The motion of bedload particles is diffusive and occurs within at least three scale ranges: local, intermediate and global, each of which with a distinctly different diffusion regime. However, these regimes, extensions of the scale ranges and boundaries between them remain to be better defined and quantified. These issues are explored using a Lagrangian model of saltating grains over the uniform fixed bed. The model combines deterministic particle motion dynamics with stochastic characteristics such as probability distributions of step lengths and resting times. Specifically, it is proposed that a memoryless exponential distribution is an appropriate model for the distribution of rest periods while the probability that a particle stops after a current jump follows a binomial distribution, which is a distribution with lack of memory as well. These distributions are incorporated in the deterministic Lagrangian model of saltating grains and extensive numerical simulations are conducted for the identification of the diffusive behavior of particles at different time scales. Based on the simulations and physical considerations, the local, intermediate, and global scale ranges are quantified and the transitions from one range to another are studied for a spectrum of motion parameters. The obtained results demonstrate that two different time scales should be considered for parameterization of diffusive behavior within intermediate and global scale ranges and for defining the local–intermediate and intermediate–global boundaries. The simulations highlight the importance of the distributions of the step lengths and resting times for the identification of the boundaries (or transition intervals) between the scale ranges.
Bibliographical noteAcknowledgments The authors are grateful to the reviewers for thorough reviews, constructive comments and useful suggestions that have been gratefully incorporated in the final manuscript. Funding for this research was provided in part by the Institute of Geophysics of the Polish Academy of Sciences through the Project for Young Scientists No. 16/IGF PAN/2011/Mł ‘‘Dynamics and topography of riverbed forms: an analysis of experimental data and modelling of sediment transport in the light of Einstein’s theory’’, by Ministry of Sciences and Higher Education within statutory activities No. 3841/E-41/S/2015, and by
EPSRC, UK (EP/G056404/1) within the project ‘‘High-resolution numerical and experimental studies of turbulence-induced sediment erosion and near-bed transport.’’
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- Lagrangian model
- Particle diffusion
- Sediment transport