09:45 〜 10:00
[MIS07-03] Inadequacy of fluvial energetics for describin gravity current autosuspension
キーワード：Turbidity current, Energetics
Here, 70 years of empirical and direct observation research is integrated with our new flume experiments, to show that material transport by turbidity currents is fundamentally different from fluvial systems.
Results: In our study, contrary to classic flow-power models, work done keeping sediment in suspension is shown to have a strongly non-linear dependence on available flow power. The observed relationship is attributed to an underestimation of turbulent kinetic energy (TKE) production. The remarkable shortfall of TKE production questions the current local energy balance theory used for generic gravity currents and highlights the need for future research to consider non-local energy balance at system scales. The energy shortfall has direct implications for the enigmatic autosuspension processes of turbidity currents and thus these insights are crucial for the understanding of these flows.
Background: The long-runout phenomenon of turbidity currents is referred to as autosuspension. The classical self-acceleration models can provide a possible explanation to the autosuspension mechanics in steep slope where the gravitational forcing is relatively large. However, in the shallow slope seafloor, it is almost impossible for turbidity currents to achieve autosuspension without additional forcing due to the negative feedback loop between deceleration and material loss by sediment deposition. Extant models of turbidity currents have been tackling on this problem based on the flow power model, the local energy balance theory of TKE, assuming that material transport of turbidity currents is dynamically similar to open-channel flows, yet the mechanics whereby turbidity currents travel long distances over shallow slope remains an enigma.
Results: In our study, contrary to classic flow-power models, work done keeping sediment in suspension is shown to have a strongly non-linear dependence on available flow power. The observed relationship is attributed to an underestimation of turbulent kinetic energy (TKE) production. The remarkable shortfall of TKE production questions the current local energy balance theory used for generic gravity currents and highlights the need for future research to consider non-local energy balance at system scales. The energy shortfall has direct implications for the enigmatic autosuspension processes of turbidity currents and thus these insights are crucial for the understanding of these flows.
Background: The long-runout phenomenon of turbidity currents is referred to as autosuspension. The classical self-acceleration models can provide a possible explanation to the autosuspension mechanics in steep slope where the gravitational forcing is relatively large. However, in the shallow slope seafloor, it is almost impossible for turbidity currents to achieve autosuspension without additional forcing due to the negative feedback loop between deceleration and material loss by sediment deposition. Extant models of turbidity currents have been tackling on this problem based on the flow power model, the local energy balance theory of TKE, assuming that material transport of turbidity currents is dynamically similar to open-channel flows, yet the mechanics whereby turbidity currents travel long distances over shallow slope remains an enigma.