تنش برش بحراني فرسايش رسوبات چسبنده و نقش آن در طراحي كانالهاي روباز

Critical Shear Stress of Erosion of Cohesive Sediment and its Trace in Design of Open Channels

Resistance to erosion in cohesionless sediment is a result of gravity only, but erosion of cohesive sediment is quite different. Before cohesive sediment can be eroded, the interparticle bond must be broken. Thus, a critical shear stress has to be exceeded before erosion of cohesive sediment can occur. The objective of this study is to determine the critical shear stress of erosion of cohesive sediment collected from bed of six khozestan irrigation channels, including Dasht-Evan, Hendijan, Veis, Shadegan, khalaf-Abad and Voseyleh. Experimental results are shown that shear stress of erosion for these channels varied from 0.96 N/M2 in Shadegan Channel, to 2.3 N/M2 in Hendijan Channel. Comparing the design shear stress of three of them, (Veis, Dasht-Evan, and Chamran(Voseyleh)) with critical shear stresses of erosion obtain from experimental study indicate that for Veis and Dasht-Evan Channels the design values of shear stress are less than the values need to eroded the deposited sediment. Only for Chamran Channel the design shear stress is more than the critical shear stress. The main point of this study is that for designing channel in the area with fine particle size, it is very important to find the critical shear stress of erosion and use this value for correct hydraulic structures design.