Influences of topography changes on the evolution of salinity intrusion in the North Branch of the Yangtze River Estuary over the past 70 years

Estuarine topography influences hydrodynamics and salinity intrusion. The most significant feature of salinity intrusion in the Yangtze River Estuary is the salt-spilling-over (SSO) into the South Branch from the North Branch (NB), which affects the utilization of freshwater resources in the South Branch where three reservoirs were built. The topography in different decades from the 1950s to the 2020s was collected, and a numerical model was adopted to simulate the topography changes on salinity intrusion and SSO. From the 1950s to the 2020s, the topography of the Yangtze River Estuary changed greatly, especially in the NB. The channel area of the NB decreased by 55.56%, and its channel volume decreased by 57.57%. The numerical simulation results showed that the evolution of salinity intrusion in the NB and SSO experienced weakening–enhancing–weakening processes. The NB was covered by high-salinity water and experienced the SSO in the 1950s. The SSO distinctly weakened in the 1970s and 1980s, greatly enhanced in the 1990s, reached its most severe level in the 2000s, and significantly weakened in the 2010s and 2020s. From the 1950s to the 2020s, the tidal prism of the NB constantly decreased, with a reduction of 62.32% during spring–neap tide. The variations in the net water flux, net salt flux, and water diversion ratio in the upper section of the NB were consistent with the evolution processes of SSO. The evolution mechanisms of the salinity intrusion and SSO were elucidated in view of changes in the estuarine topography and dynamic factors.