Analysis of Infiltration and Overland Flow over Sloped Surfaces: Application to Roadside Swales

Abstract

Environmental impacts and regulatory requirements associated with highway runoff present design challenges. Roadside swales, or drainage ditches, improve water quality by infiltration, filtration, and sedimentation. Road runoff volume reduction through infiltration occurs as the water flows over the side slope, acting as a filter strip, or down the length of the swale channel. Therefore, roadside swales are practical solutions that can mitigate the effect that linear transportation projects have on water bodies. This dissertation advances the understanding of the infiltration performance of roadside swales for design and planning purposes. The main objectives of this thesis are: understanding the overland flow and infiltration processes over a fraction of a slope, relating surface roughness parameters to the fraction of wetted area, and quantifying the effect of equidistant parallel strip water sources on the lateral component of infiltration. Moreover, this research leads to a better understanding of overland flow and infiltration in roadside swales through field experiments, and modeling efforts that can simulate how roadside swales operate. All the field tests, performed in four different highways, showed that water flow on the side 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. The laboratory experiments aimed to formulate the relevance of fractal parameters, based on the Fourier power spectrum method, for understanding soil surface roughness, overland flow patterns, and erosion. A model has been developed for coupling a Green-Ampt-Mein-Larson infiltration submodel with kinematic wave submodels for both overland flow down the side slope and open channel flow for flow in the channel. The side slope of a roadside swale is the main part contributing to the loss of runoff by infiltration and the channel primarily conveys the water. Finally, a simplified roadside swale calculator has been developed with a reduced set of input parameters. The calculator can estimate the total percentage of annual volume infiltrated, supporting informed decision-making on how to account for the infiltration benefits of roadside swales.