The Role of Near-Bed Turbulence in the Inception of Particle Motion

by A.N. Papanicolaou

4. Conclusions

In the present investigation, the characteristics of near-bed turbulence were examined for three roughness regimes; the isolated, the wake interference, and the skimming. The joint frequency distributions of  and , (where u' and w' are the turbulent intensities in the horizontal and vertical direction respectively) which were constructed here, clearly demonstrate that the frequency of occurrence and magnitude of turbulent events, under incipient flow conditions, varies significantly with bed roughness. In particular, for the isolated flow regime the outward and inward interactions are the most frequent events. For the skimming regime, the sweeps and ejections are the most dominant, and for the weak interference regime, all events appear to occupy the same percentage of time. Additional analysis was provided by developing the time series of the normal and shear stress components U², W², and UW of the instantaneous stress tensor at a very close proximity atop a particle surface. The results deduced here are consistent with the Sterk et al. (1998) and Nelson et al. (1995) measurements where stream-wise velocity was found to correlate well with sediment transport (for conditions well above the critical). The findings, although quite academic at this stage of the investigation, should be capable of extension to real cases to predict incipient motion of sediment in natural streams under various hydrologic and bed roughness conditions.

Abstract

 1. Introduction

 2. Experimental Facility

 3. Methodology-Results

 4. Conclusions

Next Section: Acknowledgements

 References