Relative density effects on incipient bed movement

Abstract

The Shields entrainment function, extrapolated from bed load movement to predict the threshold of particle motion, appears deficient in analyzing incipient shear data for fluid‐sediment combinations encompassing a wide range of relative densities. Equations describing incipient dynamic equilibrium, solved semiempirically, indicate additional density considerations that must be included in an attempt to correlate initial movement for all possible sedimentation density ratios. This and other experimental investigations also indicate any entrainment function to be dependent on the state of the bed and corresponding turbulence. Other factors which might be influential on the threshold of particle movement are mentioned.

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

“…As discussed in section 1, old experimental studies (e.g., Graf & Pazis, 1977;Ward, 1969) strongly indicated that the fluvial transport threshold measurements that are compiled in the Shields diagram are to a nonnegligible degree affected by particle inertia. As the Shields diagram shows a rough data collapse of the threshold Shields number Θ t as a function of the shear Reynolds number Re * , this raises the question of whether Re * is in some way associated with particle inertia.…”

“…Hence, is not, or at least not only, associated with fluid entrainment and thus incipient motion. The importance of particle inertia was proposed and indirectly shown even earlier, in a largely ignored study (only eight citations indexed by Web of Science today, half a century after publication) by Ward (). In this study, Ward () measured smaller values of for a larger particle‐fluid‐density ratio (which is a measure for particle inertia) at the same shear Reynolds number .…”

“…The importance of particle inertia was proposed and indirectly shown even earlier, in a largely ignored study (only eight citations indexed by Web of Science today, half a century after publication) by Ward (). In this study, Ward () measured smaller values of for a larger particle‐fluid‐density ratio (which is a measure for particle inertia) at the same shear Reynolds number . A slight downward trend of with even existed in the pioneering experiments by Shields ().…”

The Cessation Threshold of Nonsuspended Sediment Transport Across Aeolian and Fluvial Environments

“…Mean flow entrainment models derive a transport initiation threshold Shields number from a force balance, and/or torque balance, between mean fluid forces and resisting contact forces acting on a representative particle resting on the bed surface (Agudo et al, 2017;Ali & Dey, 2016;Bagnold, 1936Bagnold, , 1941Bravo et al, 2014Bravo et al, , 2017Claudin & Andreotti, 2006;Dey, 1999Dey, , 2003Dey & Papanicolaou, 2008;Duan et al, 2013;Durán et al, 2011;Edwards & Namikas, 2015;He & Ohara, 2017;Iversen et al, 1976Iversen et al, , 1987Iversen & White, 1982;Lee et al, 2012;Lehning et al, 2000;Ling, 1995;Lu et al, 2005;Luckner & Zanke, 2007;Recking, 2009;Rousar et al, 2016;Schmidt, 1980;Shao & Lu, 2000;Vollmer & Kleinhans, 2007;Ward, 1969;Wiberg & Smith, 1987;White, 1940;Wu & Chou, 2003). Many of these models have been proposed to reproduce the Shields diagram, which displays two kinds of fluvial thresholds: a threshold obtained from extrapolating measurements of the transport rate to (nearly) vanishing transport and visual measurements of the initiation threshold of individual transport (see section 4.3 for details).…”

The threshold for continuing saltation on Earth and other solar system bodies

“…Establishing under what conditions saltation is possible could determine an Aeolian origin for the streaks, ripples, and dunes observed on the surface of minor solar system bodies, such as moons, comets, and asteroids [Hansen et al, 1990;Sagan and Chyba, 1990;Thomas et al, 2015]. The first formulation for this initiation threshold was based on considerations of rotational equilibrium of a sphere in contact with a surface under the effects of gravity and aerodynamic drag [Bagnold, 1941]; successive approaches have included the role of buoyancy [Ward, 1969], aerodynamic lift, and interparticle cohesive forces [Ward, 1969;Iversen et al, 1976;Iversen and White, 1982;Shao and Lu, 2000], interparticle impact force [Iversen et al, 1987], and turbulent fluctuations [Lu et al, 2005]. The resulting expressions for the threshold involve complicated functions of the particle Reynolds number at the threshold wind velocity, the density ratio, and the interparticle cohesive forces.…”