Bed Load Pickup Rate and Flow Resistance for Turbid Flow on a Movable Plane Bed

Abstract

This study investigates bed load transport and flow resistance in alluvial channels that contain fine suspended sediment. In all, 123 samples were run in a laboratory flume over uniform bed materials. Different hydraulic conditions were considered in an 8.0 m-long, 0.255 m-wide, and 0.6 m-deep tilting flume. Unit flow discharge ranged from 3.176 × 10−3 to 18.640 × 10−3 m2 s−1, and the slope gradient varied from 2% to 5%. Two sediment-size classes (median diameter, d50, of 1.7 and 3.29 mm) and five suspended load concentrations (10, 20, 30, 40, and 50 g/L) were used. A preliminary analysis showed that the bed load pickup rate increases with an increase in the concentration of suspended particles. Dimensional analysis, which was performed satisfactorily on the bed load transport data, indicated that shear stress is the dominant parameter in bed load pickup. Moreover, the resistance of turbid flow in a channel with an erodible bed was studied. The net results were found to be an increase in the flow resistance over a movable plain bed by increasing suspended load concentration. In other words, increasing suspended particles increased the mobility of bed material and led to a higher friction factor. Special attention was paid to the separation of the friction coefficient between boundary resistance and mobile-bed resistance caused by the mobility of bed materials. The results showed that the damping effect of suspended sediment caused lower boundary resistance with increasing suspended load concentration. On the other hand, suspended particles increased the mobility of bed materials and led to a higher mobile-bed resistance. Analysis indicates that the relative importance of mobile-bed resistance is more than the boundary friction for the high bed load transport rate. Therefore, the damping effect of the suspended sediment on boundary friction can only be observed for low sediment transport or a fixed bed.