Impact of a Large Shallow Semi-Enclosed Lagoon on Freshwater Exchange Across an Inlet Channel

The impact of a large shallow semi-enclosed lagoon on freshwater exchange across an inlet channel is investigated using an idealized numerical model. Lagoons are often found between a river mouth and the ocean; we focus on those where the river discharge rate is small and the inlet channel is narrower and deeper than the lagoon. Tides generate freshwater and oceanic-water plumes across the channel; a stratified freshwater plume forms in the ocean from the late ebb to early flood phase, while a vertically well-mixed oceanic-water plume forms in the lagoon from the late flood to early ebb phase. The shallow depth of the lagoon increases the flow speed of the oceanic-water plume, which results in the formation of a sharp and vertically well-mixed salinity front within the lagoon. When this front moves toward the ocean during the ebb phase, vertical mixing increases where the bathymetry deepens and freshwater encounters oceanic water below. Without a dredged bottom slope, the impact of mixing would be greatly reduced within the shallow lagoon and channel, as the shallow depth would limit the subsurface intrusion of oceanic water. The narrow channel further causes the flow to converge and accelerate, enhancing both internal shear-driven and bottom boundary-layer mixing at the channel and increasing freshwater plume thickness where it enters the ocean. Sensitivity experiments showed that the role of tidal pumping in freshwater exchange across the channel increases when the lagoon area and tidal mixing increase and when the estuarine Richardson number decreases. Freshwater outflows from estuaries play an important role in supporting coastal marine environments. Large shallow lagoons are often found at river mouths with an inlet channel; this study uses idealized numerical model experiments to investigate how lagoons affect freshwater exchange across an inlet channel where the river discharge rate is low. Freshwater exchange often results in estuarine circulation wherein freshwater enters the ocean near the surface, while oceanic water intrudes into the estuary near the bottom, with weaker circulation expected for a shallow environment. We find that large lagoons force strong tidal flows across the channel, enhancing mixing between freshwater and oceanic water, especially during the ebb phase. Due to the fast flow generated over the shallow lagoon, mixing occurs when freshwater in the lagoon moves across a sloping bottom and interacts with oceanic water below. Freshwater outflow becomes thicker and tidally pulsed plumes are generated. A large lagoon with a narrow inlet induces strong tidal pumping across the channel, in turn generating plumes in the ocean and lagoonMixing is enhanced as the sharp salinity front formed within the shallow lagoon moves across the sloping bottom during the ebb phaseTidal pumping induces more freshwater exchange than estuarine circulation when the estuarine Richardson number is below unity