Modelling Exchange Flows across a Submerged Sill: A Numerical and Experimental Study

Alan Cuthbertson (Lead / Corresponding author), Jarle Berntsen, Janek Laanearu, Magdeli Asplin

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This paper presents direct qualitative and quantitative comparisons between laboratory experiments of density-driven exchange flows across an idealised, submerged sill obstruction and equivalent lab-scale numerical simulations using the Bergen Ocean Model (BOM), a three-dimensional general ocean circulation model. The laboratory experiments were conducted in a large rectangular channel with a trapezoidal sill to control exchange flows generated between adjacent saline water and freshwater basins depending on the sill submergence depth, the density difference between saline and fresh waters and their respective inflow volume fluxes into each basin. High resolution particle image velocimetry and micro-conductivity probes were used to obtain detailed velocity fields and density
profiles of the exchange flow generated across the sill under different parametric conditions. These measurements demonstrate that the saline intrusion flux across the sill is initially reduced and then eventually fully blocked under increasing net-barotropic flow conditions imposed in the upper freshwater layer, as well as for reduced sill submergence depths. These parametric dependences are also demonstrated in the equivalent BOM simulations of non-rotating sill exchange flows, although the numerical model tends to overpredict both the interfacial velocity and density gradients across the sill and the fresh-saline source flux ratio at which full blockage of the saline intrusion occurs. The BOM simulations were extended to consider rotating sill exchange flow dynamics. In particular, these additional model runs demonstrate that Coriolis forces result in an increased blockage of the saline intrusion layer compared to equivalent non-rotating exchange flows, especially when the Rossby number associated with the saline intrusion flow across the sill is considerably less than unity. This is largely attributed to the development of Ekman boundary layer dynamics and associated secondary circulations within the bi-directional exchange flows that impose strong control on the transverse distribution and extent of the lower saline intrusion flow across the sill and, hence, the parametric conditions under which full saline intrusion blockage is achieved.
Original languageEnglish
JournalEnvironmental Fluid Mechanics
Publication statusSubmitted - 25 May 2020


  • sill exchange flow, density-driven overflows, dynamic blocking, lab-scale modelling, numerical simulations, ocean modelling

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    Cuthbertson, A., Berntsen, J., Laanearu, J., & Asplin, M. (2020). Modelling Exchange Flows across a Submerged Sill: A Numerical and Experimental Study. Manuscript submitted for publication.