The role of velocity, pressure, and bed stress fluctuations in bed load transport over bed forms: numerical simulation downstream of a backward-facing step

M. W. Schmeeckle

doi:10.5194/esurf-3-105-2015

Abstract

Abstract. Bed load transport over ripples and dunes in rivers exhibits strong spatial and temporal variability due to the complex turbulence field caused by flow separation at bedform crests. A turbulence-resolving flow model downstream of a backward-facing step, coupled with a model integrating the equations of motion of individual sand grains, is used to investigate the physical interaction between bed load motion and turbulence downstream of separated flow. Large bed load transport events are found to correspond to low-frequency positive pressure fluctuations. Episodic penetration of fluid into the bed increases the bed stress and moves grains. Fluid penetration events are larger in magnitude near the point of reattachment than farther downstream. Models of bed load transport over ripples and dunes must incorporate the effects of these penetration events of high stress and sediment flux.

Highlights

The details of turbulent flow over dunes and ripples in rivers and oceans have been described by field and laboratory experiments, as well as high-resolution, turbulence-resolving numerical simulations (Shimizu et al, 1999, 2001; Nelson et al, 2006; Zedler and Street, 2001; Omidyeganeh and Piomelli, 2011; Grigoriadis et al, 2009; Stoesser et al, 2008; Chang and Constantinescu, 2013)
The experiments of Nelson et al (1995) simultaneously measuring sediment flux and near-bed fluid velocity over a flat bed and downstream of a backward-facing step showed that the relationship between near-bed fluid Reynolds stress and bed load transport was not simple, and the spatially varying distribution of velocity fluctuations relative to the shear velocity must be considered in formulating transport relationships over ripples and dunes
A local depth-integrated downstream sediment flux, qsx, is calculated by summing the product of each particle volume and velocity that is found in a 0.01 × 0.01 m horizontal area of the bed and dividing by the local bed area

Summary

Introduction

The details of turbulent flow over dunes and ripples in rivers and oceans have been described by field and laboratory experiments (see Best, 2005, for an extensive review), as well as high-resolution, turbulence-resolving numerical simulations (Shimizu et al, 1999, 2001; Nelson et al, 2006; Zedler and Street, 2001; Omidyeganeh and Piomelli, 2011; Grigoriadis et al, 2009; Stoesser et al, 2008; Chang and Constantinescu, 2013). The experiments of Nelson et al (1995) simultaneously measuring sediment flux and near-bed fluid velocity over a flat bed and downstream of a backward-facing step showed that the relationship between near-bed fluid Reynolds stress and bed load transport was not simple, and the spatially varying distribution of velocity fluctuations relative to the shear velocity must be considered in formulating transport relationships over ripples and dunes. They found that there was not a simple monotonic relationship between instantaneous, downstream, near-bed velocity and sediment flux. The hypothesis of Grass and Ayoub (1982) needs significant modification to be useful downstream of separated flows

Methods

Results

Discussion

Conclusion