Sediment transport in high‐speed flows over a fixed bed: 2. Particle impacts and abrasion prediction

Abstract

Single bed load particle impacts were experimentally investigated in supercritical open channel flow over a fixed planar bed of low relative roughness height simulating high‐gradient non‐alluvial mountain streams as well as hydraulic structures. Particle impact characteristics (impact velocity, impact angle, Stokes number, restitution and dynamic friction coefficients) were determined for a wide range of hydraulic parameters and particle properties. Particle impact velocity scaled with the particle velocity, and the vertical particle impact velocity increased with excess transport stage. Particle impact and rebound angles were low and decreased with transport stage. Analysis of the particle impacts with the bed revealed almost no viscous damping effects with high normal restitution coefficients exceeding unity. The normal and resultant Stokes numbers were high and above critical thresholds for viscous damping. These results are attributed to the coherent turbulent structures near the wall region, i.e. bursting motion with ejection and sweep events responsible for turbulence generation and particle transport. The tangential restitution coefficients were slightly below unity and the dynamic friction coefficients were lower than for alluvial bed data, revealing that only a small amount of horizontal energy was transferred to the bed. The abrasion prediction model formed by Sklar and Dietrich in 2004 was revised based on the new equations on vertical impact velocity and hop length covering various bed configurations. The abrasion coefficient kv was found to be vary around kv ~ 105 for hard materials (tensile strength ft > 1 MPa), one order of magnitude lower than the value assumed so far for Sklar and Dietrich's model. Copyright © 2017 John Wiley & Sons, Ltd.

Alluvial cover controlling the width, slope and sinuosity of bedrock channels

“…3) that includes both tools and cover effects and that is consistent with current process understanding (e.g. Beer et al, 2017;Fuller et al, 2016;Sklar and Dietrich, 2004), as well as quantitative field and laboratory measurements (Auel et al, 2017;Beer and Turowski, 2015;Chatanantavet and Parker, 2009;Inoue et al, 2014;. The model presented here thus improves upon existing 1-D reach-scale models both in the plausibility of the underlying assumptions and, as has been shown in Sect.…”

“…As before, Q s is the upstream bedload sediment supply. Note that in the original saltation-abrasion model (Sklar and Dietrich, 2004), k depends explicitly on hydraulics, but, consistently, in all of the field and laboratory studies where all relevant parameters have been measured, this dependency has not been found (Auel et al, 2017;Beer and Turowski, 2015;Chatanantavet and Parker, 2009;Inoue et al, 2014;. At steady state, C = C c .…”

“…2), we can relate channel width to vertical erosion rate using one of the established models for incision (e.g. Auel et al, 2017;Sklar and Dietrich, 2004). I assume a sediment-flux-dependent erosion law, including tools and cover effect, of the form…”

“…Based on laboratory flume measurements, Auel et al (2017) gave an equation (their Eq. 19) for particle speed as a function of shear stress τ , including various previous measurements, both over bedrock and alluvial beds.…”

Sediment transport in high‐speed flows over a fixed bed: 1. Particle dynamics

“…The present study analyses particle transport modes, velocities, hop lengths and hop heights. Particle impact velocities and impact angles, Stokes number, restitution and dynamic friction coefficients and their application for an abrasion prediction model are separately covered in an accompanying paper (Auel et al 2017).…”