Preliminary exploration of the spatiotemporal variation characteristics and causes of the main tidal amplitude in the Changjiang Estuary

The process and intrinsic mechanism of tidal wave propagation in the Changjiang Estuary are very complex under the interaction of tidal current and the comprehensive impact of sediment trapping at the river mouth. Using long-term hourly tidal level data, North Channel flood and drought hydrological data, and upstream Datong flow data from six tidal gauge stations along the "downstream estuary gate - North Channel sediment trapping - upstream Xuliujing" section, the t-tide analysis tool and channel resistance coefficient formula were used to analyze the spatial and temporal variations and causes of the amplitudes of the main tidal constituents (M2, S2, N2, K1, O1, M4) in the Changjiang Estuary under the influence of runoff and sediment trapping at the river mouth. The study shows that the amplitudes of the major astronomical constituents in the Changjiang Estuary exhibit a characteristic of first increasing and then decreasing as the tidal waves enter the estuary from the open sea, and the amplitude of the shallow water constituent (M4) gradually increases. The positive effect of runoff on the amplitude of the tidal constituents is manifested by the local water level rise caused by runoff, which increases the amplitude of the tidal constituents, while the negative effect is reflected by the interaction between runoff and tidal currents, which reduces the amplitude of the tidal constituents. Due to the difference in the proportion of positive and negative effects of runoff and tidal currents in different water areas of the Changjiang Estuary, the negative (positive) effect of runoff on the amplitude of the tidal constituents becomes more significant as it gets closer to the estuary (outer estuary). In the transitional water area between the two, the amplitude of the tidal constituent (M2) shows a "first increasing and then decreasing" trend with the increase of runoff, and the critical runoff is approximately 45,000 m³/s. The maximum amplitude of the astronomical constituents is located in the sediment trapping area at the river mouth, and the amplitude of the tidal constituents in the flood season sediment trapping area decreases along the section at a slower rate than in the drought season. The effect of high sediment concentration near the bottom of the flood season sediment trapping area on the propagation of tidal waves in the Yangtze River Estuary should not be ignored.