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Mixing in strongly stratified bottom cavities of estuaries: an experimental study

Nino Y., Miranda J. F.

Archives of Hydro-Engineering and Environmental Mechanics

2003

Vol. 50, nr 4

401-413

Results of an experimental study on the mixing characteristics of the stratified flow inside bottom cavities in salt wedge estuaries are presented and discussed. The experiments were conducted in a laboratory flume where a cavity was formed by placing two consecutive sills over the channel bottom. The cavity was filled with a bottom layer of saline water with a sharp density interface. A constant fresh water discharge was supplied at the upstream end of the flume. An ADV and a fast conductivity probe were used to measure the turbulence and salinity structure inside the cavity. Flow visualizations were made, in addition, to keep track of the position of the density interface inside the cavity. The density interface tilts in the downstream direction with a slope that increases as the local densimetric Froude number, Frd, increases. This slope can be reasonably estimated from a linear analysis of the response of the 2-layer stratified fluid to an interfacial shear stress. The entrainment velocity, or rate of interface deepening due to mixing, decreases as Frd increases, which appears to be the consequence of a decreasing mixing efficiency related to the tilting of the interface. The present entrainment velocity data compares well with existing experimental data and models, validating the latter on an extended range of relevant parameters. By scaling these results to a prototype such as the Valdivia Estuary in Chile, it is concluded that purging of the saline water trapped inside of bottom cavities cannot be completed during a tidal period.