An analysis of the characteristics of rough bed turbulent shear stresses in an open channel |
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Authors: | A Keshavarzy J E Ball |
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Institution: | (1) Water Research Lab, University of New South Wales, King Street, 2093 Manly Vale, Australia |
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Abstract: | Entrainment of sediment particles from channel beds into the channel flow is influenced by the characteristics of the flow
turbulence which produces stochastic shear stress fluctuations at the bed. Recent studies of the structure of turbulent flow
has recognized the importance of bursting processes as important mechanisms for the transfer of momentum into the laminar
boundary layer. Of these processes, the sweep event has been recognized as the most important bursting event for entrainment
of sediment particles as it imposes forces in the direction of the flow resulting in movement of particles by rolling, sliding
and occasionally saltating. Similarly, the ejection event has been recognized as important for sediment transport since these
events maintain the sediment particles in suspension.
In this study, the characteristics of bursting processes and, in particular, the sweep event were investigated in a flume
with a rough bed. The instantaneous velocity fluctuations of the flow were measured in two-dimensions using a small electromagnetic
velocity meter and the turbulent shear stresses were determined from these velocity fluctuations. It was found that the shear
stress applied to the sediment particles on the bed resulting from sweep events depends on the magnitude of the turbulent
shear stress and its probability distribution. A statistical analysis of the experimental data was undertaken and it was found
necessary to apply a Box-Cox transformation to transform the data into a normally distributed sample. This enabled determination
of the mean shear stress, angle of action and standard error of estimate for sweep and ejection events. These instantaneous
shear stresses were found to be greater than the mean flow shear stress and for the sweep event to be approximately 40 percent
greater near the channel bed.
Results from this analysis suggest that the critical shear stress determined from Shield's diagram is not sufficient to predict
the initiation of motion due to its use of the temporal mean shear stress. It is suggested that initiation of particle motion,
but not continuous motion, can occur earlier than suggested by Shield's diagram due to the higher shear stresses imposed on
the particles by the stochastic shear stresses resulting from turbulence within the flow. |
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Keywords: | Turbulence sediment fluvial river bursting process statistics |
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