Physical Study of Flow Velocity and Shear Stress in a Wide Rigid Boundary Channel

Hydraulic characteristics of wide channels differ from narrow channels due to the higher ratios of width to depth, b/h. In this research, using a physical model of a rigid boundary channel having 60m length, 1.5m width and a bed slope of 0.001 with b/h ratio of 12 to 56, hydraulic characteristics of wide channels including: stage-discharge relationship, velocity and shear stress distributions were experimentally considered. The results indicate that the maximum velocity in the wide channel was occurred nearby the water surface. Investigation of the vertical velocity distribution reveals that the flow velocity follows the well-known logarithm distribution law. The also results show that the shear stress was maximized in the centerline of channel section, and the dimensionless shear stress was increased by increasing the ratio of b/h. In ratios of b/h less than 30, the dimensionless bed shear stress value is less than 0.9 and in case of b/h greater than 30, it is greater than 0.9. A comparison of the results for wide channels and channels with an optimal width of b/h=2, reveals that the relationship between b/h and shear stress in narrow channels is linear, while in wide channels a power relationship is governing. Moreover, in wide channel sections, the percentage of shear force on the walls (%SFW), due to the low depth of flow which is less than 10 and negligible, so that to contribute the walls to the shear stress can be ignored for design purposes.