Experimental Investigation of The Effect of Bed’s Coarse Grain Sediments on the Critical Shear Stress for Deposition of Suspended Sediments

In this research, the deposition process of suspended sediment in presence of bed sediment has been investigated. The experiments were carried out in the circular flume for three values of shear stress, six types of beds (one smooth bed and five coarse-grained sediments) with initial concentrations of 5, 10 and 20 g/l. The results showed that, for the same initial concentration and same rotation velocity of flume, the deposition of suspended sediments in the bed containing sediments is higher than the smooth bed. However, for specific flume rotation velocity, the bed coarse-grained increased the average of flow shear stresses. Also, the results showed that the bed sediments generally increased the critical shear stress for all deposition relative to the smooth bed. It can be explained that in the bed with coarse grain sediments, in the flow with bigger turbulence, full deposition conditions for suspended sediments still exist. The results of this study showed that for threshold critical shear stress in a smooth bed, for shear stress of less than 1.28 N/m2, sediments are in the state of the deposition threshold, but for bed containing coarse grain sediment was observed in which the suspended sediments was deposited in each flow shear stress and trapped among bed sediments. Also, it can be stated that due to the phenomenon of trapping suspended sediments in the bed containing coarse grain sediments, cannot be considered threshold critical shear stress for deposition of suspended sediments.