Bedload Movement: From Initiation of Motion to High Transport Rates

This dissertation examines sediment transport phenomena in various scenarios that range from the near-threshold particle entrainment to the highly intense bedload transport. The dissertation focuses on the roles that turbulent flows and the bed geometrical configurations play in triggering the initiation of particle motion at the near-threshold flow conditions. The approach used to distinguish their specific roles from each other relies on the simplified models, in which either the bed configurations are maintained unchanged for identifying the important turbulence properties associated with particle entrainment, or the flow conditions are rendered to be devoid of turbulent fluctuations in underlining the bed configuration effects. In the cases that emphasize turbulent flows, the flow intensity relative to the particle resistance can be increased progressively to engage a transition from the near-threshold to the highly intense flow conditions. The degree of bed mobility is expected to change accordingly from the intermittent particle entrainment on a nearly quiescent bed to the continuous bedload movement that is reminiscent of a moving carpet covering the entire sediment bed. Such a significant transition in the bed mobility involves a varied time scale suitable for characterizing the bed material activities at different transport conditions, which in turn calls into question the effectiveness of the conventional, long-term averaged parameters, such as the time-averaged bed shear stress, for reflecting the mechanism of bedload transport. This dissertation performs several experimental and numerical analyses to approach the aforementioned topics, each of which represents a unique component that can contribute towards a better understanding of the sediment transport problems