An extremely severe saltwater intrusion event in the Changjiang Estuary: An analysis based on observations

Saltwater intrusion in estuaries is primarilycontrolled by river discharge and tide, leading to its occurrence typically during dry seasonunder low river discharge. However, an extremely severe saltwater intrusion occurredanomalouslyduring the flood season of 2022 in the Changjiang Estuary. This paper collected and analyzed the saltwater intrusion observed by longitudinal and synchronous surveys in the South Branch (SB) and the North Channel (NC) of the Changjiang Estuary in two massiveobservations during the event—the period of severe saltwater intrusion (from 29 September to 7 October) and the period of water release from basin cascade reservoirs (from 16 to 21 October). In the first observation, the 0.45 (the salinity standard for drinking water) isohaline at the surface extended to as far as 3.5 km downstream of the Sutong Bridge. Such a serious saltwater intrusion has never occurred, even in the dry season. The types of saltwater intrusion included frontal intrusion and saltwater spilling over (SSO) from the North Branch into the South Branch. The intensity of saltwater intrusion varied with the tidal range. In the second observation, the saltwater intrusion weakened sharply due to the artificial water release of basin cascade reservoirs. The surface 0.45 isohaline retreated to ~2.0 km upstream of the water intake of Qingcaoisha Reservoir, which shifted seaward by a maximum distance of ~26.5 km. The types of saltwater intrusion included frontal intrusion, SSO, and from the South Channel (SC) into the NC. Flux analysis indicated that both water and salt were transported landward at most of the sites observed in the first observation. Lagrange residual dominated the salt transport at most of the sites, while tidal pumping exhibited a predominant influence on the salt transport near the bifurcation of the NC and the SC. An extremely severe saltwater intrusion in the flood season of 2022 was observed, and the emergent measures of water release from the basin cascade reservoirs ensured the freshwater supply of the estuarine reservoirs, easing the water supply crisis in Shanghai.