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

Abstract

Particle dynamics are investigated experimentally in supercritical high‐speed open channel flow over a fixed planar bed of low relative roughness height simulating flows in high‐gradient non‐alluvial mountain streams and hydraulic structures. Non‐dimensional equations were developed for transport mode, particle velocity, hop length and hop height accounting for a wide range of literature data encompassing sub‐ and supercritical flow conditions as well as planar and alluvial bed configurations. Particles were dominantly transported in saltation and particle trajectories on planar beds were rather flat and long compared with alluvial bed data due to (1) increased lift forces by spinning motion, (2) strongly downward directed secondary currents, and (3) a planar flume bed where variation in particle reflection and damping effects were minor. The analysis of particle saltation trajectories revealed that the rising and falling limbs were almost symmetrical contradicting alluvial bed data. Furthermore, no or negligible effect of particle size and shape on particle dynamics were found. Implications of experimental findings for mechanistic saltation‐abrasion models are briefly discussed. Copyright © 2017 John Wiley & Sons, Ltd.

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

“…In Figure (a), the particle impact and rebound angles are presented. The long and flat trajectories disclosed by Auel et al () are also evident here revealing low values of the impact and rebound angles of γ im = 3.1 ± 0.8° and γ out = 4.0 ± 1.2°, respectively. The rebound angle is always slightly larger than the impact angle independent of particle size and material, which has also been found in the data of Niño et al () despite the large data scatter ( γ im = 18 ± 10°, γ out = 30 ± 20°) due to their mobile alluvial bed conditions (0.21 ≤ k s / h ≤ 0.38).…”

“…In Figure (b), Equation is plotted for comparison revealing that it results in 3.2 to 8.4 times higher values compared with Equation in the range 10 0 ≤ T* ≤ 10 2 . The reason for the differences is the assumption of the limb ratio in Equation , which does not account for relatively flat particle trajectories as found by Auel et al ().…”

“…Consequently, different physical effects seem to cancel each other: The faster the flow velocity the faster the particle impact velocity (Figure (a)). However, both particle impact angles decrease (Figure (b)) and particle saltation lengths increase (Auel et al , Table ) with increasing flow velocity opposing the increasing impact energy, that is, particles impact faster but with milder angles and less often.…”

“…It was found that the particle impact velocity linearly scales with the average particle velocity and the vertical impact velocity increases with excess transport stage. Particle impact and rebound angles are low and decreased with transport stage supporting the finding of flat and long trajectories (Auel et al , ). Little damping effects but high normal restitution coefficients exceeding unity (on average e z = 2.4) were found.…”

“…The goals of the present study are to understand particle dynamics and their impacts on the bed and to further develop abrasion prediction models applicable to both bedrock and man‐made hydraulic structures. To achieve these goals, the flow turbulence characteristics (Auel et al ., ), particle dynamics (Auel et al ., ) and impact characteristics (present manuscript) in high‐speed flows were systematically investigated by means of laboratory experiments.…”