Abstract:

[Objective]As a critical deep-water navigation channel, the stability of the water-sediment structure in the North Passage of the Yangtze River Estuary directly affects channel maintenance and navigational safety. This study aims to (1) identify the main driving factor behind the baroclinic effects (salinity vs. temperature) caused by water density in the North Passage, (2) quantitatively reveal the differential influence of baroclinicity on flood and ebb current velocities across different water layers (surface vs. bottom), (3) clarify how baroclinic effects reshape the vertical distribution of sediment content (particularly the tidal-averaged sediment content and its vertical gradient), and (4) investigate the similarities and differences in baroclinic effects between the main channel and adjacent shoals, thereby deepening the understanding of the physical mechanisms governing water-sediment dynamics in the North Passage, providing more precise theoretical support for channel management and sediment deposition prediction. [Methods] An advanced three-dimensional high-resolution coupled mathematical model of hydrodynamics-sediment-salinity was established to accurately represent the complex topography, tidal forcing, runoff input, and salt-freshwater mixing processes in the North Passage of the Yangtze River Estuary. Key components of the model included hydrodynamic module, salinity transport module, sediment module, and density calculation. To isolate and quantify the baroclinic effects, a baseline scenario and a baroclinicity-off scenario were designed (in which the baroclinic term induced by density gradients was artificially disabled, while maintaining identical topography, tides, runoff, and sediment parameters). By comparing the velocity fields (especially the vertical structure) and sediment content fields (vertical distribution and tidal average) under the two scenarios, the net influence of baroclinicity on the water-sediment structure in the North Passage was precisely determined. [Results] (1) Salinity difference was the absolute dominant factor in generating water body density gradients and significant baroclinic effects in the North Passage, while the influence of temperature was negligible.(2) Differentiated vertical influence on flow velocity structure: Baroclinic effects significantly enhanced the bottom-layer flood current velocity, with the most pronounced influence observed in the middle and lower sections of the North Passage. In contrast, its influence on surface-layer flood velocity was relatively small or slightly weakening. Baroclinic effects generally weakened the ebb current velocity across all water layers (surface to bottom), with a particularly evident reduction in the bottom layer.(3) Spatial difference: The influence of baroclinic effects on flow velocities (including flood and ebb currents) was significantly stronger in the main channel than in adjacent shoals, indicating that the deep-channel topography amplified the dynamic effect of baroclinicity.(4) Influence on sediment content structure: Baroclinic effects led to a significant decrease in the average sediment content during ebb tides in the bottom layer, while conversely, the average sediment content during flood tides showed an increasing trend. Baroclinic effects profoundly altered the vertical distribution structure of sediment content, enhancing the vertical gradient of average sediment content during flood tides. This indicated that during flood tides, the difference between the bottom layer with high sediment content and the surface layer with low sediment content became more pronounced, intensifying vertical stratification. In contrast, baroclinic effects reduced the vertical gradient of average sediment content during ebb tides, indicating relatively enhanced vertical mixing or reduced differences in sediment content between layers during ebb phases.(5) Mechanistic linkage: The changes in flow velocity structure (enhanced bottom-layer flood currents and weakened ebb currents) served as the direct hydrodynamic driver for the sediment content response (increased sediment content during flood tides and decreased during ebb tides in the bottom layer). The variations in vertical gradients reflected how baroclinic effects influenced the vertical diffusion and stratification of sediment by altering vertical circulation and mixing intensity. [Conclusion] (1) Salinity is the sole key factor driving the baroclinic effects in the North Passage of the Yangtze River Estuary, and for the first time, the detailed influence patterns of baroclinic effects on layered flow velocities and vertical structure of suspended sediment content in the North Passage are systematically revealed.(2) The baroclinic effects substantially restructure the vertical momentum distribution in the North Passage by altering the vertical pressure gradient, manifesting as enhanced flood-tide dynamics and weakened ebb-tide dynamics in the bottom layer. This finding is of great significance for understanding the dynamic mechanisms of the formation and maintenance of the largest turbid zone in estuaries.(3) New insights into sediment response: The influence of the baroclinic effects on suspended sediment content exhibits significant tidal phase dependence and vertical non-uniformity, promoting greater sediment accumulation in the bottom layer during flood tides (increased sediment content and vertical gradient), and enhanced diffusion during ebb tides (decreased sediment content and vertical gradient).(4) The influence intensity of baroclinic effects on hydrodynamics exhibits distinct spatial heterogeneity. The main channel of the deep-water navigation route demonstrates more sensitive and pronounced responses to baroclinic forcing compared to the adjacent shallow shoals on both sides, highlighting the critical role of topography in modulating baroclinic effects.(5) The successful application and verification of the three-dimensional coupled hydrodynamics-sediment-salinity model, combined with the “baroclinic switch” scenario comparison method, demonstrates its effectiveness in complex estuarine studies and provides a reliable approach for precisely isolating the influence of a single physical process (e.g., baroclinicity) within a complex system. This study demonstrates that salinity-induced baroclinic effects serve as a key physical mechanism shaping water-sediment transport and deposition in the North Passage of the Yangtze River Estuary. The revealed detailed response characteristics of flow velocity and sediment content (layered, tidal-phase-specific, and spatially heterogeneous) offer theoretical and practical value for improving the prediction model for the siltation of deep-water channels in the Yangtze River Estuary, optimizing the dredging strategies for channel maintenance, and understanding the morphological evolution of estuaries. Future water-sediment modeling and management in the North Passage must fully consider the refined influence of salinity baroclinicity.

Key words: North Passage of Yangtze River Estuary, three-dimensional model, density gradient, baroclinic effect, tidal sediment