Velocity fields and particle trajectories for bed load over subaqueous barchan dunes

Abstract

This paper presents an experimental investigation of moving grains over subaqueous barchan dunes that consisted of spherical glass beads of known granulometry. Prior to each test run, a pre-determined quantity of grains was poured inside a closed conduit, and the grains settled on its bottom wall forming one conical heap. As different turbulent water flows were imposed, each heap evolved to a barchan dune, which was filmed with a high-speed camera. An image processing code was written to identify some of the moving grains and compute their velocity fields and trajectories. Our results show that the velocity of grains varies along the barchan dune, with higher velocities occurring close to the dune centroid, and that grains trajectories are intermittent. Depending on the region over the dune, we found that the velocity fields present values within 1 and 10% of the crosssectional mean velocity of the fluid. Considering the average trajectories of grains moving over a given dune, their mean displacement varies within 30 and 60 grain diameters and their characteristic velocities within 10 and 20 % of the cross-sectional mean velocity of the fluid. The displacement time varies between 30 and 90 % of the settling time, and it seems to have two asymptotic behaviors: one close to bed load inception and other far from it. When compared with This is a pre-print of an article published in Granular Matter, 21:75, 2019. The final authenticated version is available online at: http://dx.bed load over a plane bed, we observe that grains have the same mean velocity, but they travel distances up to 5 times larger, with higher densities of moving grains.

General friction law for velocity‐stress dependent phase transition in granular flow

“…Granular materials are ubiquitous and exhibit complex behaviors in a variety of natural phenomena and engineering problems, for example, segregation of nuts, 1,2 rattlesnake motion, 3 hopper flow, 4,5 silo discharge, 4,5 slope collapse, [6][7][8] dune transference, [9][10][11] and avalanche propagation. [12][13][14][15] A granular material is an assembly of distinct particles with diameters greater than 1 μm.…”

An experimental investigation of sediment kinematics and multi‐scale propagation for laboratory bed‐load dunes

“…Ashley et al . (2020) instead took a Lagrangian approach to analyze the probability distributions of travel time and hop distances for bed‐load particles over equilibrium mobile bedforms; Wenzel & De Moraes Franklin (2019), finally, presented results for particle tracking over a barchan dune, that is a different case since it involves migration and lowering of an isolated sediment heap over a smooth bed. However, measurements of the bed‐load sediment motion over bed dunes are still extremely scarce, and frequently limited in space and time.…”

“…Recently, Tsubaki et al (2018) presented detailed measurements of sediment motion patterns over two-dimensional and threedimensional bed-forms; Terwisscha van Scheltinga et al (2018Scheltinga et al ( , 2019) also adopted a Eulerian approach and provided measurements of sediment velocity over the tail of bed-load dunes in a laboratory flume, in an attempt to: "bridge the gap between measurements of bedload transport at the particle-scale and at the bedform-scale". Ashley et al (2020) instead took a Lagrangian approach to analyze the probability distributions of travel time and hop distances for bed-load particles over equilibrium mobile bedforms; Wenzel & De Moraes Franklin (2019), finally, presented results for particle tracking over a barchan dune, that is a different case since it involves migration and lowering of an isolated sediment heap over a smooth bed. However, measurements of the bed-load sediment motion over bed dunes are still extremely scarce, and frequently limited in space and time.…”

Growth of barchan dunes of bidispersed granular mixtures

“…Those studies produced valuable data that increased our understanding of many aspects of barchan morphodynamics, though with limited applicability for bedform evolution over long times and little information at the grain scale. Given the smaller and faster scales of subaqueous barchans, experimental investigations were carried out in water channels and tanks under controlled conditions, from which it was possible to obtain the initial and long-time evolutions of the barchan morphology 8,22 and the motion of individual grains on the barchan surface [23][24][25] . However, although natural beds are composed of polydisperse grains 1,10,11 , the previous experiments investigated barchans of monodisperse particles, the only bidisperse-related references being the experiments of Caps and Vandewalle 26 and Rousseaux et al 27 for size segregation within twodimensional ripples and those of Groh et al 28 for density segregation within two-dimensional dunes.…”