Analysis and modelling of Morphodynamics of the Yangtze Estuary

Abstract

The flow and sediment transport in the Yangtze Estuary are intrinsically complex because various processes and mechanisms are involved on a large range of temporal and spatial scales. In this thesis the interaction of river discharge and tidal wave with the corresponding sediment transport in the Yangtze Estuary is investigated. The objective is to gain further understanding of the processes and mechanisms dominating the sediment transport in the estuary. Based on the understanding of flow and sediment transport, a morphodynamic model is established and tested to simulate the morphological change of the Yangtze Estuary. Supported by a literature survey reviewing previous studies, the observed data (including water levels, currents, salinity, sediment concentration, sediment samples, etc.) at various stations under different conditions (spring/neap tide, dry/wet season, etc.) are first analyzed to investigate the characteristics of flow and sediment transport in the Yangtze Estuary. Subsequently, a process-based model based on Delft3D is set up for the estuary. After being calibrated and validated against measurements under various conditions, the model is used to simulate the sediment transport at the mouth bar of the Yangtze Estuary. Scenarios of model simulations are designed to account for different combinations of processes and mechanisms contributing to sediment transport. The results demonstrate that taking salinity processes into consideration is a prerequisite to understand how fine sediment has been trapped in the mouth bar area of the Yangtze Estuary in the last half century. It is also concluded that flocculation of fine-grained sediment in suspension enhances the sediment deposition in the mouth bar area. The net effect of all sediment transport processes is typical sedimentation in the wet season and erosion in the dry season, with net deposition annually. A decreasing trend in the annual net deposition has recently become visible. The deposition rate at present is down to 1/3 of the magnitude in the past.