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ARS Home » Southeast Area » Oxford, Mississippi » National Sedimentation Laboratory » Watershed Physical Processes Research » Research » Publications at this Location » Publication #75954
Title: VELOCITY AND TURBULENCE STRUCTURE OF EXPERIMENTAL GRAVITY CURRENTS AND INTERNAL SOLITARY WAVES: SEDIMENT TRANSPORT POTENTIAL AND THE FORMATION OFWAVE RIPPLES IN DEEP WATER

Author
item KNELLER, BEN - UNIVERSITY OF LEEDS
item Bennett, Sean
item MCCAFFREY, WILLIAM - UNIVERSITY OF LEEDS

Submitted to: Geological Society of America Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 1/23/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: To formulate models for flow and sediment transport in turbidity currents, the nature of velocity, turbulence and stress must be well-defined. As a first step, we have used laser Doppler anemometry (LDA) to measure the instantaneous downstream and vertical velocities in a series of turbulent salt-water gravity currents generated in a lock-exchange flume tank. This saline fluid entered a mixture of water and isopropyl alcohol, producing rapid under-flows with mean downstream velocities in excess of 0.15 m/s. A two-dimensional picture of the fluid motions within the current was constructed by measuring a number of identical flows with the laser mounted in different positions, allowing the creation of detailed maps of the mean flow. Maps of mean flow vectors illustrate the spanwise rotation within the head. Turbulent velocities were highest within the head, both in the lowest part of the front of the current and associated with the layer between the dense under-flow and the overlying fluid. This is in contrast to the location of the highest mean velocities concentrated in the mid-flow region. Peaks of turbulence also occurred in association with the arrival of reflections from a ramp and from the flume end-wall, producing a series of internal waves. Calculations illustrate that these flows and reflections can not only entrain but suspend sediment. In natural turbidity currents, such reflections could produce symmetrical ripples in environments below storm wave base.

Technical Abstract: To formulate predictive models for flow and sediment transport in turbidity currents, the time- and space-varying nature of velocity, turbulence and stress must be well-defined. As a first step, we have used laser Doppler anemometry (LDA) to measure the instantaneous downstream and vertical velocities in a series of turbulent subcritical saline gravity currents generated in a lock-exchange flume tank. This saline fluid entered a mixture of water and isopropyl alcohol of equal refractive index, producing rapid under-flows with mean downstream velocities in excess of 0.15 m/s. A two-dimensional picture of the fluid motions within the current was constructed by measuring a number of identical flows with the laser mounted in different positions, allowing the creation of detailed maps of mean flow, turbulent velocities and Reynolds stress. Lagrangian maps of mean flow vectors illustrate the spanwise vorticity within the head. Turbulent velocities and Reynolds stresses were highest within the head, both in the lowest part of the nose and associated with the shear layer between the dense under-flow and the ambient. This is in contrast to the location of the highest mean velocities concentrated in the mid-flow region. Peaks of turbulence, Reynolds stress and shear velocity also occurred in association with the arrival of reflections from a ramp and from the flume end-wall, producing a series of internal solitary waves. Competence determinations illustrate that these flows and reflections can not only entrain but suspend sediment. In natural turbidity currents, such reflections could produce symmetrical ripples in environments below storm wave base.