PIANC Smart Rivers 2022

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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 key factors 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 using Delft3D, and the shallow water assumption is applicable due to the wide and shallow channel morphology in the LYR. The k-epsilon model was chosen as the turbulent model to simulate the turbulent flow processes. The model has structured grids in the horizontal direction and is divided into 10 layers in the vertical direction based on the sigma-division method. The model simulates the flow characteristics during high, medium, and low tides in flood and dry seasons. The numerical simulation shows that there is a significant difference in the large-scale flow structure between the flood and the dry seasons. In the flood season, the flow at the entrance of the KW is extremely turbulent due to two factors, the complex topographic boundary of the upstream reaches, and the topographic narrowing effect at the entrance at the KW that causes flow separation. A large-scale vortex shedding phenomenon, similar to wake flow, is formed downstream of the topographic narrowing at the channel entrance. The dislodged vortex can last up to 3-5 times the river width downstream. The vortex shedding causes strong flow mixing and thus significantly increases flow turbulence, which strengthens the sediment transport capacity and leads to an increase in the amount of sediment flux into the KW. In addition, because the river width widens in the middle part of the KW, a large-scale gyratory flow structure is developed near the sandbar at Sanyiqiao, and the scale of this structure completely covers the sidebar. This gyratory flow structure tends to introduce sedimentation. The prementioned two factors, increasing inlet sediment transport capacity and the development of gyratory flow structure, lead to sedimentation in the sidebar at Sanyiqiao and the middle portion of the waterway. In the dry season, both of these factors are greatly weakened. On the one hand, the sediment transport rate at the entrance decreases, and on the other hand, the gyratory flow structure at the side sandbar is significantly reduced. The results of this study provide a vital reference for the engineering works of waterway regulations in tidal river reaches.

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