The Changjiang Estuary: A highly turbid estuary in transition

Estuaries are the core area of land-sea interactions and have significant ecological and economic value. In estuaries, hydrodynamics and sediment dynamics are the crucial processes governing geomorphology, navigability, and primary production. Since the turn of the 20th century, increased human activities (e.g., damming, dredging, and reclamation) have subjected estuaries to significant pressure and prompted changes in hydrodynamics and sediment dynamics. Some estuaries, for example, experienced a transition from low- to hyper-turbidity after channel deepening. This transition is particularly common in tide-dominated estuaries such as the Ems and Loire estuaries. However, the reactions of estuaries with a high runoff (such as the Changjiang/Yangtze Estuary) are unclear, and the transition and underlying processes of such estuaries are further complicated by declined fluvial sediment supply. By integrating acoustic and optical sensors, this PhD dissertation developed a wide-range and high-precision sediment concentration observation system to monitor high sediment concentrations in the Changjiang Estuary. Over the past three decades, observations revealed a transition in suspended sediment concentrations, with increasing concentrations near the bed and decreasing concentrations near the water surface. The drag reduction induced by suspended sediment was assessed by a bottom-mounted tripod system. Moreover, this dissertation clarified the mechanism controlling the formation of concentrated benthic suspensions, namely the positive feedback between stratification, turbulence damping, and hindered setting. The comparison between sediment transport processes before and after the Deep Waterway Project reveals that estuarine circulation is the primary force driving sediment import from the sea, whereas tidal pumping results in the along-estuary extension of the estuarine turbidity maximum zone. These findings enhance our knowledge of the response of estuarine hydrodynamics and sediment dynamics to human interventions and provide a theoretical basis for the effective management of estuarine systems.