Validating a universal model of particle transport lengths with laboratory measurements of suspended grain motions
TLDR
In this article, a continuous model is presented that predicts particle travel distances for saltation and suspension based on Rouse number and relative grain roughness using a series of 8 video cameras in a plexiglass flume.Abstract:
The mechanics of sediment transport are of fundamental importance for fluvio-deltaic morphodynamics. The present study focuses on quantifying particle motions and trajectories across a wide range of flow conditions. In particular, a continuous model is presented that predicts particle travel distances for saltation and suspension based on Rouse number and relative grain roughness. By utilizing a series of 8 video cameras in a plexiglass flume direct measurements of the distributions of particle travel distances (excursion lengths) were obtained. To this end, experiments were carried out in dark under black lights with fluorescent painted plastic and quartz sand particles. For relatively high Rouse numbers indicating bed load dominant transport regime (P ≥ 2.5), particle motion is governed by the effect of gravitational forces (settling velocities) and measured excursion lengths closely follow a Gaussian distribution. For P = 2.5, particle motion is equally subjected to both gravitational and turbulent forces. Consequently, measured excursion lengths exhibit a bi-modal distribution with two distinct peaks. As turbulent fluctuations increase and dominate particle motion over gravity (P < 2.5), distributions of excursion lengths become unimodal and negative-skewed with mean values deviating from the modes. The predicted trend of linearly increasing excursion lengths with decreasing Rouse numbers is consistent with measured excursion lengths across a wide range of Rouse numbers (P = 1.8 – 8.9). Furthermore, measured excursion lengths are observed to fit within the predicted range of excursion lengths with no significant difference between measured excursion lengths of plastic and quartz sand particles.read more
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Journal ArticleDOI
Methods of Digital Video Microscopy for Colloidal Studies
John C. Crocker,David G. Grier +1 more
TL;DR: In this article, a set of image processing algorithms for extracting quantitative data from digitized video microscope images of colloidal suspensions is described, which can locate submicrometer spheres to within 10 nm in the focal plane and 150 nm in depth.
Journal ArticleDOI
Calculations of the Critical Shear Stress for Motion of Uniform and Heterogeneous Sediments
TL;DR: In this article, an expression for the critical shear stress of noncohesive sediment is derived from the balance of forces on individual particles at the surface of a bed, where the initial motion problem for mixed grain sizes additionally depends on the relative protrusion of the grains into the flow and the particle angle of repose.
Journal ArticleDOI
Erosion And Transport Of Bed-Load Sediment
TL;DR: In this article, a general derivation is given of the macro-equations of mass and linear-momentum balance that govern the mo'mentum transfer from a Newtonian fluid to rigid particles in a fluid-solid mixture.
Journal ArticleDOI
The nature of saltation and of ‘bed-load’ transport in water
TL;DR: In this paper, it has been shown that suspension by fluid turbulence of mineral solids larger than those of medium sands does not become appreciable until the bed shear stress is increased to a value exceeding 12 times its threshold value for the bed material considered.
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Bedload transport of fine gravel observed by motion-picture photography
Abstract: Motion pictures taken at Duck Creek, a clear stream 6.5 m wide and 35 cm deep near Pinedale, Wyoming, provide detailed, quantitative information on both the modes of motion of individual bedload particles and the collective motions of large numbers of them. Bed shear stress was approximately 6 Pa (60 dynes cm−2), which was about twice the threshold for movement of the 4 mm median diameter fine gravel bed material; and transport was almost entirely as bedload. The displacements of individual particles occurred mainly by rolling of the majority of the particles and saltation of the smallest ones, and rarely by brief sliding of large, angular ones. Entrainment was principally by rollover of the larger particles and liftoff of the smaller ones, and infrequently by ejection caused by impacts, whereas distrainment was primarily by diminution of fluid forces in the case of rolling particles and by collisions with larger bed particles in the case of saltating ones. The displacement times averaged about 0.2−0.4 s and generally were much shorter than the intervening repose times. The collective motions of the particles were characterized by frequent, brief, localized, random sweep-transport events of very high rates of entrainment and transport, which in the aggregate transported approximately 70% of the total load moved. These events occurred 9% of the time at any particular point of the bed, lasted 1–2 s, affected areas typically 20–50 cm long by 10–20 cm wide, and involved bedload concentrations approximately 10 times greater than background. The distances travelled during displacements averaged about 15 times the particle diameter. Despite the differences in their dominant modes of movement, the 8–16 mm particles typically travelled only about 30% slower during displacement than the 2–4 mm ones, whose speeds averaged 21 cm s−1. Particles starting from the same point not only moved intermittently downstream but also dispersed both longitudinally and transversely, with diffusivities of 4.6 and 0.26 cm2 s−1, respectively. The bedload transport rates measured from the films were consistent with those determined conventionally with a bedload sampler. The 2–4 mm particles were entrained 6 times faster on finer areas of the bed, where 8–16 mm particles covered 6% of the surface area, than on coarser ones, where they covered 12%, even though 2–4 and 4–8 mm particles covered practically the same percentage areas in both cases. The 4–8 and 8–16 mm particles, in contrast, were entrained at the same rates in both cases. To within the statistical uncertainty, the rates of distrainment balanced the rates of entrainment for all three sizes, and were approximately proportional to the corresponding concentrations of bedload.