EXPERIMENTAL INVESTIGATION OF COHESIVE SEDIMENTS EROSION IN THE PRESENCE BED COARSE-GRAIN SIZE SEDIMENTS

This paper studies the erosion of cohesive sediments in case of bed having coarse-grain size. Experiments were conducted in an annular flume with six different bed material grain sizes (D50=2.8 mm to D50=48 mm) at five different shear stresses and 24 hours of self-consolidated cohesive sediment placed on the bed material. The hydraulic parameters were measured using an Acoustic Doppler Velocimeter (ADV). The results showed that despite increase in flow shear stress in bed containing coarse grains, the amount of erosion of deposited cohesive sediments on the bed materials significantly decreased in comparison with the case of the smooth bed. Final concentrations of eroded cohesive sediment in the flow for smooth bed and coarse-grain size bed (D50 = 48.2 mm) were observed 47 and 1.9 g/l, respectively, showing that the coarse-grain size bed caused a reduction in the rate of cohesive sediment erosion up to 96% in comparison with the smooth bed, while the shear stress in case of the rough bed was 5.5 times more than that of smooth bed. The significant reduction in the erosion rate might be because of cohesive sediment trapping inside the void space of the bed materials so that the trapped fine sediment would not be re-suspended into the flow. In this work, the critical shear stress for erosion of deposited cohesive sediment was obtained as 0.12 N/m2 for smooth bed and 0.16 and 0.92 N/m2 for bed material with grain sizes of 2.8 and 48 mm, respectively.