Simulations to examine the effect of enhanced turbulence levels on bed load transport

摘要

The effect of enhanced turbulence levels on the magnitude of bed load transport rates is examined by computer simulations for both hydraulically smooth and rough beds. A method is demonstrated to estimate the root mean squared (RMS) bed shear stress for hydraulically rough beds. This method as well as an existing numerical integration method to calculate bed shear stresses in turbulent smooth bed conditions are used to simulate high frequency temporal shear stresses and bed load transport rates. The simulation method is shown to give similar temporally-averaged results to an existing approach of van Rijn for a smooth bed. The present work suggests that reduced or enhanced shear stress fluctuations can significantly affect the magnitude of the temporally-averaged bed load transport rates for both the smooth and the rough bed cases. The results also demonstrate that the enhancement effects are of greatest significance near the threshold of sediment motion. The turbulent shear stress characteristics developed in the present work for a hydraulically rough bed were added into van Rijn's wave-current bed load transport model and used to study enhanced transport rates. The results indicate that the random turbulent shear stress fluctuations have a significant effect on the magnitude of bed load transport rates for enhanced turbulence levels as might be experienced in the presence of structures.