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Title page for ETD etd-11102016-151557


Type of Document Dissertation
Author Fathel, Siobhan L.
Author's Email Address siobhan.fathel@gmail.com
URN etd-11102016-151557
Title Experimental Analysis of Bed Load Sediment Motions Using High-Speed Imagery in Support of Statistical Mechanics Theory
Degree PhD
Department Environmental Engineering
Advisory Committee
Advisor Name Title
David J. Furbish Committee Chair
Calvin F. Miller Committee Member
George M. Hornberger Committee Member
James H. Clarke Committee Member
Keywords
  • probabilistic methods
  • bed load sediment
  • ensemble distributions
  • experimental geomorphology
  • sediment transport
Date of Defense 2016-09-30
Availability unrestricted
Abstract
Bed load sediment particles move as complex motions over the surface of a stream bed, accelerating and decelerating in response to the near-bed turbulence and due to particle-bed interactions. Detailed measurements of individual sand grains moving on a streambed allow us to obtain a deeper understanding of the characteristics of particle motions and evaluate spatial and temporal properties of particle diffusion, entrainment and disentrainment. I track bed load particle motions, measured start-to-stop, from high-speed (250 Hz) imaging of uniform, coarse-grained sand from two flume experiments, which have different mean fluid velocities near the bed. This work utilizes rich data sets to provide foundational support for a statistical mechanics approach to bed load transport. First, using these data, we characterize the underlying ensemble distributions of key measures of particle motions (particle velocities, accelerations, hop distances, and travel times). These distributions best represent the probabilistically expected behavior of sediment motions consistent with the macroscopic sediment and flow conditions, and thus provide a clear target for further analyses of transport, including statistical mechanics theory and numerical simulations. We then investigate the diffusive contribution to the sediment flux to demonstrate that conventional measures of particle spreading reveal different attributes of bed load particle behavior depending on details of the calculation. Our results indicate that while there are similarities between bed load transport and Brownian systems, care is needed in suggesting anomalous behavior when appealing to conventional measures of diffusion formulated for ideal particle systems. Finally, we analyze patterns and controls on sediment entrainment and disentrainment. Here we focus on entrainment and disentrainment in relation to near-bed fluid velocity measurements. This suggests that the connection between the fluid and particle entrainment is complex and will require a clearer understanding of the factors that jointly influence entrainment at the particle scale. Furthermore, this highlights and reveals important aspects of the mechanical behavior of particles during entrainment and disentrainment, and ultimately shows that these processes involve more than 'starting' and 'stopping'.
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