Saltwater Intrusion and Emergency Freshwater Release in the Changjiang River Estuary Under Extreme Drought Conditions

Releasing freshwater from upstream reservoirs is a reasonable strategy to mitigate saltwater intrusion, however, its effectiveness may vary depending on weather events. Previous studies have primarily examined the effects on saltwater intrusion of seasonal regulation in reservoir discharge, with a limited attention given to the synoptic scale. This study applied the ECOM-si to quantitatively analyze the impacts of the October 2022 emergency freshwater release on saltwater intrusion, also elucidating the mechanisms by which cold fronts (defined as northerly winds with speeds exceeding 10 m/s) impaired the effects of that release. The release reduced landward advective salt flux, shifting the 0.45 psu isohaline 17 km downstream during the neap tide period. On October 21, the salinity at the Qingcaosha Reservoir (QCSR) intake point fell to 0.45 psu, creating a 12.75-hr window for freshwater intake. Cold fronts greatly diminished the effectiveness of freshwater release, shortening the water intake period by 24.14 hr. During the cold front period, northerly winds induced landward Ekman transport, creating a horizontal recirculation pattern with inflow through the North Channel (NC) and outflow through the South Channel (SC). The net landward water flux per unit width in the NC reached −1 m²/s. During the first cold front, steady shear salt flux contributed most significantly, with a magnitude of −70 ton/s, while advective salt flux dominated during the second cold front, reaching −239 ton/s. Without the cold fronts, the potential water intake time could have increased to 36.89 hr.