The recent morphological evolution in the upper mouth sandbar of the North Branch and the impacts on its upper reach in the Changjiang Estuary

Due to the new-born sandbank found in the upper river mouth of the North Branch in Changjiang Estuary, the bathymetry data during 2010—2020 was used to investigate its evolution and trends in the last decade. Additionally, a hydrodynamic numerical model combined with an empirical formula was used to further explore the effects on the hydrodynamics and bathymetry after the sandbank was further silted up or dredged. Results indicate that, in the past 10 years, the area of the sandbank has increased by nearly 5 times, and its elevation has gradually developed from less than 1 m to more than 3 m. In the recent years, the sandbank has stopped growing larger, but still gradually grown taller, and exhibited a trend of transformation from the lower tidal flat to the higher one. If the sandbank further silts up, the water diversion ratio of the North Branch will be further reduced. At the same time, the velocity of the main channel from Haimen Port to Rixin River will decrease, which may lead to siltation and the restrict of inflow in the North Branch. The nearshore water speed around Yuejin Port will increase, and may enhance the erosion there, which may threaten the safety of sea dikes. If the sandbank is dredged off, the water diversion ratio of the North Branch will increase, which takes advantageous to alleviate the S-shaped river regime in the upper North Branch. At the same time, the velocity from Haimen Port to Rixin River will increase, the main channel will be deepened, which will promote the water diversion ratio in the North Branch, decrease the nearshore water speed around Yuejin Port, induce the local siltation, and finally benefit the safety of sea dikes.