Abstract
Abstract. The Princeton Ocean Model (POM) is used to investigate the intratidal variability of currents and turbulent mixing and their impact on the characteristics and evolution of the plumes of two neighbouring rivers, the Zhuoshui River and the Wu River, at the central eastern coast of Taiwan Strait. The two estuaries are located close to each other and their conditions are similar in many respects, and yet the two plumes exhibit significantly different behaviour. We explain this through differences of the bottom topography in the areas adjacent to the two river mouths. The Zhuoshui River runs into a shallow area that is permanently exposed to strong tidal mixing, while the Wu River mouth is located in a deeper, stratified area outside the region of intense mixing. This destruction of the plume by tidal mixing is confirmed by the results of numerical modeling with POM. The spatial and temporal variability of turbulent kinetic energy, the rates of its production by shear and destruction rate by buoyancy in the study, as well as the horizontal diffusivity, are analysed with the emphasis given to the dependence of the turbulence parameters on the bottom topography on the one hand and their influence on the river plumes on the other. The results of the study support the central hypothesis of this paper: the dynamic behaviours of the Zhuoshui and Wu plumes are different because their evolution occurs under different regimes of bottom-generated turbulent mixing. Further, we use a Lagrangian particle tracking model in combination with POM to investigate the effect of the tidal wetting-and-drying (WAD) near the Zhuoshui River estuary, and demonstrate that WAD leads to significant reduction of the plume extent and surface salinity deficit near the river mouth. We use observational data from a short field campaign in the study area to tune and validate the model experiments.
Highlights
Understanding the physics of river plumes is important for predicting the pathways and fate of pollutants and other terrigenic material carried by river water
A peculiar feature of tidal dynamics in this area is the asymmetry of the sea surface height (SSH) curve and current velocity, so that near the Wu River (Site 1), the period of falling tide lasted less (∼ 1 h) than that of the rising tide
A high-resolution Princeton Ocean Model was used to investigate the intratidal variability of currents and turbulent mixing and their impact on the characteristics and evolution of the plumes of two neighbouring rivers, the Zhuoshui River and the Wu River, at the central eastern coast of Taiwan Strait, where the shallow sand Chang-Yuen Ridge is a significant feature of the bottom topography having considerable impacts on the local dynamics
Summary
Understanding the physics of river plumes is important for predicting the pathways and fate of pollutants and other terrigenic material carried by river water. Due to continuity requirements imposed by the coast, the offshore-directed surface currents and onshore-directed bottom currents lead to coastal upwelling tending to displace the plume offshore (de Boer et al, 2009) This effect on river plumes has been investigated in a number of field and model studies (Simpson and Souza, 1995a, b; Souza and James, 1996; de Boer et al, 2006, 2009; Burchard and Schuttelaars, 2012). We use the Princeton Ocean Model to investigate the influence of mixing governed by bottom-generated turbulence on the dynamics and evolution of the Zhuoshui and Wu plumes and identify processes primarily responsible for differences in the development of the two plumes.
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