*4.5. Analysis of Spillover on Morphological Dynamics*

The riverbed evolution around the bifurcation of the North and the South Branches, which closely relates to the spillover of water and sediment, is qualitatively analyzed, based on the simulation results and the aforementioned spillover mechanics.

When the landward floodtide high-concentration flow goes through the entrance reach of the North Branch, some of the sediment deposits on the riverbed on its landward journey through the upper reach of North Branch and the other part spills over to the South Branch. Overall, for the North Branch, the deposition at the entrance reach will facilitate the shrinkage of its upper reach and further prevent upstream inflows from entering during the ebbtide duration. This conclusion is consistent with field observations and former studies [5,30,35–38].

The entrance reach from the bifurcation to the QLG in the North Branch has a compound floodplain-channel cross-section (see Figure 3), where riverbed evolution may have the following properties. On the one hand, most of the floodtide flow goes through the main channel, and considerate

flow intensity is kept there. As a result, it is not easy for the sediment to deposit in the main channel. On the other hand, according to the simulated velocity field of the floodtide flow, the flow intensity of the floodplain is much weaker than that of the main channel, which implies that widespread sediment deposition may happen there.

The morphological dynamics of the North Branch can be summarized as the entrance reach of North Branch will experience successive sediment deposition (the possible sediment deposition mainly happens in the floodplain), leading to shrinkage of the branch.
