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

Abstract

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.

The Influence of Surface Stress Fluctuation on Saltation Sand Transport Around Threshold

“…Interestingly, Figure 5a reveals that, for a sufficiently low resistance level, aerodynamically sustained sand transport can occur below θ *it . That such a behavior may be possible was, indeed, recently suggested by Pähtz et al (2020). However, these authors stated that an aerodynamically sustained sand transport regime below θ *it should be bedload rather than saltation.…”

The Design and Calibration of Instrumented Particles for Assessing Water Infrastructure Hazards

“…It is the most used criterion in the literature to determine whether there is initiation of motion of a sediment particle. Based on the results shown in Table 5, Shields parameter values for the three runs of the experiment are around the regime of incipient flow conditions predicted by a plethora of experiment studies (see [31,32], amongst others). Additionally, Nikuradse's equivalent sand roughness, which is an indicative parameter of the friction matched against an equivalent sand-grain roughness of height k s , is estimated for the three runs of the experiment and results are presented in Table 5.…”

“…There are a number of tools developed to monitor the actual occurrence of scour around hydraulic infrastructure [30][31][32]. However, many geomorphological and hydrological hazards, such as scour around hydraulic infrastructure or riverbed and riverbank destabilisation, typically develop very fast relative to our capacity to take action once they are initiated or detected.…”

Turbulence‐Based Model for Subthreshold Aeolian Saltation

“…The influence of hysteresis on saltation is not currently accounted for in most estimates of sediment flux on Earth or Mars, but sediment transport between fluid‐ and impact‐threshold shear velocities clearly contributes to sediment flux on both planets (Baker et al., 2018; Comola et al., 2019). We emphasize, further, that for the purpose of this article we assume equivalence between the fluid and continuous transport threshold, noting that the efficacy of this assumption is itself the topic of ongoing work (Pähtz et al., 2020).…”