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Dynamical Behavior of the Metalimnion in Stratified Lakes

Author(s): Y. Nino; J. Helmbrecht; F. Lopez; G. Buscaglia

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Keywords: Stratified lakes; Metalimnion; Wind-induced mixing; Interface tilting; Seiches; Laboratory experiments; 3D numerical simulations

Abstract: Results of both, an experimental and a numerical study on the behavior of the density stratification in water bodies under the action of a surface shear stress, such as that exerted by the wind, are presented. The experimental study was conducted in a tank with a conveyor belt that applies a shear stress on the surface of a two-layer salinity-stratified volume of water. The experimental results were conducted under unsteady surface shear stress conditions. Video images of the flow were analyzed to study the shear-induced mixing and tilting of the density interface. The numerical study consisted of a 3D simulation of the hydrodynamics of Lake San Roque, driven by measured hydro-meteorological data. The experimental results show the existence of a mode 2 response of the density stratification to surface shear. The tilting of the density interface responds to the variable Richardson number induced by the surface shear and results to be similar to that measured in similar experiments with steady shear conditions. The rate of mixing induced by the unsteady shear, once the flow conditions are established, is similar to that observed in the final stage of mixing in the steady case, for corresponding values of the Richardson number. The interface tilting obtained from the simulations agrees better with the predictions of the classical theory than that of the experimental results. The values of the entrainment velocity obtained from the simulations are generally larger than expected according to different entrainment relationships. A definite explanation for this result is currently being investigated.


Year: 2001

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