Shreve 9506415 The long-term goal of the project is to develop quantitative predictive relations between fluid flow and bedload transport that incorporate the role of turbulence more precisely than those currently available. Such relations should apply to the broad spectrum of nonuniform or unsteady flows that occur in streams and beneath waves: and their development should provide new insight into the mechanics of formation of a variety of erosional and depositional features from ripples and dunes to bars and channels. The proposed research will therefore emphasize the parts of the coupled flow-sediment problem that most directly bear on how the morphology and stratigraphy of these features originate and evolve. It will entail a combined experimental and theoretical approach. The experimental work will use two-component laser-doppler velocimetry, high -speed motion-picture photography, and a newly developed force transducer to (1) make time-series measurements of near-bed fluid velocities, bedload sediment transport, and bed-particle forces noninvasively in both nonuniform and unsteady flows at turbulence-resolving frequencies, (2) study the turbulence structure over spatial variation in bed roughness, and (3) make synoptic measurements of bedload particle motions, fluid-velocity profile, and turbulence structure by particle tracking. The theoretical work will use the experimental data to (1) elucidate the equations of motion for near-bed sediment particle trajectories by evaluating the formulas for the lift and drag forces, assessing the relative magnitudes of the terms in the equation, and testing the effects of the turbulence fluctuations (instead of using time average velocities, (2) use the verified equations and measured fluctuating velocity fields to compute bedload particle trajectories for statistical comparison with actual trajectory speeds, heights, and lengths, and (3) apply nonlinear forecasting techniques to study the relations between the measured time series of the near-bed velocities and the corresponding bed-particle forces and bedload transport. The proposed research basically addresses two fundamental problems: (1) relating the forces exerted on particles on the bed to the temporal and spatial distribution of the near-bed fluid velocities and (2) relating the entrainment and motion of those particles to the forces. The experimental work will elucidate the processes involved in both relations, provide ground truth for any general model of entrainment and transport of bedload, and supply new data on the role of turbulence in bedload transport. The theoretical work will aid in developing a more accurate description of bedload transport in natural flows, verify the formulas for lift, drag, and other forces on bedload transport by turbulent flows. Together, the two approaches should significantly enlarge current understanding of the physics of bedload transport of sediment by both streams and waves.
