Experiments in Hydrodynamic Sediment Transport: Precision Measurements of Erosion in a Turbulent Fluid Flo

This thesis reports experimental measurements of hydrodynamic transport of sediment by a turbulent fluid flow. We use a racetrack-shaped closed-lid flume to generate turbulent shear flows in water that flow over a bed of millimeter-scale glass grains. We image both the fluid flow and the surface of the granular bed, and we use particle tracking velocimetry to measure velocity statistics of the grains in the bed and of the fluid flow acting on those grains. We interpret these statistics to further our understanding of the criteria governing the onset of erosion and sediment transport, and to begin to unravel the influence of turbulence on this onset.;We describe the design, construction, and characterization of a purpose-built racetrack flume, capable of generating a large range of fluid flow conditions. We optimize this apparatus to balance the scales necessary for generating geophysically relevant fluid flows with the scales needed to carefully measure behavior of both fluid and grains at the grain scale. We design this flume to allow for future upgrades of its capabilities, though we also include a thorough discussion of its limitations.;Using this flume, we investigate the longstanding topic of the onset of sediment transport under a turbulent fluid flow. We find that as we increase fluid flows above the threshold for grain motion, the increase in sediment flux is dominated by an increasing fraction of transported sediment rather than an increasing sediment velocity, and that there is no sharp transition in this fraction of moving grains at a critical flow rate. We further investigate the statistics of both transported and non-transported grains in this system, and find that fluctuations in velocity of the non-transported grains make it impossible to unambiguously separate `static' and `mobile' grains at a given velocity threshold. Instead, we address the full velocity distribution of the grains with a statistical mixture model approach, capturing grain transport as well as grain velocity fluctuations without transport. This model suggests that near the onset of sediment transport in turbulent fluid flows, nuanced statistical measures are required to accurately quantify grain motion.