Pollutant removal ability of grassed surface water channels and swales: Literature review and identification of potential monitoring sites

This report primarily consists of a literature review that assesses the current state of knowledge with regard to the pollutant removal ability of swales and grassed channels for highway drainage, and seeks to identify gaps in the current knowledge. However, a dearth of published data on the nature of highway run-off necessitated the addition of a second Phase to the project, to develop a specification for undertaking field monitoring to provide sufficient data for the purposes of the Highways Agency. Three alternative approaches were proposed for achieving this objective. The requirement for field monitoring necessitated the identification of potential sites. Works from a previous and related study had revealed only a small number of possible monitoring sites on grassed channel locations on the Network. Consequently a questionnaire was circulated to the Maintaining Area Contractors (MACs) with the aim of identifying potential monitoring sites. At the same time a Discussion Paper was circulated around Government bodies soliciting views on grassed channels and swales performance, desirability and current design practice. A synopsis of the comments received and the sites identified are contained in the report.