The design of vegetative constructed wetlands for the treatment of highwayrunoff

The Environment Agency for England and Wales are responsible for assessing the effects of highway runoff and for monitoring the treatment systems/proc edures which have been introduced for the reduction of deleterious effects. The Agency is looking into the improvement of surface water management in terms of best management practices and plans to work in partnership with th e Highways Agency to achieve this aim. Among the treatment options being co nsidered are constructed wetlands. Draft Guidelines have been developed to provide information on their design. This paper describes procedures for ca rrying out an Environmental Sensitivity Analysis to determine whether treat ment by a constructed wetland is appropriate. Information on water quality and quantity is required as well as the sensitivity of the receiving enviro nment. The legislative position, particularly in relation to the discharge quality of the water and the conservation status of the receiving environme nt, needs also to be considered. The factors that will determine the most a ppropriate wetland design criteria include traffic loadings, road drainage area, land availability, cost and the size/extent and type of the receiving water body. The following structures are recommended for incorporation in the overall design; oil separator and silt trap, spillage containment, sett lement pond, vegetative wetland and final settlement tank. The operation an d maintenance procedures and the monitoring requirements for a functioning wetland are described. (C) 1999 Elsevier Science B.V. All rights reserved.