A surface water treatment SuDS evaluation tool using the SuDS manual

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 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.




  • CIRIA, SuDS manual (2015)
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HR Wallingford provides training in the use of UK SuDS tools, plus planning and design of SuDS to meet current standards and best practice. 

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