An explanation of the internal flow field of stream potholes from the perspective of fluid mechanics

Numerous questions concerning the formation mechanisms of stream potholes remain unresolved. To address these issues, research on stream potholes must be carried out with adequate methodological transparency in order to explicitly elucidate the hydrodynamic mechanisms occurring within the potholes. This study numerically simulates vortices within stream potholes developed in open-channel flows by employing a fluid-mechanical model comprising a three-layered cylindrical container that represents the atmosphere, the stream channel, and the potholes, respectively. Simulation results demonstrate that the flow field structure of vortex water in potholes with varying geometries (hemispherical and cylindrical shapes) and dimensions (different aspect ratios) is nearly identical, arising from a combined effect of horizontal rotational and vertical secondary flows. These swirling flow patterns regulate the movement of particles within potholes, determine the distribution of shear stresses along the sidewalls and base, and simultaneously influence morphological changes in the potholes. This contributes to elucidating the underlying mechanisms of stream pothole formation and addressing key uncertainties associated with their growth. Field evidence is utilized to substantiate the simulation results.