Gradation effects on sediment transport in oscillatory sheet flow

The main goals of this study are to obtain more insight and quantitative data of the selective transport process under sheet flow conditions. Experiments were carried out in the Large Oscillating Water Tunnel of WL DELFT HYDRAUUCS for five different hydraulic conditions. The sand that was used consisted of a mixture of 50% fine (0. 13mm) and 50% coarse (0.32mm) sand. The total transport rates are calculated with a mass balance equation. The transport rates of each fraction are calculated with a mass balance equation for each fraction, with use of a layer model. Comparison between these data and data from former experiments shows that the fine fraction is hindered by the coarse fraction for larger velocities. In case of sinusoidal waves with a current the coarse fraction is more easily transported than in case of uniform sand. For second order Stokes waves, the coarse fraction is not influenced by the fine fraction. Verification of three existing sediment transport models (Bailard, 1981; Ribberink, 1994; Dibajnia & Watanabe, 1992) shows that the model of Bailard gives good result for coarse fractions, the total and fine fraction transport rates are overestimated with a factor 1.5. However, the trend of the datapoints is rather well. The model of Ribberink is able to predict most transport rates well. For this sand mixture, correction factors can be applied. The correction factor of Day is the correction factor with the highest influence on the transport rates. For most conditions the transport rates are predicted in the best way with the correction factor of Day. The model of Dibajnia & Watanabe overpredicts most of the transport rates, however it is less than a factor 2. This model is the only model that is able to predict negative transport rates. Unfortunately the negative transport rates are not predicted well. In order to take gradation effects of sediment that was supposed to be uniform into account, the size fraction method is used for the three models. Each fraction is split up in 4 fractions. This size fraction method is only an improvement for most conditions for the model of Dibajnia & Watanabe.Hydraulic EngineeringCivil Engineering and Geoscience