Water quality assessment for SuDS developments (SuDS manual)

The Water quality assessment for SuDS developments (SuDS manual) tool provides an indication of the suitability of different combinations of SuDS components to mitigate water quality risks (to both groundwater and surface water) to an acceptable level.


As a part of the rewriting of the SuDS Manual (2015), a simple risk assessment method (the Simple Index Assessment method) was developed to address some of the deficiencies of the original ‘levels of treatment’ approach specified by the previous version of the Manual.  The Scottish Environment Protection Agency (SEPA) then supported an initiative to produce this water quality treatment tool. It is conceptually very similar to the [original tool] [LINK] produced by HR Wallingford in 2006; the differences are 

  • the indices for land use pollution indices 
  • the method of calculation of the reduction in pollutant load due to SuDS treatment. 

When to use this tool?

There is no requirement to demonstrate the effectiveness of SuDS schemes in providing treatment to authorities other than in making a suitable risk assessment of the treatment requirements of a site in accordance with the SuDS Manual (2015). However an objective method of indicating the effectiveness of one or more scheme options will help in demonstrating the scheme’s suitability in providing surface water treatment.

The Water quality assessment for SuDS developments (SuDS manual) tool is a simple way of comparing the treatment effectiveness of various SuDS schemes based on the assumption of various land use loading conditions and a proportional reduction of pollutants depending on the SuDS types. 

How to use the tool?

The tool is available as an excel spreadsheet, which all registered users can download and save locally for free. Once saved locally, the tool is only valid for 20 days. After that, you will need to download the spreadsheet again from this website. 

The user can edit all treatment coefficient values for each SuDS type and all pollution loading factors for land use types  Although default factors and values are provided, all are subject to significant uncertainty for a specific site, so there is a significant element of subjective judgement that needs to be applied in using this tool.

What values will the tool give?

The tool provides an indication of the likely suitability of the proposed SuDS components in mitigating water quality risks to an acceptable level for both groundwater and surface water.


The level of hazard for most housing estate elements is not dependent on size – but land use (i.e. roofs, driveways etc). The exception is for roads : low traffic roads are described are low risk and the < 300 movements applies to all use types. Therefore, to be classified as low risk, general access roads should be low traffic with the same level of traffic as expected in cul de sacs and home zones. This would lead to roads for small developments where traffic is less than 300 movements per day on that road being classed as low risk and, for larger developments where more traffic is expected, the risk classification is medium. In a large development, different roads may therefore have a different risk category.

The 300 traffic movements a day suggested as a threshold for moving to a Medium hazard categorisation is only indicative of the type of level at which pollution hazards may increase. It is not based on robust scientific evidence and actual hazard changes will be dependent on a huge range of factors, e.g. the type of vehicles using the road will have a significant influence – for sites with relatively higher proportions of HGVs using the roads, hazards will be greater.

For residential developments, where HGV use is low, and where traffic movements are estimated as around the 300 mark then reasonable judgement needs to be exercised. Normal road use rather than any ‘worst case scenario’ should probably be adopted, but this will depend on the definition of ‘worse case’ and how likely that might be to occur, every day, during the lifetime of the development. If significant infill is anticipated and road use is likely to increase during the design life of the drainage system, then this should also be taken into account.’