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The tidal reaches are characterized by unsteady and non-uniform flow (UNF), which is significantly different from the commonly assumed steady and uniform flow (SUF) in hydraulics. The SUF shows invariant temporal and spatial flow characteristics, and thus flow acceleration is absent in a prismatic channel. However, for the UNF, the variation of flow velocity and depth in both temporal and spatial scales causes the loss of flow energy, and thus increases the flow resistance. In order to clarify the variation of flow resistance and its influencing factors in tidal reaches, this study investigates the flow resistance characteristics under UNF conditions. In this study, a typical tidal section of the Lower Yangtze River (LYR) – Kouanzhi Waterway (KW) – was selected as the study area, where the temporal variation of water surface along the river course at different tide levels, the bathymetry of multiple cross-sections, the distribution of cross-sectional flow velocity and its temporal variation were measured in detail. Based on these field measurement data, the contribution terms to the energy slope were calculated and evaluated, by decomposing the momentum equation. The calculated contributing terms include water surface gradient, local acceleration, and convective acceleration. The results showed that the local acceleration and convective acceleration have a substantial impact on the energy slope during specific time periods, which was found to be more significant than the findings in previous studies. The results show that the local acceleration term is more significant than the convective acceleration term except when the water surface slope is close to zero, and its contribution is significant throughout the flood tide and the initial ebb tide periods. The above research results are of great significance for the investigation of flow resistance mechanisms and numerical simulations in tidal rivers.

River flow in the Lower Yangtze River (LYR) is influenced by the combined effect of runoff and tides, and the complex flow conditions tend to cause sediment deposition in waterways, which in turn affects navigation conditions. In order to improve the understanding of the river mechanism of tidal reaches, this study selected a typical reach in the LYR – the Kouanzhi Waterway (KW) – as an example to investigate the flow characteristics that affect sediment transport processes. Sedimentation annually occurs at the entrance of the KW and a large sidebar constantly increases in size, causing the reduction of navigation depth and width, but the causes of the sedimentation are still unclear. In this study, a three-dimensional (3D) model of the KW was established based on the Delft3D, and the k-epsilon model was chosen to simulate the turbulent flow. The model simulates the flow processes during flood tides in flood and dry seasons. The results of the numerical simulation show that a significant difference exists in the large-scale flow structures between the flood and the dry seasons. In the flood season, flow at the entrance of the KW is extremely turbulent and a large-scale vortex shedding phenomenon is formed downstream of the channel entrance, which causes a great sediment transport rate at the entrance. In addition, because the river width widens in the middle part of the KW, a large-scale circulation flow structure is developed near the Sanyiqiao sidebar. The circulation is supposed to trap and deposit sediment on the sidebar. In the dry season, the intensity of flow turbulence is greatly reduced, which causes the sediment transport rate at the entrance decreases accordingly. In addition, the circulation flow structure at the Sanyiqiao disappeared, and thus the sidebar is supposed to be under erosion during this period. The results of this study provide a vital reference for the engineering works of waterway regulations in tidal river reaches.

In view of the navigation problem of Dongbei Waterway in the lower reaches of the Yangtze River, the evolution of riverbed and the characteristics of the obstacles are analyzed. The engineering layout scheme is determined by various methods such as model test. In the design of engineering structure, the geological conditions of the project area are analyzed through geological exploration. The applicable engineering structure form is determining, the advanced and mature bank protection and beach protection structure is adopted in the regulation of the Yangtze River channel, and the optimized structural design is adopted to strengthen the stability of project and deal with problems such as geology and strong erosion. The environmental design material and the green structure type are adopted in the structural design to meet the ecological benefits of the project, which can provide reference for the engineering design of similar channel improvement project.