عنوان مقاله [English]
The importance of sediments studies for safety design of hydraulic structures has been drawn attention by river engineers. Determining the critical shear stress for suspended sediment is of highly significant in sediment hydraulics. Therefore in this research, the deposition process of suspended sediment in the presence of bed sediment has been investigated. Deposition experiments were carried out in the circular flume for three values of shear stress, six types of beds (smooth bed and five coarsegrained sediments) with initial concentrations of 5, 10 and 20 g/l. The results showed that, for the same initial concentration and equal velocity of flume rotation, the deposition of suspended sediments in the bed containing sediments is higher than that of the smooth bed. However, for specific flume rotation velocity, the bed coarse-grained increased the average of flow shear stresses. It was also found that bed sediments generally increased the critical shear stress for all deposition relative to the smooth bed. Accordingly, 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 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.