Size-Gradation Effect on Sand Transport Rates under Oscillatory Sheet-Flows

New experiments with graded sand were conducted in the large oscillating tunnel of Delft Hydraulics. The measured transport rates show that size gradation of sand particles has a small effect on the total net transport rates. However, the transport rates of individual grain-size fractions are strongly affected due to the interactions between the sizes. Finer sizes in the mixture are less transported as part of a mixture than as `uniform' material. Moreover, the transported sand mixture is coarser than the original sand bed. Comparing the new experimental results with three different transport models show that using the hiding/exposure correction factor of Day with the model of Ribberink leads to better predictions for the composition of the transported sand. The model of Dibajnia & Watanabe overestimates the transport rates of the two fractions under investigation. The model of Bailard overestimates the transport rates and the contribution of the fine fraction in the transported sand.