Plunging Flow and Coherent Turbulent Structures in a Straight Pool-Riffle

Abstract

Pool-riffle units are naturally occurring topographical elements of rivers. Various hypotheses have been proposed to describe how the hydrodynamics in pool-riffle units may lead to scour and sediment transport. However, most hypotheses have focused on time averaged properties, and a physical mechanism for observations of high near-bed velocities has not been proposed. In this study, we apply Large Eddy Simulations to examine the dynamics of flow over straight riffles. We show that a high velocity core of fluid can alternately skim or plunge toward the bed as a function of the Froude number (Fr) at the crest of the riffle, even within a subcritical flow regime. Coherent turbulent structures are generated in the pool and induce high magnitude fluctuations of shear stress and bed pressure. Four different turbulent structures are described and the concept of vortex resistance is used to discuss how turbulence can induce the observed bifurcation in pool hydrodynamics. Despite the simple geometry, the hydraulic scaling means that the results provide a novel physical explanation of near-bed high velocity flow observations in natural pool-riffles. The results are also significant because turbulent shear stresses in the head of a pool exceed what occurs elsewhere in the channel, including the riffles. Further research should examine the conditions under which plunging flow may occur and turbulence may lead to sediment entrainment in other riffle pool geometries.