Abstract
River engineers have long been challenged by the need to predict sediment transport, especially over armored riverbeds. This study investigates the statistical properties of bed load transport over an armored bed layer with cluster microforms in laboratory experiments. Particle clusters on the sediment bed were formed by widely graded particles under constant flow. A series of key kinematic parameters computed from particle trajectories recorded by a digital camcorder, including mean squared particle displacement (MSD), particle number activity, particle velocities, step length, and rest period, were analyzed. The scaling growth of the MSD with time showed that the particle diffusion regime was superdiffusive at small time scales, but became subdiffusive at larger time scales. The particle number activity follows a negative binomial distribution, and the probability distributions of streamwise and transverse particle velocities displayed heavy asymptotic tails, which indicates the particle clusters might exert a dual impact on bed load transport: some particles are accelerated in the preferential paths between particle clusters, while others were obstructed by the particle clusters. In addition, the bed load diffusion regime varied with observation time scales. The findings of this study can gain insight into the bed load transport processes over armored riverbeds.
Highlights
In nature, a gravel stream bed that has a wide particle size distribution often develops an armored surface layer with various cluster microforms [1,2,3]
We argue that the particle clusters over armored bed layers exert a impact on bed load sediment particles: some particles are accelerated in the preferential paths between dual impact on bed load sediment particles: some particles are accelerated in the preferential paths particle clusters, while others were obstructed by the clusters to produce negative streamwise particle between particle clusters, while others were obstructed by the clusters to produce negative velocities
This study aims to explore the stochastic nature of bed load transport over an armored bed with cluster microforms
Summary
A gravel stream bed that has a wide particle size distribution often develops an armored surface layer with various cluster microforms [1,2,3]. Over the last two decades, river engineers have been challenged by the need to predict sandy sediment transport over armored gravel beds. Such phenomena are often observed in streams after dam removal or wildfires [5]. Despite the substantial efforts, including hiding-exposing correction [6] and surface-based transport mechanisms [7], our ability to estimate bed load flux on mixed-size gravel beds remains unsatisfactory [8]. A major obstacle is to quantify the impact of particle clusters on bed load transport
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