Vortex Trapping of Suspended Sand Grains Over Ripples.

Coastal hydrodynamics and morphodynamics integrate the effects of small‐scale fluid‐sediment interactions; yet, these small‐scale processes are not well understood. To investigate sediment trapping by turbulent coherent structures or vortices, the transport of coarse sand over ripples was analyzed in a small‐oscillatory flow tunnel with phase‐separated Particle Image and Tracking Velocimetry. Results from one of the first direct measurements of vortex‐trapped sand grains under oscillatory flows are presented. The vortices mobilized sand grains along the ripple slopes just prior to flow reversal and transported the suspended sediment grains. During several flow cycles, some sand grains were temporarily trapped in the vortex, prescribing semi‐circular trajectories off‐center from the vortex core in quadrants of the vortex that were closest to the ripple slope, as illustrated by Nielsen (1992, https://doi.org/10.1142/1269). Comparisons of the horizontal sediment grain velocity with the horizontal fluid velocity yielded a linear relationship with a slope of 0.87. The vertical grain velocities also varied linearly with the vertical fluid velocity with a slope of approximately 1 and an offset of −0.08 m s−1 ${\mathrm{s}}^{-1}$. The offset is close to the still water settling velocity for coarse sand grains, as hypothesized during vortex trapping. Additionally, estimates of the off‐center distance, between the centers of the semi‐circular sediment paths and vortex cores, compared well with the ratio of the settling velocity to the radian frequency of the vortex yielding a linear regression slope of 0.99. Improved understanding of vortex trapping effects on sediment dynamics may decrease uncertainty in model predictions of large‐scale coastal hydrodynamics and sediment transport. Plain Language Summary: Large‐scale coastal processes integrate the effects of small‐scale fluid‐sediment interactions; yet, these small‐scale processes are not well understood. Nielsen (1992, https://doi.org/10.1142/1269) hypothesized that suspended sand grains over ripples could get caught in a vortex under certain hydrodynamic conditions, in a process called vortex trapping. In this article, we present results from one of the first direct measurements of vortex‐trapped sand grains under oscillatory flows. Vortices along the ripple slope mobilized sand grains prior to flow reversal and transported the suspended grains. During several flow cycles, some sand grains were temporarily trapped in the vortex and moved along semi‐circular paths off‐center from the vortex core in sections of the vortex that were closest to the ripple slope, as illustrated by Nielsen (1992, https://doi.org/10.1142/1269). Comparisons of the sediment grain velocity with the fluid velocity yielded a linear relationship. Additionally, the distance between the center of the semi‐circular sediment paths and the center of the vortex cores agreed with the theory. Improved understanding of the effects of vortex trapping on sediment dynamics may decrease uncertainty in the predictions of large‐scale coastal hydrodynamics and sediment transport models, which do not typically account for this small‐scale process. Key Points: Observations of vortex‐trapped grains suggest delayed settling of advected grains, as well as delayed advection of grains mobilized from the bedQuantitative comparisons of vortex‐trapped sand grains compared well with theoretical formulations by Nielsen (1992, https://doi.org/10.1142/1269) for a forced vortexImproved understanding of vortex trapping effects on sediment dynamics may decrease uncertainty in large‐scale coastal model predictions [ABSTRACT FROM AUTHOR]