A surface water treatment SuDS evaluation tool using the WaND method
The Water quality assessment for SuDS developments (WaND) tool provides an indication of the ability of different combinations of SuDS components to treat the surface water runoff discharged to surface waters. It produces a performance indicator based on the site areas and impervious surfaces of different land use types, and the sensitivity of the receiving waterbody.
Surface water treatment is a fundamental requirement of SuDS systems. This tool was developed by HR Wallingford as an outcome of the EPSRC WaND project (2006). This project provided a simple objective method to evaluate the relative effectiveness of different SuDS schemes in providing water quality treatment to surface water runoff.
There is no requirement to demonstrate the effectiveness of SuDS schemes to authorities in providing treatment 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.This tool is a simple way of comparing the water quality 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. The results are based on weighting of the treatment effectiveness based on the areas drained and the sequential arrangement of the SuDS units. It also takes into account the sensitivity of the receiving waters.
This tool, although different to the SuDS treatment assessment tool developed for the second version of the SuDS Manual, uses the same principles and provides a simple alternative method of analysis.
The tool is available as an excel spreadsheet, which all registered users can download and saved locally for free. Once saved locally, the tool will only work for 20 days. After that, you will need to download the spreadsheet again.
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, the actual effectiveness of any SuDS arrangement is very dependent on the design of the units, so there is a significant element of subjective judgement that needs to be applied in using this tool.
The tool provides a single indicator value between 0 and 1 (the lower the value the less polluting the impact of the surface water runoff) of the effectiveness of the SuDS scheme.
HR Wallingford provides training in the use of UK SuDS tools, plus planning and design of SuDS to meet current standards and best practice.