SIMULATION OF SORTING IN THE CROSSSHORE DIRECTION AND TOPOGRAPHY CHANGE OF BEACH COMPOSED OF FINE AND COARSE SANDS

摘要

Numerical models for predicting topography change caused by sediment transport so far are generally carried out by using representative grain size. But actually bed material in the field often has wide grain size distribution. In a beach where grain size distributes widely, the direction of bed material transport depends on its grain size because the mode of transport of fine bed material is different from that of coarse bed material in the cross-shore direction every now and then. The purpose of this study is to simulate sorting process of the bed materials in the cross-shore direction due to difference of transport direction between fine and coarse materials and topography change by applying SBEACH model (Larson and Kraus, 1989). Although representative grain size such as the mean grain size is originally used in SBEACH, in this study the model is extended to apply to the beach composed of mixed bed materials. In this expanded model, sediment transport rate and transport direction of each grain size are estimated separately on the presumption that the beach is uniformly composed of bed material of each grain size. Total sediment transport rate is evaluated as the summation of the product of transport rate and content rate of each grain size. This expanded model can take account of grain size distribution for sediment transport rate. The change in the grain size distribution at the surface layer is calculated by conservation of mass of bed material of each grain size in the exchange layer. We carried out simulation to reproduce the hydraulic model tests. Simulated grain size distributions gave qualitative explanation for the observed one by the hydraulic model tests. Calculated results showed that the coarse materials and fine materials tend to move to the counter direction just like the measured results in model tests.