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ARS Home » Southeast Area » Oxford, Mississippi » National Sedimentation Laboratory » Watershed Physical Processes Research » Research » Publications at this Location » Publication #97363
Title: VELOCITY STRUCTURE, TURBULENCE AND FLUID STRESSES IN EXPERIMENTAL GRAVITY CURRENTS

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

Submitted to: Journal of Geophysical Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/1999
Publication Date: N/A
Citation: N/A

Interpretive Summary: In lakes, reservoirs, and the ocean, sandy and muddy currents of water periodically flow down the sloping bottom along the bed, underneath the still body of water, and carry with it vast quantities of sediment. These currents, called turbidity currents, are one of the primary mechanisms for the transport and dispersion of agriculturally-derived sediments and pollutants in reservoirs. Similar currents also occur during volcanic eruptions, and can devastate life, property, and the environment. Predicting how these currents form, how often they flow into reservoirs, how they spread out on the bottom of a reservoir, and how they pick up and deposit sediment require an understanding of the currents physical characteristics. At present, these processes are poorly understood. This investigation sought to obtain detailed experimental data on the physical characteristics of turbidity currents in a laboratory channel. Laboratory measurements resulted in improved knowledge of bottom-sediment movement and entrainment and aided in the derivation of a new equation to predict the distribution of velocity within a turbidity current. This new knowledge will contribute to the improvement of reservoir models used by federal agencies in their assessment of sedimentation rates, sedimentation patterns, and pollutant dispersal in lakes and reservoirs.

Technical Abstract: Gravity currents are of considerable environmental and industrial importance as hazards and as agents of sediment transport, and the deposits of ancient turbidity currents form some significantly large hydrocarbon reservoirs. Prediction of the behavior of these currents and the nature and distribution of their deposits require an understanding of their turbulent structure. To this end, a series of experiments was conducted with turbulent, subcritical, brine underflows in a rectangular lock-exchange tank using laser Doppler anemometry. The velocity maximum within the gravity current occurs at 0.2d. The shape of the velocity profile is governed by the differing and interfering effects of the lower (rigid) and upper (diffuse) boundaries, and can be approximated with the law of the wall up to the velocity maximum and a cumulative Gaussian distribution from the velocity maximum to the ambient interface. Mean motion within the head consists of a single large vortex and an overall motion of fluid away from the bed. The distribution of turbulence and turbulent kinetic energy within the current is heterogeneous and controlled by the location of large eddies that dominate the turbulent energy spectrum and scale with flow thickness.