[H-CG25] 混濁流:発生源から堆積物・地形形成まで

[HCG25-08] Statistical Analyses of Vertical Flow Profile show Turbidity Currents and Open-Channel Flow are Fundamentally Different

*福田 壮二郎¹、Bastianon Elena²、McCaffrey Bill³、成瀬 元⁴、Dorrell Robert¹ (1.Center for Environmental Fluid Dynamics, University of Hull, UK、2.Energy and Environment Institute, University of Hull, UK、3.Institute of Applied Geoscience, School of Earth and Environment, University of Leeds、4.京都大学大学院理学研究科)

キーワード：混濁流、鉛直プロファイル、堆積物輸送、Flow Power、 主成分分析

Over the past 100 years research into submarine sediment-laden buoyancy-driven flows, turbidity currents, have assumed that they are similar to open-channel flows. Here, we integrate 70 years of research into turbidity currents hydrodynamics, to show that the dynamics of turbidity currents are likely fundamentally different from open channel flow. We gathered the vertical velocity and density profile from available dataset of flume experiments of quasi-steady density currents and conducted statistical analyses. As resutls, we found that the stratification of velocity and density are well constrained by each other rather than by a specific flow parameters such as Froude number and Rouse number. Also the comparison between turbidity currents and fluvial rivers indicates that turbidity currents do not follow standard sediment-load transport models of open-channel flows, and thus that turbidity currents are not “rivers on the seafloor” as previously postulated. In open-channel flow the work-done keeping sediment in suspension, and thus sediment concentration, is directly proportional to flow power. Our results show this relationship does not hold for the turbidity currents. That experimental suspended sediment load is not directly proportional to flow power in turbidity currents, the historic Knapp-Bagnold constraint, opens new questions on sediment erosion, transport and deposition by buoyancy driven flows. The further analysis of flow parameters and vertical flow profile suggest a potential cause of this different trend of sediment-load transport model: the experimental turbidity currents, especially for the relatively dilute currents are unstable and cannot be regarded as quasi-equilibrium state. In other words, the sediment capacity might be strongly affected by whether a flow is erosional or depositional.