Evolution of Bed Shear Stress in Open-Channel Flow at Smooth-to-Rough and Rough-to-Smooth Transitions

Abstract The evolution of bed shear stress in open-channel flows due to a sudden change in bed roughness was investigated for rough-to-smooth (RTS) and smooth-to-rough (STR) transitions. The velocity field was measured in the longitudinal–vertical plane at high spatial resolution along the centerline of the channel using the particle image velocimetry technique, and the bed shear stress was determined from the measured velocity profiles and flow depths using various methods. For the RTS transition, the bed shear stress adjusted almost immediately to the new bed roughness when the flow depth was essentially constant after the transition but continued to increase under an S2 water surface profile until approximately uniform flow was established downstream. The velocity gradient method, the Spalding wall function, and the logarithmic law yielded similar results for the friction velocity on the smooth bed. For the STR transition, the choice of the displacement height was found to be of critical importance in determining the distribution of bed shear stress on the rough bed, including the overshooting behavior near the roughness discontinuity. A logarithmic region began to form at the transition and grew with distance downstream. The logarithmic law yielded results consistent with those for the RTS transition, and with published data in external and closed conduit flows, when the equivalent roughness of the rough bed was determined separately under uniform flow conditions.