Toward flow forces acting on a step-pool unit

The flow forces on step-pool units are important to understand the physical processes and flow resistance partitioning in step-pool channels, and build the basis for better prediction of channel evolution and more advanced design of artificial step-pool system. However, the flow forces acting on step-pool units are understudied and poorly understood. To fill this knowledge gap, we applied the approach combining physical experiment and computational fluid dynamics simulation to a step-pool unit made of natural grains at six flow conditions. The topography of the step-pool unit was split into topography components (TCs) covering the entire unit length with the same width. The flow forces from both pressure and shear stress in XYZ directions were examined for the TCs. The results illustrate significant transverse variability of the flow forces from both the shear stress and pressure at all the three directions. The flow forces in both X and Y directions are closely related to the flow structures and morphology in the unit. The ratios between skin and form drag have large variations at low flows while show a relatively limited range of 0.05–0.1 at high flows, suggesting a small proportion occupied by the skin resistance in the total flow resistance in the step-pool channel. The drag and lift coefficient generally increase with discharge and the drag coefficient of the unit is around 0.3 at high flows, which can be used in evaluating the stability of the step-pool units in a sequence.