Jamming in the pocket: the rhythmic interactions between turbulence, bed structure, and sediment motion

Abstract:

Thresholds of sediment motion are integral to bedload flux and channel evolution estimates. The onset of sediment motion can temporally vary in a given river because of the effects of long-term flow history on broad-scale particle arrangement (e.g. protrusion, orientation). However, the interplay between short-term turbulence fluctuations, local particle arrangement, and sediment motion are relatively unexplored. We used a Discrete Element Method model of a test sphere in a fixed pocket surrounded by smaller mobile spheres. All initial sphere positions were identical between simulation runs. We applied either constant or fluctuating external forces on the test sphere in each run; the same probability density function (pdf) of fluctuating forces was employed with only the force sequence differing between runs. For the constant driving force runs, application of driving forces slightly greater than the critical value (needed for motion) resulted in bed shear strengthening and slower test sphere motion. For the fluctuating driving force runs, the test sphere either moved or remained stuck (jammed) in its pocket depending on the applied force sequence. Both shear strengthening and particle jamming were explained by subtle changes in the test-sphere pocket geometry (e.g. contact number), which were driven by the slight rearrangement of the smaller spheres. Our results demonstrate that: 1) the exact sequence of turbulence fluctuations can influence particle pocket geometry and whether particle motion occurs, and 2) randomly sampled applied forces from a pdf may not capture these complex interactions. Temporal variations in the onset of motion are not only driven by long-term flow histories that cause larger scale bed changes but also by small changes to particle pocket geometries that may be driven by turbulence fluctuations.

To help expand equity, diversity, and inclusion in STEM fields, we helped to develop and provide scholarships for Women Outdoors with Science Camps for middle and high school students at the University of Idaho McCall Outdoor Science School. Activities included exploration of STEM identity, visits with women scientists who discussed career paths and their science, multi-day river rafting, measurements of water quality and stream conditions, and student presentations.