Water Quality Benefits of Grass Swales in Managing Highway Runoff

Grass swales are a vegetated stormwater management technology that can remove surface runoff contamination through sedimentation, filtration by the grass blades, infiltration to the soil, and likely some biological processes. Two full-scale grass swales in the median of a four-lane highway were monitored during 18 storm events to characterize the overall performance of grass swales as a stormwater management technology and to evaluate the effect of the shallow-sloped grass pre-treatment area adjacent to the swale in most designs. The study was designed as an input/output comparison between the water quantity and quality captured directly from the roadway and the effluent from the swales. Both swales exhibited significant removal of suspended solids (65-71% based on EMCs) and zinc (30-60% based on EMCs). No significant difference in treatment efficiency is noted between the swale with a pretreatment area and the grass swale alone.

In an effort to reduce the effects of non-point source pollution, which includes overland runoff from agricultural, industrial, and urban areas and accounts for almost 50% of the total water pollution in the developed world (Novotny 1994), engineers and highway administrations have placed an emphasis on cost-effective methods to control runoff pollution. One such LID technology that has been employed for the conveyance of stormwater runoff in highway designs for many years is grass swales. Water quality enhancements can be realized in these swales through sedimentation (due to the low velocity induced by the vegetation), filtering by the grass blades, infiltration, and likely some biological processes. Swales are commonly used on highway projects because they represent an aesthetically pleasing method for linearly conveying runoff. While recent studies have revealed them as an effective stormwater management technology, good performance data and mechanistic understanding of swale design parameters are not available.

Little consistent information on water quality improvements for swales is available, in large part because of the complexity of swale operation. Swales receive flow laterally through vegetated side slopes, which can greatly improve incoming water quality. Infiltration throughout the swale surface area can reduce flow volume and improve quality. Thus, swales have several points of water input and output, which can complicate simple performance analyses. This is supported by a summary report on swale performances from several states, which has shown sediment removals ranging from -85% (i.e., sediment increases) to 98% (Schueler 1994). These results indicate that many variables can contribute to the pollutant removal efficiency.

A recent study comparing the performance of grass swales to other stormwater best management practices was performed by Barrett (2005). This study evaluated 42 events over 6 grass swales. Results indicated that grass swales removed metals (zinc 54%, copper 24%) and suspended solids (48%) successfully. However, significant export of nutrients (nitrate -28%, orthophosphorus -242%) was noted. Yu et al. (2001) compiled results from grass swales from their work and that of others. Length was found to be the most important parameter in insuring good swale performance. A swale length of over 100 meters has proven to be successful in providing very good removal of suspended solids and other pollutants. The authors recommended using slopes less than 3%. Phosphorus removal was scattered, apparently because of phosphorus being contributed by the vegetation in the swale. The use of check dams was found to significantly improve treatment efficiency.

As part of increased focus on stormwater management practices capable of treating non-point source runoff and the great amount of uncertainty regarding the performance and pollutant removal mechanisms of grass swales, a pilot project was constructed. The goals for this project were to systematically quantify the effects of some operational parameters for water quality improvement using grass swales. This project had two objectives. The first focused on the overall efficiency of a grass swale on roadway runoff pollutant removal. The second examined the effect of the shallow sloped grass pre-treatment area adjacent to the grass swale.

In order to analyze the pollutant removal capability of the swales and to measure the effect of a pretreatment area, two nearly identical swales were designed and constructed; one swale with a pretreatment area adjacent to the swale and one swale receiving runoff directly from the roadway surface. A third sampling site, a concrete channel which received runoff directly from the roadway, was assumed to be equivalent in quantity and water quality to the inputs for the two swales. The study system was constructed to concurrently monitor representative inflow and outflow from the grass swales, allowing the determination of pollutant removal efficiency. Water quality parameters examined included those considered as being most problematic from roadway runoff – suspended solids and zinc. Flow rates were recorded to determine the effect of swales on stormwater quantity and so that total pollutant mass reduction could be calculated.

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