Flow and Sedimentation Characteristics of Tidal Waterways – with the Kouanzhi Waterway in the Lower Yangtze River as an Example

Abstract

AbstractRiver 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.