Influence of Vertical Motion on Initiation of Sediment Movement

Abstract

This paper makes an attempt to answer why the observed critical Shields stress for incipient sediment motion deviates from the Shields curve. The measured dataset collected from literature show that the critical Shields stress widely deviates from the Shields diagram's prediction. This paper has re-examined the possible mechanisms responsible for the validity of Shields' diagram and found that, among many factors, the vertical velocity in the sediment layer plays a leading role for the invalidity of Shield's prediction. A closer look of the positive/negative deviation reveals that they correspond to the up/downward vertical velocity, and the Shields diagram is valid only when flow is uniform. Therefore, this diagram needs to be modified to account for hydraulic environments when near bed vertical velocities are significant. A new theory for critical shear stress has been developed and a unified critical Shields stress for sediment transport has been established, which is valid to predict the critical shear stress of sediment in both uniform and nonuniform flows.

Formula for Sediment Transport Subject to Vertical Flows

“…1 forth (Yang 2009a, b, c). Its influence on mass transport also needs 203 to be spelled out clearly 17 (Yang 2013;Alfadhli et al 2014).…”

Sediment Transport and Morphodynamics Induced by a Translating Monopolar Vortex

“…However, particles are then transported due to the the radially inward secondary motion toward the vortex core, where significant upward flow velocities occur. As discussed by Alfadhli et al (2014), vertical velocities influence the particle transport rate in sediment beds due to the changes in critical shear stress, increasing the transport if upward and decreasing it if downward. Therefore, in contrast with studies of the radial exerted shear stress at the bottom boundary layer of a rotating flow (Caps & Vandewalle, 2003;González-Vera et al, 2018;Thomas, 1994;Thomas & Zoueshtiagh, 2007), the vertical velocity induced by the pressure gradient effect (as described by Greeley et al, 1981;Greeley & Iversen, 1987) could significantly enhance the suspension of particles.…”

Retrofitting of Pressurized Sand Traps in Hydropower Plants

“…Since the Shields number, particle Reynolds number, and Euler number are equal in model and prototype for the 1:1 velocity and sediment scaling, the authors argue that this approach may be more accurate compared with the more usual Froude scaling approach, where these dimensionless numbers are not similar in model and prototype without additional scaling techniques, such as lightweight sediment material, that introduce additional scaling errors. The 1:1 sediment scaling also mitigates the problem of cohesion is natural material is used [27].…”