons). Then build the SuDS management train in flow order. The total mitigation index must equal or exceed the hazard index for all three groups for the train to pass.
Building the treatment train
The first component is the primary treatment and later ones add further mitigation. The calculator sums the mitigation indices and tells you whether the train is adequate, or whether you need an extra component such as a filter strip, swale or pond. It also handles the metals uplift for surfaces like zinc or copper roofs.
Surface water or groundwater discharge
The mitigation table differs depending on whether you are discharging to surface water or to ground by infiltration. The calculator applies the right table for your discharge destination, so the check matches your outfall.
Further reading: green SuDS: nature-based drainage · source control: SuDS at plot level · gov.uk SuDS technical standards.
Frequently asked questions
A method in The SuDS Manual (C753) for checking water quality. It scores the pollution hazard of the land use against the mitigation provided by the SuDS components, for three pollutant groups.
When the total mitigation index meets or exceeds the hazard index for all three pollutant groups. The calculator does the comparison and flags any shortfall.
Yes. Infiltration to ground uses a different mitigation table from a discharge to surface water, because the risk to groundwater is treated more cautiously.
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