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United States Department of Agriculture

Agricultural Research Service

Title: Macroturbulence and Fluid Stresses in Experimental Gravity Currents: Implications for Sediment Transport and Deposition in the Ocean

Authors
item Kneller, Ben - UNIVERSITY OF LEEDS
item Bennett, Sean
item Mccaffrey, Bill - UNIVERSITY OF LEEDS

Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: November 1, 1996
Publication Date: N/A

Interpretive Summary: We have investigated the velocity structure of simple density under-flows in the laboratory, in a preliminary attempt to model aspects of sediment transport and deposition associated with turbidity currents in the deep ocean and in lakes. By using a optically-similar fluids, we were able to apply laser Doppler anemometry to salt-water flows. Instantaneous at-a-point velocities were measured in the vertical and downstream directions at a number of points within the flows. The flows were fully turbulent. Analysis of the velocity time series reveals large eddies within the under-flow, with dimensions similar to the flow thickness. Turbulent flow of an under-flow is quite different to that of river and streams because the under-flow have two points of contact: the bed and the upper surface next to the ambient flow. Flow velocity within the flows show major fluctuations associated with the passage of these large eddies, and also with the arrival of reflections from the other side of the laboratory channel. Predictable consequences for the structure of sediments deposited from turbidity currents include: erosional surfaces within deposits of some sustained flows, marking the passage of large eddies; and wave ripples in deep water, where internal waves have been generated by the interaction of turbidity currents with sea-floor topography.

Technical Abstract: We have investigated the velocity structure of simple quasi-two-dimensional density under-flows in the laboratory, in a preliminary attempt to model aspects of sediment transport and deposition associated with turbidity currents in the deep ocean. By using a refractive index matching technique, we were able to apply laser Doppler anemometry to brine flows. Instantaneous at-a-point velocities were measured in the vertical and downstream directions at a number of points within the flows. Downstream velocities ranged from 0.14 m/s. The flows were fully turbulent and subcritical (flow Richardson number greater than unity). Analysis of the velocity time series reveals dominant long-period eddies, with dimensions similar to the flow thickness. The vertical frequency distribution of this macroturbulence, and also of the microturbulence, is quite different to that of shear flows. We believe this difference arises because the macroturbulence in density under-flows is generated at the upper boundary of the flow, unlike shear flows, where it is largely generated at the bed. Both boundary shear stress and Reynolds stress within the flows show major fluctuations associated with the passage of these eddies, and also with the arrival of reflected bores and internal solitary waves in confined currents. Predictable consequences for the structure of sediments deposited from turbidity currents include: erosional surfaces within deposits of some sustained flows, marking the passage of large eddies; and wave ripples in deep water, where internal waves have been generated by the interaction of turbidity currents with sea-floor topography.

Last Modified: 10/1/2014
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