A model for predicting the grain size distribution of an armor layer under clear water scouring

Clear water scouring often occurs in the channel downstream of a dam and changes the grain size distribution of the channel bed, which impacts the bed resistance and flood conveyance capability. Based on the proposed spatial-mean rolling probability of the non-cohesive sediment and the effects of the constantly decreasing sand content during riverbed armoring, a model was established to predict the grain size distribution of an armor layer under clear water scouring. Based on iterative calculation and the new spatial-mean rolling probability, the predicted grain size distribution of an armored bed was found to be in good agreement with measurements from laboratory experiments and field studies. This indicates that the new model is capable of accurately predicting the grain size distribution of an armor layer. Finally, the model was applied to predict the grain size distribution of the bed load and the depth of bed erosion in combination with an existing estimator of the thickness of the active layer in a channel bed. A high friction velocity transported more coarse sediment downstream and resulted in more significant scouring.