Miles Traer - Dissertation Defense
Reconciling laboratory studies and field-scale flow inferences of turbidity currents using numerical modeling
Abstract: Turbidity currents constitute a major means by which sediment is transferred from near-shore to deep-water environments, and are responsible for the formation of favorable hydrocarbon reservoir rocks. In my thesis, I use numerical models to examine and how inferences of flow processes at the laboratory-scale might affect our understanding of turbidity current dynamics at the field-scale. Turbidity currents flow due to the excess density provided by suspended sediments. Measurements of turbidity currents in natural systems have proven difficult due to the destructive nature of the flows. As a result, researchers have primarily investigated turbidity current dynamics using physical models at the laboratory-scale, and numerical models at the field-scale to understand the processes that control the excess density of the flows. Given the difficulties inherent in scaling the turbulent behavior of these flows in physical models, my thesis focuses on the empirical relationships used in numerical modeling studies that describe how sediment and clear-water entrainment rates relate to flow turbulence. Additionally, I expand the widely-used four-equation model for turbidity currents to account for previously neglected processes that exchange mass and momentum between the flow and its surroundings, allowing for a more complete description of turbidity current dynamics.
To examine the processes that exert primary influences on the excess density, I began by quantifying the uncertainties within the empirical relationships that describe sediment and clear-water entrainment. The range of statistically sampled relationships, particularly the clear-water entrainment relationships, obscured turbidity current evolution beyond 200-500 m, suggesting that consideration of a wide range of entrainment parameters might be necessary to accurately predict the range of plausible field-scale flow dynamics. I then simulated flows through a series of well-studied natural systems and found that all modeled turbidity currents reached physically unrealistic flow heights (10-200x the channel depth) when laboratory-derived clear-water entrainment rules were used. This suggests that the net-clear-water entrainment rate is too high, either due to the neglect of alternate processes that remove mass and momentum from the flow, or due to improper parameterization of the clear-water entrainment relationship at the field-scale. I completed this thesis by expanding the numerical model to account for flow stripping and overspill processes that remove fluid and sediment mass from the flow and help regulate flow height and velocity. Simulated flows not only maintained more realistic flow conditions, but reached a dynamic equilibrium. This dynamic equilibrium, particularly Richardson sub-critical equilibrium, might help elucidate the long runout distances of turbidity currents on low slopes inferred from extensive natural systems.