Infiltration capacity of roadside filter strips with non-uniform overland flow

The side slope to a roadside swale (drainage ditch) constitutes a filter strip that has potential for infiltration of road runoff, thereby serving as a stormwater quantity and quality control mechanism. A total of thirty-two tests were performed during three seasons in four different highways located in the Minneapolis-St. Paul metropolitan area, MN to analyze the infiltration performance of roadside filter strips and the effect of fractional coverage of water on infiltration. Three different application rates were used in the experiments. All the tests showed that water flow on the lateral slope of a roadside swale is concentrated in fingers, instead of sheet flow, at the typical road runoff intensities for which infiltration practices are utilized to improve surface water quality. A linear relationship between flux of water from the road and fraction of wetted surface was observed, for the intensities tested. The average percentage infiltration of the medium road runoff rate (1.55 × 10⁻⁴ m²/s, without direct rainfall) experiments performed in fall was 85% and in spring 70%. For the high road runoff rate (3.1 × 10⁻⁴ m²/s, without direct rainfall) tests the average amount of water infiltrated was 47% and for the low road runoff rate (7.76 × 10⁻⁵ m²/s, without direct rainfall) tests it was 69%, both set of tests performed in spring and summer. The saturated hydraulic conductivity of swale soil was high, relative to the values typical of laboratory permeameter measurements for these types of soils. This is believed to be due to the macropores generated by vegetation roots, activity of macrofauna (e.g. earthworms), and construction/maintenance procedures. The trend was to have more infiltration when the saturated hydraulic conductivity was higher and for a greater side slope length, as expected. The vegetation, type of soil and length of the side slope are important to consider for constructing and maintaining roadside swales that will be efficient as stormwater control measures. These measurements indicate that the filter strip portion of a roadside swale typically infiltrates a substantial fraction of road runoff. However, the measurements do not incorporate the influence of direct rainfall upon the infiltration into filter strips.