Fluid‐Driven Transport of Round Sediment Particles: From Discrete Simulations to Continuum Modeling. Issue 7 (5th July 2022)

• Record Type: Journal Article
• Authors: Zhang, Qiong; Deal, Eric; Perron, J. Taylor; Venditti, Jeremy G.; Benavides, Santiago J.; Rushlow, Matthew; Kamrin, Ken
• Is Part Of: Journal of geophysical research. Volume 127:Issue 7(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-07-05
• Subjects: sediment transport -- discrete simulations -- continuum modeling -- dimensional analysis; Geomorphology -- Periodicals; 551.3
• DOI: 10.1029/2021JF006504
• Languages: English
• ISSNs: 2169-9003

Abstract

Abstract: Bedload sediment transport is ubiquitous in shaping natural and engineered landscapes, but the variability in the relation between sediment flux and driving factors is not well understood. At a given Shields number, the observed dimensionless transport rate can vary over a range in controlled systems and up to several orders of magnitude in natural streams. Here, we (a) experimentally validate a resolved fluid‐grain numerical scheme (Lattice Boltzmann Method‐Discrete Element Method or DEM‐LBM), and use it to (b) explore the parameter space controlling sediment transport in simple systems. Wide wall‐free simulations show the dimensionless transport rate is not influenced by the slope, fluid depth, mean particle size, particle surface friction, or grain‐grain damping for gentle slopes (0.01–0.03) at a medium to high fixed Shields number. (c) Examination of small‐scale fluid‐grain interactions shows fluid torque is non‐negligible for the entrainment and sediment transport near the threshold. And (d) the simulations guide the formulation of continuum models for the transport process. We present an upscaled two‐phase continuum model for grains in a turbulent fluid and validate it against bedload transport DEM‐LBM simulations. To model the creeping granular flow under the bed surface, we use an extension of the Nonlocal Granular Fluidity model, which was previously shown to account for flow cooperativity from grain‐size‐effects in dry media. The model accurately predicts the exponentially decaying velocity profile deeper into the bed. Plain Language Summary: Sediment transport caused by particles rolling, sliding, and hopping on a river bed is called bedload transport. Semi‐empirical formulas to predict bedload sediment flux from the driving factors, known as the transport relation, can be highly inaccurate. Simulations where the sediment particles are fully resolved are carried out to find if the predictions can be improved by considering more particle parameters. After being validated against flume experiments, the numerical scheme is used to simulate bedload transport under many conditions, and its results show that at a fixed relative hydrodynamic driving force, varying river slope (on gentle slopes), fluid depth, mean particle size, particle surface sliding friction coefficient, and grain‐grain damping coefficient cause almost no variation of the transport rate. The simulations also shed light on the microscopic mechanisms such as how the fluid torque on particles helps initiate rolling and subsequent grain transport. We further use the numerical scheme to guide development of an alternative framework that can predict the flow profiles for the fast transport as well as the gradual transport beneath the bed surface without resolving the individual particles, which is a more tractable way to model large‐scale bedload sediment transport problems. Key Points: Particle‐scale bedload transport simulations agree with flume experiments Simulations probe the small‐scale mechanisms and parameter dependences in the transport relation Particle simulations guide continuum model development for turbulent sediment transport and bed creep