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Research at St Andrews

Blockage of saline intrusions in restricted, two-layer exchange flows across a submerged sill obstruction

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Author(s)

Alan Cuthbertson, Janek Laanearu, Magda Carr, Joel Sommeria, Samuel Viboud

School/Research organisations

Abstract

Results are presented from a series of large-scale experiments investigating the internal and near-bed dynamics of bi-directional stratified flows with a net-barotropic component across a submerged, trapezoidal, sill obstruction. High-resolution velocity and density profiles are obtained in the vicinity of the obstruction to observe internal-flow dynamics under a range of parametric forcing conditions (i.e. variable saline and fresh water volume fluxes; density differences; sill obstruction submergence depths). Detailed synoptic velocity fields are measured across the sill crest using 2D particle image velocimetry, while the density structure of the two-layer exchange flows is measured using micro-conductivity probes at several sill locations. These measurements are designed to aid qualitative and quantitative interpretation of the internal-flow processes associated with the lower saline intrusion layer blockage conditions, and indicate that the primary mechanism for this blockage is mass exchange from the saline intrusion layer due to significant interfacial mixing and entrainment under dominant, net-barotropic, flow conditions in the upper freshwater layer. This interfacial mixing is quantified by considering both the isopycnal separation of vertically-sorted density profiles across the sill, as well as calculation of corresponding Thorpe overturning length scales. Analysis of the synoptic velocity fields and density profiles also indicates that the net exchange flow conditions remain subcritical (G < 1) across the sill for all parametric conditions tested. An analytical two-layer exchange flow model is then developed to include frictional and entrainment effects, both of which are needed to account for turbulent stresses and saline entrainment into the upper freshwater layer. The experimental results are used to validate two key model parameters: (1) the internal-flow head loss associated with boundary friction and interfacial shear; and (2) the mass exchange from the lower saline layer into the upper fresh layer due to entrainment.
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Details

Original languageEnglish
Pages (from-to)27-57
Number of pages31
JournalEnvironmental Fluid Mechanics
Volume18
Issue number1
Early online date23 Mar 2017
DOIs
StatePublished - Feb 2018

    Research areas

  • Exchange flows, Net-barotropic flows, Saline intrusion blockage, Sill obstruction, Stratified flow, Mixing and entrainment, Internal-flow hydraulic modelling

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