A Survey of Ditches Along County Roads for Their Potential to Affect Storm Runoff Water Quality

Abstract

Twenty years of research have demonstrated that the water quality of stormwater runoff can improve after flowing in a well-vegetated channel, relatively slowly, at a depth below the vegetation height. These channels are commonly called “biofiltration swales.” Roadside ditches that are vegetated also may have the potential to provide the same water quality benefits as biofiltration swales by removing pollutants. Conversely, ditches that are devoid of vegetation are subject to erosion and could be significant sources of sediments and other pollutants. If the potential benefits are to be realized, and the pollutant source avoided, ditch condition and maintenance must be consistent with not only conveyance but also water-quality objectives. Because no systematic data have been collected that describe ditch characteristics with respect to water-quality considerations, Snohomish and King counties commissioned the Center for Urban Water Resources Management to evaluate ditch status in the two jurisdictions and to consider how road maintenance crews might maximize their potential for water-quality performance. The goal of this investigation was to develop strategies for improving runoff treatment and reducing downstream sediment loading from existing ditches, while retaining their hydraulic function of conveying roadway runoff. The principal focus was to guide maintenance actions, but it was anticipated that design of future ditches should also benefit. This report documents one aspect of the investigation— a systematic survey of ditches during the summer and fall of 1998, designed to evaluate the water-quality performance of ditches in the two counties’ road networks. The survey encompassed 113 ditch segments in Snohomish County and 87 segments in King County, ranging in length from 200 to 600 feet. Single-family residential is by far the dominant land use in the catchments adjacent to the ditches surveyed, a circumstance representative of areas in the two counties with roadside ditches. Specific measurements and observations were made at several transects in each ditch, extending across the width and spaced along the length of each segment. In total, 1000 transects were surveyed for this project, emphasizing the data collection and analysis of those factors that were anticipated to be both beneficial and detrimental to improving water quality.