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.
This study investigates the Reynolds shear stress distribution in steady and unsteady non-uniform flows. Specifically, it deals with how to express the deviation of this turbulence characteristic from that of uniform flow line; it is found that flow acceleration can well represent the deviation of Reynolds shear stress from its standard linear distribution. By connecting the flow acceleration with Reynolds shear stress, the study demonstrates empirically that the linear distrubiton of Reynolds shear stress can be observed when the flow acceleration is zero; the concave distribution of Reynolds shear stress can be observed when the flow acceleration is negative or when the flow velocity is decreased along the channel; the convex distribution of Reynolds shear stress can be observed when the flow acceleration is positive or the flow velocity is increased along the channel. These empirical results have been verified using published experimental data and good agreement between the predicted and observed profiles has been achieved. ABSTRACTThis study investigates the Reynolds shear stress distribution in steady and unsteady nonuniform flows. Specifically, it deals with how to express the deviation of this turbulence characteristic from that of uniform flow line; it is found that flow acceleration can well represent the deviation of Reynolds shear stress from its standard linear distribution. By connecting the flow acceleration with Reynolds shear stress, the study demonstrates empirically that the linear distribution of Reynolds shear stress can be observed when the flow acceleration is zero; the concave distribution of Reynolds shear stress can be observed when the flow acceleration is negative or when the flow velocity is decreased along the channel; the convex distribution of Reynolds shear stress can be observed when the flow acceleration is positive or the flow velocity is increased along the channel. These empirical results have been verified using published experimental data and good agreement between the predicted and observed profiles has been achieved.
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