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'.
| Identifer | oai:union.ndltd.org:VANDERBILT/oai:VANDERBILTETD:etd-11102016-151557 |
| Date | 11 November 2016 |
| Creators | Fathel, Siobhan |
| Contributors | George M. Hornberger, Calvin F. Miller, James H. Clarke, David J. Furbish |
| Publisher | VANDERBILT |
| Source Sets | Vanderbilt University Theses |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.library.vanderbilt.edu/available/etd-11102016-151557/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to Vanderbilt University or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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