The Physics of Sediment Transport Initiation, Cessation, and Entrainment Across Aeolian and Fluvial Environments

Predicting the morphodynamics of sedimentary landscapes due to fluvial and aeolian flows requires answering the following questions: Is the flow strong enough to initiate sediment transport, is the flow strong enough to sustain sediment transport once initiated, and how much sediment is transported by the flow in the saturated state (i.e., what is the transport capacity)? In the geomorphological and related literature, the widespread consensus has been that the initiation, cessation, and capacity of fluvial transport, and the initiation of aeolian transport, are controlled by fluid entrainment of bed sediment caused by flow forces overcoming local resisting forces, whereas aeolian transport cessation and capacity are controlled by impact entrainment caused by the impacts of transported particles with the bed. Here the physics of sediment transport initiation, cessation, and capacity is reviewed with emphasis on recent consensus-challenging developments in sediment transport experiments, two-phase flow modeling, and the incorporation of granular physics' concepts. Highlighted are the similarities between dense granular flows and sediment transport, such as a superslow granular motion known as creeping (which occurs for arbitrarily weak driving flows) and system-spanning force networks that resist bed sediment entrainment; the roles of the magnitude and duration of turbulent fluctuation events in fluid entrainment; the traditionally overlooked role of particle-bed impacts in triggering entrainment events in fluvial transport; and the common physical underpinning of transport thresholds across aeolian and fluvial environments. This sheds a new light on the well-known Shields diagram, where measurements of fluid entrainment thresholds could actually correspond to entrainment-independent cessation thresholds. Plain Language Summary Loose sediment grains can be transported by blowing wind (aeolian) or water flowing in a riverbed (fluvial). These processes are responsible for shaping much of the natural world, but they involve the combination of several very complex physical systems, like turbulent fluid flow near a rough boundary and the mechanical behavior of granular materials. Thus, there is no consensus about the minimum wind or water speeds required to initiate and sustain sediment transport. Additionally, wind and water-driven sediment transport are obviously similar, suggesting that it should be possible to capture both under one description. Recent advances in experiments and computer simulations have helped scientists to answer some key questions about why sediment transport is initiated and sustained. This article reviews many of these recent discoveries, focusing on three key topics: (1) the mechanical behavior of granular materials; (2) how turbulence in the fluid helps to move grains; and (3) the role of inertia of mobile grains. We show that a deeper understanding of these topics helps to resolve some major inconsistencies in our understanding of why sediment transport is initiated and sustained and may help to unify sediment transport by wind and water under a single theoretical description.