Grain size and gradation effects on sediment transport under sheet-flow conditions, data analysis

Abstract

This study consists of three sections: - experiments on sand transport in oscillatory sheet flow with two sands (D50 = 0.21 and 0.32 mm) - verification of three existing sediment transport models - extension of these models to include the gradation of the sediment The experiments are performed in the Large Oscillating Water Tunnel (see also data report: Z2137, part I). Comparison between these data and data from earlier experiments with finer sand shows the effect of the grain size and the unsteadiness on the transport rate for relatively uniform sand. Verification of three existing sediment transport models (Bailard, 1981; Ribberink, 1994; Dibajnia & Watanabe, 1992) shows that the model of Bailard largely overpredicts the measured transport rates, especially for fine sand. The model of Ribberink shows reasonably good results for all sands, except when unsteady effects become too important. In the model of Dibajnia & Watanabe unsteady effects are included. Therefore the net transport rates for fine sand are predicted well by this model, but the net transport rates for coarser sand are overpredicted. The gradation of the sediment is included into the models by using a size-fraction method. Moreover, hiding and exposure corrections are included in the model of Ribberink. Based on the results of the grain-size effect in uniform sand, a sensitivity study is performed, which shows the following results: Bailard predicts larger net transport rates than for uniform sand due to an overprediction in transport rate for the fine fractions. The transported material is finer than the bed material. For Dibajnia & Watanabe the transported material is coarser than the bed material and the net transport rate may either increase or decrease. For Ribberink the net transport rate is almost the same as for uniform sand and the transported material is hardly different from the bed material. The width of the gradation has a large effect on the difference between the net transport of uniform sand and that of a mixture with the same mean grain diameter.