This study had three objectives and associated work components: Component 1 - assess routine highway ditch cleaning alternatives ("Service Levels") for water quality benefits; Component 2 - survey biofiltration swales to evaluate conditions promoting water quality benefits; and Component 3 - assess restabilization and revegetation options for use after ditch cleaning and for restoring biofiltration swale vegetation.
Component 1 tested the water quality effects of three Service Levels in freeway ditches: (1) excavated to original elevation and shape along the upstream three-quarters of the length and then sodded, (2) excavated along the entire length and straw-covered, and (3) excavated along the upstream three-quarters of the length and then straw-covered. Component 2 surveyed representative swales along central Puget Sound area highways for a variety of geometric, hydraulic and vegetative characteristics. Survey data were analyzed to develop maintenance, design, and construction guidelines. In Component 3 vegetation establishment from seed was assessed in replicate plots in a freeway ditch with the assistance of restabilization aids: (1) coconut fiber blanket, (2) straw held in place with stapled jute mat, (3) straw without covering, and (4) polyacrylamide (PAM). Cost-benefit analyses were performed in Components 1 and 3.
The overall best Service Level for water quality benefits was excavating the first three quarters and retaining vegetation in the remainder. The ditch treated in this manner was capable of reducing total suspended solids (TSS) by approximately 40%, total phosphorus by about 50%, and total and dissolved Cu and Zn each by roughly 20 to 25%. It is recommended as the standard procedure when cleaning ditches that discharge to a natural receiving water. Analysis of survey data showed that biofiltration swales with broad side slopes, wide bases, and total storage volumes equivalent to 3 in. of runoff from the impervious drainage area consistently supported good vegetation cover and showed few signs of damage. For assisting grass growth, straw held in place with stapled jute mat had a clear advantage in effectiveness over the alternatives and a slight economy advantage over the coconut mat.