Influence of Vertical Motion on Initiation of Sediment Movement

Ishraq Alfadhli^*, Shu-Qing Yang, Muttucumaru ivakumar

School of Civil, Mining and Environmental Engineering, University of Wollongong, Wollongong, Australia.

DOI: 10.4236/jwarp.2014.618150                

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.

Keywords

Critical Shear Stress, Nonuniform Flows, Shields Diagram, Vertical Velocity, Decelerating/Accelerating Flows

Conflicts of Interest

The authors declare no conflicts of interest.

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