Roadside grassed drainage ditches (roadside swales) improve water quality by infiltration, filtration, and sedimentation. Road runoff volume reduction through infiltration occurs as the water flows over the side slope (which acts as a filter strip) in a direction perpendicular to the roadway and down the length of the channel parallel to the roadway. A coupled overland flow-infiltration model that accounts for shallow concentrated flow has been developed. This numerical model is used to estimate the infiltration performance of roadside swales using the Green-Ampt-Mein-Larson assumptions to calculate infiltration along with a kinematic wave model for overland flow that accounts for concentration of flow into “fingers”. The roadside swale model was validated using the data collected in twelve simulated runoff tests in three different highways located in the Minneapolis-St. Paul metropolitan area, MN. Based on global sensitivity analyses, the volume infiltrated by a roadside swale is most sensitive to the saturated hydraulic conductivity (Ksat). This computational model allowed us to create a simplified calculator, where repeated simulations of the model were run to quantify the relationship between rainfall depth and percentage infiltration for different Ksat and ratios of width of the roadside filter strip over width of the road (Wswale/Wroad). The result is the percentage of the annual rainfall infiltrated by the roadside swale or percentage of events captured. The calculator will also display a figure representing annual infiltration performance, corresponding to a specific location based on historical rainfall data, versus saturated hydraulic conductivity.

Infiltration in roadside drainage ditches

2019 Fall National Meeting

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