Mechanisms Controlling the Distribution of Net Water Transport in Estuarine Networks

Abstract

Net water transport (NWT) in estuaries is important for, for example, salt intrusion and sediment dynamics. While NWT is only determined by river runoff in single channels, in estuarine networks, it results from a complex interplay between tides and residual flows. This study aims to disentangle the various contributions of these physical drivers to NWT in estuarine networks and investigate the sensitivities of NWT to variable forcing conditions, interventions, and sea level rise (SLR). To this end, a processes‐based perturbative network model is developed, which accounts for the vertical flow structure to resolve density‐driven flow driven by a vertically uniform along‐channel salinity gradient. Other identified drivers are river discharge and three tidal rectification processes: Stokes transport and its return flow, momentum advection, and velocity‐depth asymmetry. The model is applied to the Yangtze Estuary. NWT due to tidal rectifications and density‐driven flow can be comparable to river discharge. Specifically in the North Branch, the direction of NWT may differ from the direction of river discharge. Varying river discharge mainly affects NWT 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 (Deep Waterway Project), affected the NWT mostly through the density‐driven flow (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.

System‐Wide Effects of Local Bed Disturbance on the Morphological Evolution of a Bifurcating Channel Network

“…With the intensive human intervention in the many deltas across the world, our results can also help understand the long‐term morphological evolution of the channel network and inform the mitigation of the relevant hazards and risks, such as salt water intrusion, flooding and river avulsion (Cox et al., 2021; Luo et al., 2007; Shaw et al., 2021; Wang et al., 2015). For example, the spatially uneven navigation dredging and sand mining in the channel network of the Pearl River Delta and the Yangtze River Estuary have led to the alterations of the water partitioning at the bifurcations (Chen et al., 2022; Luo et al., 2007; Wang et al., 2022), which can potentially affect the spatial patterns of salt water intrusion and flood risks (Biemond et al., 2023; Qiu et al., 2022). The dredging activities in the lower Atchafalaya River have resulted in an altered water partition at the Mississippi‐Atchafalaya bifurcation (Shaw et al., 2021).…”

“…Furthermore, the system‐wide morphological responses pertain to the “process connectivity” in deltaic channel networks (Passalacqua, 2017), that is, the bed changes in one of the branches are highly coupled with the other branch. Therefore, it is critical to explore the long‐term morphological responses to human intervention in the entire channel network rather than focusing on a single channel reach (Chowdhury et al., 2023; Cox et al., 2021; Wang et al., 2022). The long timescale for recovering the equilibrium (Figures 5 and 7) can lead to significantly cumulative alterations of the water and material supplies to the downstream habitats (Hariharan et al., 2023).…”

Turbidity maxima in estuarine networks: Dependence on fluvial sediment input and local deepening/narrowing with an exploratory model

“…Hence, the sediment transport in Channel 2 is only influenced by the changing net water transport, primarily due to river flow and density-driven flow. The net water transport due to river flow scales with depth, whilst the net water transport due to density-driven flow scales with depth squared (Wang et al, 2022). For a relatively small depth of Channel 1 (H 1 <11 m), the river flow contribution to the sediment transport capacity T in Channel 2 is more important than the densitydriven flow contribution (Supplementary Figure S3A).…”

“…In this particular setting, the most dominant contributions to T are the residual transports by densitydriven flow (baroclinic, magenta) and river flow (cyan). The solution for density-driven flow in a single-channel estuary is given in Geyer and MacCready (2014) and references therein, and Wang et al (2022) presents the solution for a channel network. The density-driven flow imports sediment from the sea in Channel 1 and exports sediment into the sea through Channel 2.…”