Exploration of the Mechanisms for the Low Sensitivity of Deposition Flux to Upstream Sediment Reduction in the North Passage, Yangtze Estuary

Deltas are densely populated industrialized regions, and home to important ports and navigation channels. Due to human interferences, the Yangtze Estuary has experienced a significant reduction in sediment load caused by the Three Gorges Dam (TGD), as well as adjustment to local morphodynamics by the Deep-water Navigation Channel (DNC). While the dramatic reduction in sediment triggers the increased channel erosion, the deposition flux of the DNC located at the estuary mouth has little change. To explore the physical mechanism of this phenomenon, a two-dimensional model is used to establish the relationship between sediment load and the sediment budget of channels based on the bathymetry in 2016. Model results show that the tidal reach and the inner estuary have a negative sediment budget. And the seasonal characteristics of water and sediment fluxes become less obvious downstream. Sensitivity analysis shows that the influence of upstream sediment load on deposition flux decreases along the channel, with a transition from deposition to erosion occurring in the tidal reach. For the last-level bifurcation, the annual siltation of sediment in the North Passage (NP) decreases by 4.5% with low sensitivity. This is attributed to the reduction of sediment load partially mitigated by riverbed erosion and cascade bifurcations. In addition, the lateral sediment supply, which accounts for 68% of the sediment input in the NP, is stable. Overall, this study strengthens the understanding of the relationship between sediment load and artificially deepened systems, thus allowing for better management of estuarine sediment and navigation channel.