Extreme Saltwater Intrusion in the Yangtze Estuary under Compound Drought, Wind Forcing, and Sea-Level Rise

Coastal megacities are increasingly exposed to estuarine saltwater intrusion (SWI) as a result of climate change and extreme hydro-meteorological events. Shanghai, a water-stressed megacity with a population exceeding 24 million, relies on the Yangtze Estuary for more than 80% of its freshwater supply, rendering it highly sensitive to  saltwater intrusion. In this study, we apply the three-dimensional hydrodynamic model UFDECOM-i, developed by our research group based on the ECOM-si. The model incorporates two-way nested unstructured quadrilateral grids, enabling an improved representation of complex estuarine bathymetry and hydrodynamic interactions among multiple bifurcated channels during extreme conditions.We first simulate the extreme SWI event that occurred in the summer of 2022, driven by a basin-wide record-breaking megadrought. Our simulations show that the concurrence of exceptionally low river discharge and persistent northerly winds led to severe summer salinization in the estuary. As a consequence, the Qingcaosha Reservoir experienced 98 consecutive days during which water was unfit for intake, far beyond its designed operational resilience. Further diagnostic analysis reveals that reduced river discharge weakened the seaward freshwater flushing, while wind-induced landward Ekman transport generated an anomalous horizontal estuarine circulation. This circulation pattern is characterized by inflow through the North Channel and outflow through the South Channel, and it intensified the direct transport of high-salinity water toward critical water intake locations.In addition, we quantitatively assess the influence of future sea-level rise, SLR, on SWI intensity. Simulations under a 1.17 m SLR scenario derived from RCP8.5 projections reveal a pronounced non-linear amplification of saltwater intrusion intensity. The sensitivity of estuarine salinity to river discharge reduction increases significantly. Notably, the impact of SLR on salinity enhancement is approximately twice that of discharge decline. Under the 1.17 m SLR scenario, the duration of water unfit for intake at the Chenhang Reservoir increases from 76.17 days to 115.49 days, corresponding to a 51.6% increase from present-day levels. These results provide a quantitative basis for assessing future freshwater security in the Yangtze Estuary and offer scientific support for adaptive water resource management and refined diversion strategies under a changing climate, aligning with the United Nations Early Warnings for All initiative aimed at strengthening climate-resilient water security.