Numerical Investigation of the Morphological Dynamics of a Step-and-Pool Riverbed Using DEM-MPS

Abstract

In mountain streams, riverbeds with a series of coarse gravel steps are common. The sequence flow pattern with a chute and pool is usually formed on series of steps, where a transition region from supercritical to subcritical flow appears between steps. In the transition region, a discontinuous water surface with hydraulic jumps is observed and considerable water surface fluctuations occur. These fluctuations significantly affect the bed material. Accurate measurement of the flow velocities under such violent flow conditions is difficult owing to shallow water conditions, air entrainment, and flow nonlinearity. Therefore, detailed flow structures around the steps were not previously understood. To conduct dynamic investigations of formation/collapse processes of the series of steps, numerical simulations were conducted using the discrete element method (DEM) and the enhanced moving particle semi-implicit (MPS) method. Simulation results suggest that the developed DEM-MPS coupling model will be a promising tool to investigate the morphological dynamics of the step series. Furthermore, the potential relationship between water surface elevation and the collapse of steps is implied by the simulation results.