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