TITLE:
Influence of Vertical Motion on Initiation of Sediment Movement
AUTHORS:
Ishraq Alfadhli, Shu-Qing Yang, Muttucumaru ivakumar
KEYWORDS:
Critical Shear Stress, Nonuniform Flows, Shields Diagram, Vertical Velocity, Decelerating/Accelerating Flows
JOURNAL NAME:
Journal of Water Resource and Protection,
Vol.6 No.18,
December
23,
2014
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.