Study on the changes in water and sediment transport in the north passage following the third phase of the Yangtze Estuary deepwater channel project

Based on topographic and hydrological data from 2010 to 2021, this study analyzes the changes in water and sediment transport in the North Passage following the implementation of the third phase of the Yangtze Estuary deepwater channel project. Post-completion, the overall water depth of the upper and middle sections of the North Passage increased, with the −8 m contour line widening primarily between 2010 and 2017, and the −10 m contour line continuously expanding. The lower section experienced relatively minor depth changes. The average tidal current velocity in the middle and lower sections showed no significant trend, while the sediment concentration in most water layers decreased, with the average sediment concentration in the middle layer decreasing by nearly 50%. The longitudinal water and sediment flux in the North Passage was significantly higher than the lateral flux, with the proportion of lateral flux decreasing since 2010. The net flux of current in the North Passage was higher in the surface layer and lower in the bottom layer. Over the years, a vertical circulation of water flux has been observed in the middle and lower sections, with net lateral flux moving northward in the surface layer and southward in the bottom layer. The net sediment flux was higher in the bottom layer and lower in the surface layer. Longitudinally, the net sediment flux in the surface layer pointed downstream, while the bottom layer showed less regularity. Laterally, post-2015, the net sediment flux in the surface layer at the middle and lower sections generally pointed to the north side of the channel, while the bottom layer pointed south, forming a vertical circulation pattern with the surface layer.