ANALYSIS OF THE CHARACTERISTICS OF ROUGH BED TURBULENT SHEAR STRESSESIN AN OPEN-CHANNEL

Entrainment of sediment particles from channel beds into the channel f low is influenced by the characteristics of the flow turbulence which produces stochastic shear stress fluctuations at the bed. Recent studi es of the structure of turbulent flow has recognized the importance of bursting processes as important mechanisms for the transfer of moment um into the laminar boundary layer. Of these processes, the sweep even t has been recognized as the most important bursting event for entrain ment of sediment particles as it imposes forces in the direction of th e flow resulting in movement of particles by rolling, sliding and occa sionally saltating. Similarly, the ejection event has been recognized as important for sediment transport since these events maintain the se diment particles in suspension. In this study, the characteristics of bursting processes and, in particular, the sweep event were investigat ed in a flume with a rough bed. The instantaneous velocity fluctuation s of the flow were measured in two-dimensions using a small electromag netic velocity meter and the turbulent shear stresses were determined from these velocity fluctuations. It was found that the shear stress a pplied to the sediment particles on the bed resulting from sweep event s depends on the magnitude of the turbulent shear stress and its proba bility distribution. A statistical analysis of the experimental data w as undertaken and it was found necessary to apply a Box-Cox transforma tion to transform the data into a normally distributed sample. This en abled determination of the mean shear stress, angle of action and stan dard error of estimate for sweep and ejection events. These instantane ous shear stresses were found to be greater than the mean flow shear s tress and for the sweep event to be approximately 40 percent greater n ear the channel bed. Results from this analysis suggest that the criti cal 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 di agram due to the higher shear stresses imposed on the particles by the stochastic shear stresses resulting from turbulence within the flow.