Sand particle velocities over a subaqueous dune slope using high‐frequency image capturing

Abstract

This work presents measurements and analysis of sand particle velocities over a subaqueous dune with median sand diameter of 0.85 mm. Time‐lapse images of the mobile bed and an automated particle image velocimetry (PIV)‐based cross‐correlation method are used to obtain mean velocity of sand particles. This technique is shown to be consistent with measurements obtained with manual tracing. The measurements indicate an increase in mean particle velocity over a dune slope. Three regions are distinguished over the dune slope: (1) region of fluctuating particle velocity, (2) region of increasing particle velocity, and (3) region of maximum particle velocity. The observations are aligned with experimental and numerical modelling studies, indicating fluctuations in flow velocity over a dune stoss slope. We furthermore show that the standard deviation of the mean particle velocity is affected by the slope location and decreases from the lower slope towards the upper slope. The particle velocity variability is discussed in the context of general onset and cessation of sediment transport, the effect of the reattachment zone, sweep‐transport events, and the existence of superimposed bedforms. With this work we bridge the gap between measurements of bedload transport at the particle‐scale and at the bedform‐scale. © 2019 John Wiley & Sons, Ltd.

Probability Distributions of Particle Hop Distance and Travel Time Over Equilibrium Mobile Bedforms

“…Tracked particle paths reveal significant qualitative differences between the plane bed and bedform experiments. Notably, particle behavior clearly depends on position relative to bedform features in a manner that is reminiscent of the backward facing step experiments of Leary and Schmeeckle (2017) and the particle velocity fields reported by Tsubaki et al (2018) and Terwisscha van Scheltinga et al (2019). Particle transport direction is highly variable in the region of flow separation immediately downstream of the bedform crest.…”

“…(2018) and Terwisscha van Scheltinga et al. (2019). Particle transport direction is highly variable in the region of flow separation immediately downstream of the bedform crest.…”

“…Previous studies of particle motion find that particle velocities are conditionally dependent on the local topographic configuration due to the coupling of topography, flow, and sediment transport (Terwisscha van Scheltinga et al., 2019; Tsubaki et al., 2018). Topographically induced correlations in flow velocity exist over spatial scales that are comparable to the bedform length; in contrast, we find that the average hop distance is much smaller than a bedform length.…”

“…As an example, consider a particle that is entrained on a stoss slope that is oriented obliquely relative to the mean flow direction. This topographic configuration usually results in flow being redirected laterally (Best, 2005; Venditti et al., 2005b), causing a corresponding lateral component of sediment movement (Tsubaki et al., 2018; Terwisscha van Scheltinga et al., 2019) that is possibly amplified by gravitational effects (Parker et al., 2003). Because particle motions are short relative to the spatial scales of topography, this particle is likely to spend the entire interval from entrainment to disentrainment on this oblique slope.…”

A PIV-based method to measure spatial gradients in bedload transport over a dune

“…The PIV-based measurements of mean particle velocity and measurements of mean particle activity from IoD of the bedload bridge the gap between individual particle motion studies and (non-local) sediment transport flux measurements. The measurements of the streamwise particle velocities over the dune slope showed an increase in mean streamwise particle velocity from the lower slope (0.06 m/s) to the dune crest (0.095 m/s) [21]. In Fig.…”