In modern society, land development can increase the amount of stormwater runoff and its associated pollutant loads to receiving waters (Barrett et al., 1998). Several authors have studied stormwater runoff carries off dissolved, colloidal and solids components from the urban surfaces (e.g. Chang and Crowley, 1993 and Ball et al., 2000). A porous medium is a material containing large number of randomly located voids (pores) of sizes greater than the microscopic interatomic scale such as soil, rocks, sponge, etc. Transport phenomena at the interface between a fluid and porous medium has remained an important topic of research because of their fundamental interest in understanding the phenomena of infiltration into the porous medium and its wide field of applications in industry and environmental analyses, for example, urban hydrology, river hydraulics, pollution and remediation of soils, water treatment, etc. As in urban hydrology field, the erosion and clogging phenomena of road cause a lot of economic losses and even threaten the people's lives, because these are the results of an effect of sediment transport caused by flow after rains, a better understanding of details of flow structures near the surface of road (a kind of porous medium) will help to improve the design of road and provide optimized solutions. Flows over a porous medium are the flows combined both over and through the porous medium. Because of the interaction between surface flow and seepage flow inside porous region, this interaction affects both the flow discharge and the sediment discharge. In recent years, due to the development of the computational capacity, a complex computational fluid dynamics (CFD) could be performed. Meanwhile the discrete element method (DEM), developed by Cundall and Strack (1979), has been used extensively to investigate the behavior of granular solids subjected to a variety of loading conditions with the goal of understanding and predicting quantitative results.
