Site assessment of road-edge grassed channels for highway drainage

Worldwide, grass-lined channels (or swales) have long been used for the drainage of surface runoff from roads and motorways. Recently, the emphasis on sustainability and on minimisation of impact on the environment have prompted the spread of swales as a drainage option which also provides flow attenuation and improvement of the quality of the discharged water. However, swales and other traditional grass-lined channels are typically too deep and steep-sided, from a vehicle safety view point, for use adjacent to carriageways. A new environmentally friendly drainage system was therefore devel-oped by HR Wallingford and TRL (Transport Research Laboratory) in col-laboration with the Sports Turf Research Institute for the UK Highways Agency (HA). During an eight year long study, road-edge grassed chan-nels were evaluated in terms of grass type specification, geometric charac-teristics, safety, hydraulic resistance and performance on site through field trials. These trials are the subject of this paper and provided important in-formation for the specification, construction and maintenance requirements of grassed channels as well as for their hydraulic capacity and environ-mental qualities. As a result of these very positive trials, the design docu-ment Advice Note HA119 [1] was produced giving guidance to highway drainage engineers on the design, construction and maintenance of grassed channels