Investigating the rate of cohesive sediments deposition using a rotational flume

Aggregation of cohesive sediments increases the rate of sediment deposition. To investigate the cohesive sediment deposition processes, experiments were conducted in a rotational flume in the Hydraulic Laboratory of Shahrekord University. Sediment samples were taken from a small dam reservoir located near Shahrekord city (as a subbasin of north Karun watershed). The sediments were composed of 31% clay, 58% silt and 11% sand. For each experiment, the velocity and shear stress profiles were measured using acoustic doppler velocimeter (ADV) and suspended sediment concentration was measured using drying and weighting method. Duration of each experiment was 180 minutes and the required time to reach the equilibrium concentration ranged from 30 to 60 minutes. The results show that a linear relationship exists between the bed shear stress and the relative equilibrium concentration. Based on the time variations of the sediment concentration, the deposition rate of the sediments was calculated and a parabolic relationship was derived between the deposition rate of sediments and the equilibrium relative concentration. The threshold of non-settling of sediments has also been investigated. Minimum bed shear stress at threshold deposition of sediment for initial sediment concentration of 5, 10 and 20 gm/l were 0.827, 0.938 and 0.98 N/m2, respectively. This means that the minimum shear stress directly depends on the initial concentration of the sediment.