关键词: 河势变化, 咸潮入侵, 纳潮量, 分流比, 盐通量

Abstract:

Studying changes in estuarine saltwater intrusion patterns can deepen the understanding of its mechanism and help better ensure the safe use of freshwater resources. Based on measured topographic and salinity data, this paper analyzes changes in river regime and saltwater intrusion patterns in the Changjiang Estuary. A numerical model is used to reveal the causes of these pattern changes. From 2007 to 2021, major reclamation projects in the estuary led to channel narrowing and significant local topographic changes. Due to severe siltation on the south side of the lower North Branch and the emergence of a new sand body at its upper end, the channel volume decreased by -33.33% and -13.26% (with the negative sign denoting northward transport from the south), respectively. Salinity observations in the dry seasons of 2007 and 2021 indicate a significant weakening of saltwater intrusion and of the North Branch saltwater backflow into the South Branch. In the North Channel, saltwater intrusion weakened under prevailing climatic winds but intensified under strong northerly winds. Overall, the saltwater intrusion pattern in the Changjiang Estuary has changed. Numerical simulations, considering multi-year monthly mean river discharge and wind, show that from 2007 to 2021, saltwater intrusion weakened significantly in the North Branch, weakened in the upper and most middle reaches of the North Channel, but intensified significantly in the South Channel, North Passage, and South Passage. The saltwater backflow from the North Branch into the South Branch was greatly reduced, resulting in weakened saltwater intrusion in the South Branch, which is conducive to water intakes at source areas. In the North Branch, the tidal prism decreased by 2.88×10⁸ m³ and 1.98×10⁸ m³ during spring and neap tides in February, respectively. The net water flux into the South Branch decreased by 423 m³·s^-1 and 369 m³·s^-1, and the net salt flux into the South Branch decreased by 10.06 kg·s^-1 and 1.10 kg·s^-1. The variation of net unit-width salt flux in the upper North Branch also indicates a major reduction in salt transport to the South Branch. Significant North Branch backflow observed in 2007 had nearly disappeared by 2021. In the North Channel, during February spring and neap tides, the tidal prism decreased by 1.92×10⁸ m³ and 1.86×10⁸ m³, while the net water flux increased by 857 m³·s^-1 and 1379 m³·s^-1. The water diversion ratio increased by 12.79% and 7.79%, and the net seaward salt flux decreased by 2.28 kg·s^-1 and 5.42 kg·s^-1. These changes explain the weakening of saltwater intrusion in the upper and most middle reaches of the North Channel. The obvious increase in the North Channel’s water diversion ratio (corresponding to a decrease in the South Channel’s ratio) also accounts for the intensification of saltwater intrusion in the South Channel, North Passage, and South Passage. A localized salinity rise in a small part of the North Channel during spring tide is caused by a significant increase in the net salt flux from the North Passage into the North Channel. This paper reveals the hydrodynamic causes of the changing saltwater intrusion pattern in the Changjiang Estuary through changes in net water flux, net salt flux, tidal prism, and water diversion ratio.

Key words: river regime change, saltwater intrusion, tidal prism, water diversion ratio, net salt flux