Hansson et al. (2005) made an attempt to illustrate the effect of a rain shower and fracture zone permeability on the subsurface flow pattern using a two-dimensional computer model; thus making the simulation domain more like reality (Fig. 11.6). The properties of the materials used in the various layers of the model road fulfil the requirements of the Swedish road design guide. In addition, it was assumed that the asphalt layer of the road had plenty of fractures over a relatively short distance, “a fracture zone”, which thus enabled the use of an equivalent homogeneous porous media model. More details about material properties, driving data etc. can be found in Hansson (2005).
Notice (Fig. 11.6) that the fracture zone captures most, if not all, of the upstream surface runoff for the light rainfall event. The heavier rainfall causes a significantly larger infiltration in the road shoulder since the infiltration capacity of the fracture zone, or the granular base layer beneath it, was exceeded. As a consequence, a larger fraction of the total surface runoff reached the road shoulder, and the region of the roadside where both rainfall and surface runoff infiltrated was considerably expanded laterally. This result is qualitatively supported by the findings of Flyhammar and Bendz (2003) who measured concentrations of various solutes in the shoulder and beneath the asphalt cover in a Swedish road partly built with alternative materials, generated from waste and residuals. These materials contained plenty of solutes, and the leaching pattern is similar to the simulated water flow pattern, although the leaching patterns exhibited a large variability between solutes.
