Hydrodynamics and suspended sediment dynamics in estuarine channel networks: an idealised modelling approach

Abstract

Estuaries are bodies of water that connect river to sea. Many of them are composed of multiple interconnected channels and thus constitute a so-called estuarine channel network. They are important for both ecology and economy. Freshwater and fluvial sediments are continuously discharged into the system by the river flow. Additionally, the tidal flow causes water and sediments being periodically imported from and exported into the open sea. Water and sediments are exchanged at the junctions of the channels. Therefore, estuarine channel networks feature complex hydrodynamics and sediment dynamics. The latter gives rise to the estuarine turbidity maximum (ETM), where the suspended sediment concentration attains a local maximum. The aim of this thesis is to understand more about the response of the hydrodynamics and the ETM dynamics in estuarine channel networks to natural environmental conditions, as well as due to changes in these conditions due to e.g. climate change or anthropogenic measures. To understand the dependence of their along-channel and vertical structure on forcings, geometry characteristics and sea level changes, an idealised process based model that resolves the flow vertical structure is developed and applied to the Yangtze Estuary. Increasing river discharge enhances the friction for tides by increasing both internal and bottom stresses. Changes in tidal forcing are correlated with the friction for both tide and river. A shortcut channel reduces the water level difference in adjacent channels. SLR results in larger friction parameters, faster propagation of tides, and more even distribution of river water transport. To disentangle the various contributions of physical drivers to net water transport (NWT) in estuarine networks and to investigate the sensitivities of net water transport to above-mentioned change, the model is further developed to resolve density-gradients. NWT due to tidal rectifications and density-driven flow can be comparable to river discharge. Varying river discharge mainly affects NWT due to river as tide-river interaction is weak and density-driven flow is shown to be insensitive to salt intrusion. Conversely, variations in tidal amplitude strongly affect NWT related to tidal rectification and density-driven flow. The deepening (narrowing) of one channel affected the NWT mostly through the density-driven flow (momentum advection). Furthermore, NWT distribution in the Yangtze is insensitive to SLR up to 2 m because the effects of SLR on transport due to different drivers compensate each other. ETM locations in an idealised three-channel network on fluvial sediment input and the local deepening and narrowing of a seaward channel is investigated. Sensitivity results show that, keeping river discharge fixed, a larger fluvial sediment input leads to the upstream shift of ETMs and an increase in the overall sediment concentration. Both deepening or narrowing of a seaward channel may influence the ETMs in the entire network. Furthermore, the effect of either deepening or narrowing of a seaward channel on the ETM locations in the network depends on the system geometry and the dominant hydrodynamic conditions. Therefore, the response of the ETM location to local geometric changes can only be understood by analysing the dominant sediment transport mechanisms.