Experimental Study of the Coarse Surface Development Effect on the Bimodal Bed-Load Transport under Unsteady Flow Conditions

Gravel-bed rivers display a surface layer that is coarser than the substrate lining below this armored surface. The armored layer that is developed under certain flow conditions may be attributed as water-worked sediment bed. Both the threshold of incipient motion and the bed-load transport are affected by the presence of this layer. In this study, the coarse surface development and its effect on the sediment transport were investigated experimentally in a rectangular flume. Four antecedent flow rates were combined with an input hydrograph, without sediment feeding. The reference shear stress of individual fractions and the grain-size distribution were determined after the development of the coarse surface due to the antecedent flow. Then a triangular-shaped hydrograph was generated for both the intact surface and the coarse surface cases. It was seen that the maximum bed-load transport values obtained in unsteady flow experiments were highly dependent on the coarse surface formed by the antecedent flow. The interrelation between the armor ratio and the total bed load was also sought. It was revealed that there existed a nearly linear relation between the armor ratio and the dimensionless total bed load with a correlation coefficient of 0.99. This strong interdependence implies that the knowledge of the armor ratio is of basic and utmost importance to predict accurately the bed load to be transported. The values of the Einstein bed-load transport parameter were plotted versus those of the dimensionless shear stress. When the global sediment transport was considered, a clockwise hysteresis appeared in the case of the initial intact bed surface, whereas a counterclockwise hysteresis arose when the coarse surface was developed. As for the fractional sediment transport, in the case of the intact initial surface the clockwise hysteresis was encountered more frequently, but for the armored bed experiments the coarser the bed surface, the more dominant was the counterclockwise hysteresis, meaning that the quantity transported during the falling limb of the hydrograph was higher than that transported during the rising limb.