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Abstract Wave‐ and current‐supported turbidity currents (WCSTCs) are one of the sediment delivery mechanisms from the inner shelf to the shelf break. Therefore, they play a significant role in the global cycles of geo‐chemically important particulate matter. Recent observations suggest that WCSTCs can transform into self‐driven turbidity currents close to the continental margin. However, little is known regarding the critical conditions that grow self‐driven turbidity currents out of WCSTCs. This is in part due to the knowledge gaps in the dynamics of WCSTCs regarding the role of density stratification. Especially the effect of sediment entrainment on the amount of sediment suspension has been overlooked. To this end, this study revisits the existing theoretical framework for a simplified WCSTC, in which waves are absent, that is, along‐shelf current‐supported turbidity current. A depth‐integrated advection model is developed for suspended sediment concentration. The model results, which are verified by turbulence‐resolving simulations, indicate that the amount of suspended sediment load is regulated by the equilibrium among positive/negative feedback between entrainment and cross‐shelf gravity force/density stratification, and settling flux dissociated with density stratification. It is also found that critical density stratification is not a necessary condition for equilibrium. A quantitative relation is developed for the critical conditions for self‐driven turbidity currents, which is a function of bed shear stress, entrainment parameters, bed slope, and sediment settling velocity. In addition, the suspended sediment load is analytically estimated from the model developed.
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Direct Numerical Simulations of Miniature Along‐Shelf Current‐Supported Turbidity Currents: Conceptual Investigation of Velocity Structure and Drag Coefficient
Abstract Alongshore current‐supported turbidity currents (ACSTCs) are a subclass of wave‐ and current‐supported turbidity currents. They are one of the agents responsible for the dispersal of the river‐borne sediments on the continental shelf, which constitutes a major phenomenon controlling the geomorphic evolution of ocean‐basin margins over geological time. Therefore, parameterization of the sediment flux associated with ACSTCs will help its implementation in operational models and quantify the sediment flux budgets on the continental shelf. The velocity structure of ACSTCs and the amount of sediments suspended by them are crucial to determine the suspended sediment flux. This study investigates the velocity structure of a simplified miniature ACSTC over an erodible bed composed of fine sediments. Direct numerical simulations are conducted for various bed erosion parameters and sediment settling velocity. The role of sediment‐induced stable density stratification on the velocity structure of ACSTCs is analyzed. The simulation results indicate that density stratification and the drag coefficient are functions of the product of sediment settling velocity and sediment concentration. The velocity profile was found to deviate toward the alongshore direction with strengthening density stratification, which enhances the drag coefficient. By using the Monin‐Obukhov theory, the drag coefficient associated with the cross‐shelf propagation of ACSTCs is formulated as a function of the Reynolds number, sediment concentration, and sediment settling velocity.
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- Award ID(s):
- 2023676
- PAR ID:
- 10360296
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 126
- Issue:
- 8
- ISSN:
- 2169-9275
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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