Distribution of Tractive Force in Open Channels

The designs of alluvial channels by the tractive force method requires information on the distribution of wall shear stress over the wetted perimeter of the cross-section. The present study was undertaken in order to provide some details on actually measured shear distributions and, hence, to check the validity of currently available design information. The latter is entirely analytical in origin and is based either on the assumption of laminar flow or on over-simplified models of turbulent channel flow. The experiments were carried out in a smooth-walled laboratory flume at various aspect ratios of the rectangular cross-section. Wall shear stress was determined with Preston tubes which were calibrated by a method exploiting the logarithmic form of the inner law of velocity distribution. Results are presented which clearly suggest that none of the present analytical techniques can be counted upon to provide any precise details on tractive force distributions in turbulent channel flow.