Mechanism of the Impact of Floods and Typhoons on the Morphological Evolution of the Yangtze Estuary

In the last two decades, the Yangtze Estuary has undergone significant changes under the influence of reduced sediment inflow and estuary engineering. This study investigates the influence of floods and typhoons on sediment concentration and the morphological evolution of shoals and channels in the Yangtze Estuary. The analysis is conducted through the utilization of topographic data measured pre- and post-flood events and observations of hydro-sedimentary changes during typhoons. By using a generalized estuary mathematical model, this study examines the interplay between varying tidal ranges, tidal divisions, runoff volumes, and regulation projects on the erosion and deposition of shoals and channels in bifurcated estuaries. The results show that due to the implementation of river and waterway regulation projects, the impact of the 2020 flood on the main channel and shoal was significantly less than that of the 1998 flood. The swing amplitude of the South Branch main channel decreased. However, local river sections such as the Southern Waterway of Baimao Shoal exhibited erosion. During typhoons, sediment concentration in the 20 cm above the bottom increased significantly and was closely related to wave processes, with a weakened correlation to tidal dynamics. After typhoons, high shoals in South Passage above 0 m were silted up, while the terrain on one side of the tail of Jiuduan Shoal in the downstream deep-water area was generally scoured due to strong wave action. The generalized mathematical model of the bifurcated estuary revealed that M2 tidal component contributed most to the erosion and deposition evolution of estuary shoals and channels, with floods exhibiting characteristics of sedimentation on shoals and erosion on channels. With the implementation of a branch rectification project, branch resistance increased, diversion decreased, and the riverbed changed from pre-project erosion to post-project sedimentation, with an increase in erosion in non-project branches.