Seasonal hydrodynamic interactions between tidal waves and river flows in the Yangtze Estuary

Estuaries are transition regions between marines and inland rivers where hydrodynamic processes are controlled by oceanic tidal waves and fluvial discharges. Moreover, human activities, such as the growing number of water conservancy projects, have an increasing impact on estuarine hydrodynamics. However, these natural variations and anthropogenic impacts are not yet fully understood, especially for large estuaries. In this study, the interactions between tidal waves and riverine flows in the Yangtze Estuary were investigated using the methods of numerical modelling and harmonic analysis. The focus of this study is on the characteristics of tides in the coastal area, seasonal variations in hydrodynamic process and impacts of reservoir regulation on tidal constituents in the tidal river. The results indicate that in the coastal area, semi-diurnal and diurnal tidal constituents propagate in different directions along the coast and that the tidal waves in the coastal area are stationary, i.e. the amplitude and phase do not change in time. However, in the tidal river, significant seasonal variations in the water surface slope, water level fluctuations and damping process can be identified. In the dry season, the tidal signal can reach Datong station, >600 km from the estuary mouth, whereas in the wet season, the signal's reach is approximately 350 km. From the dry to wet seasons, the maximum decrease in the amplitude of the dominant M2 tide is approximately 0.3 m. The amplitudes of the tidal constituents do not decrease monotonically because of the complex interactions among the bed friction, river discharge and channel width convergence as well as energy transmission between the tidal constituents. Because the Three Gorges Reservoir is used to regulate seasonal discharge, the increase in the accumulated amplitudes of the six major tides can reach up to 0.1 m in the tidal river reach.