Parameter space study of granular bed erosion and bedform development

Sediment transport created by fluid flows is responsible for shaping a big portion of the natural world. It is relevant to many agricultural and ecological problems. Unfortunately, sediment transport has a very large parameter space, making the concrete understanding of even the simpler questions, like for example when the grains will start to move, a very challenging task. A lot of work had been done on predicting the onset of grain motion on simplified versions of granular beds. Most researchers had developed models that could accurately predict the onset of motion for unimodal beds. But, the real world is more complicated than that. And many times the simplified models struggle to produce accurate predictions. So, for our first project, we developed a tool that detects the centers of individual grains in RGB images. Using this tool together with a predictive multiframe tracking algorithm developed by my advisor, we extended the work that had be done in measuring and analyzing the statistical properties of grain motion near their onset, from unimodal to bimodal granular beds. Besides confirming that a traditional Shields-number framework successfully accounts for grain-size effects in beds composed of grains of only a single size, we showed that bimodal beds are distinct. Large grains in a bimodal bed are mobilized into bedload transport by stresses that would normally be subcritical for them but that the behavior of small grains is less significantly affected. That result has implications for modeling sediment transport in natural systems where polydispersity is unavoidable. For the second project, we considered the impact of stress history on the onset of grain motion for unimodal beds. A lot of work had been done for cases where the granular bed is subject to fluid flows of the same direction. So, we decided to first confirm that critical stress depends on the stress history of the bed, and that beds will strengthen when subjected to subcritical stresses. But, we also found that this effect is strongly directional. Our experiments showed that preconditioned beds are in fact more susceptible to erosion by flows in the direction opposite to that of the conditioning flow than freshly settled beds are. This result shows that the strength of a natural erodible bed with a stress history is likely to be highly anisotropic, with significant implications for predictions of sediment transport. In our third project, we performed experiments with the goal of extending the current understanding of bedform growth. Past studies had thoroughly investigated the properties of bedform initiation and growth until equilibrium, for unimodal beds. But, only a few researchers had studied the creation and behavior of bedforms under oscillatory flows, where the unexplored parameters space remained vast. Even less work had been done for the case of bimodal beds. So, the goal of our study was to perform a comparison of the properties of bedform growth and propagation under four different scenarios, namely unimodal beds under unidirectional flows, bimodal beds under unidirectional flows, unimodal beds under oscillatory flows, and bimodal beds under oscillatory flows. Our findings suggested that flow oscillations highly enhance and speed up the process of bedform creation, while bimodality has the exact opposite effect, i.e. it considerably slows down the process of bedform development

Advisor: Ouellette Nicholas (Nicholas Testroet)
Freyberg David L.
Fringer Oliver B. (Oliver Bartlett)
Koseff Jeffrey Russell