The dynamics of bi-directional exchange flows: implication for morphodynamic change within estuaries and sea straits

Environmental and geophysical flows, including dense bottom gravity currents in the ocean and buoyancy-driven exchange flows in marginal seas,<br> are strongly controlled by topographic features.<br> These are known to exert significant influence on both internal mixing and secondary circulations generated by these flows.<br> In such cases, uni-directional or bi-directional exchange flows develop when horizontal density differences<br> and/or pressure gradients are present between adjacent water bodies connected by a submerged channel.<br> The flow dynamics of the dense lower layer depend primarily on the volumetric flux and channel cross-sectional shape,<br> while the stratified interfacial flow mixing characteristics, leading to fluid entrainment/detrainment,<br> are also dependent on the buoyancy flux and motion within the upper (lower density) water mass.<br> For submerged channels that are relatively wide compared to the internal Rossby radius of deformation,<br> Earth rotation effects introduce geostrophic adjustment of these internal fluid motions,<br> which can suppress turbulent mixing generated at the interface and result in the development of Ekman layers that induce secondary,<br> cross-channel circulations, even within straight channels.<br> Moreover, recent studies of dense, gravity currents generated in rotating and non-rotating systems,<br> respectively, indicated that the V-shaped channel topography had a strong influence on both flow distribution<br> and associated interfacial mixing characteristics along the channel.<br> However, such topographic controls on the interfacial mixing and secondary circulations generated by bi-directional exchange flows<br> are not yet fully understood and remain to be investigated thoroughly in the laboratory.<br> Also the effect of mobile bed for bi-directional exchange flows generated in deformable channels along with the physical interactions<br> between the lower dense water flow and the erodible bed sediments<br> will have a strong influence in (re-)shaping the overall channel bed topography (i.e. bed morphodynamics).<br> Consequently, the resulting temporal changes in cross-sectional channel bathymetry (i.e. through erosion and deposition processes)<br> would also be expected to have associated feedbacks on transverse asymmetries in the bi-directional exchange flow structure,<br> as well as on the internal flow stability.