Author(s): Shu-Qing Yang
Linked Author(s): Shuqing Yang
Keywords: Sediment transport, shear stress, mean velocity, vertical velocity, unsteady flow
Abstract: For sediment transport, by far, many formulae have been developed based on different assumptions, but unfortunately, almost none of them is universally applicable. Generally, there are two methods available in the literature to express sediment transport; the shear stress approach and velocity approach. The velocity approach assumes that if the mean velocity U exceeds the critical velocity, then sediment motion can be observed. The shear stress approach has been widely used by researchers as it accounts for the forces acting on a particle in horizontal plane. The earliest researcher to use the shear stress approach is Shields (1936), who related the dimensionless shear stress, or Shields number ?/ (?s-?) gd50. Both approaches assume that sediment transport rate is increased when U or ? is higher. But by comparing with experimental data, the performance of these formulae is not very satisfactory. In this paper, the measured bedload is analyzed against the famous equations available in the literature. It is found that the measured data do not always support the hypothesis that the higher parameters of shear stress or mean velocity always generate higher sediment transport rate. The mechanism of this high discrepancy has been examined and it is found that the disagreement can be attributed to the missing parameter in vertical direction, which has never been systematically measured and included in the existing sediment transport formulae due to its subtle characteristics, but its existence can be inferred from the vertical force induced by seepage, pressure variation over time and space. A new model has been established by including the new vertical parameter in the sediment transport formula and the agreement with experimental data has been improved significantly
Year: 2017