Direct skin friction measurements and stress partitioning over movable sand ripples

Abstract

The separation of skin friction from total boundary layer shear stress for flows over bed forms is critical in erosion and sediment transport studies. Close‐to‐bed velocity profiles and skin friction shear velocities have been measured over mobile sand ripples of different geometries in a series of flume experiments to investigate the effects of form drag and flow stress partitioning. As expected, higher and longer ripples produce stronger form drag and hence higher total shear velocities for a given mean flow velocity, but the skin friction shear velocity was found to decrease with the increase in ripple height and length. For a fixed ripple geometry, the ratio of the skin friction to the total shear stress is also found to increase with the flow strength. Results of the present study together with data from previous investigations are used to test and extend the stress partition model of Smith and McLean (1977) for rippled beds. The Smith and McLean model is found to give reasonable partition of the skin friction from the total shear stress over mobile sand ripples. Overall empirical relationships based on total shear velocity and ripple geometry have also been obtained to understand the mechanisms of skin friction variation and to simplify the stress partitioning in bottom boundary layer flows over sand ripples.