Date of Award
Doctor of Philosophy (PhD)
Earth & Environmental Science
Douglas J. Jerolmack
Transitions abound in geomorphology. Be it the transition from static to dynamic or the shift in a dune field from one dune morphology to another, many questions involve changes of state. This dissertation empirically studies state changes over three scales with an emphasis placed on contributing mechanical factors. First, we examine the transition from static to dynamic at the grain scale in bedload transport (hopping, rolling, and skipping of grains along a riverbed). Transport of grains is continuous at high rates but becomes unpredictable near the threshold of motion. Results show that this unpredictability is similar to how avalanches occur in a sandpile. Transport events are similar in size and merge as transport increases. Grain displacement in the system appears governed by mobile grain-bed kinetic energy exchange. Next, we study the static to dynamic transition at the scale of many grains by examining how soil mechanical properties influence the threshold of motion. Sandbed experiments indicate that, when moisture is the primary variable, shear strength is proportional to the threshold of motion. Finally, we examine a landscape scale pattern transition that arises from the physics of sediment transport interacting with vegetation in a dune field. The onset of vegetation destroys a fluid instability that is fundamental for maintaining the dune pattern prevalent in the dune fields center. Plants destroy the mechanism maintaining the dune field pattern which helps give rise to a qualitative shift in dune morphology. This work highlights the continued relevance of physically informed experiments and field studies for understanding geomorphic transitions from the grain to the landscape scale.
Lee, Dylan Bentley, "Mechanistic Contributions To Geomorphic Changes Of State" (2018). Publicly Accessible Penn Dissertations. 2895.