**1. Introduction**

The Yangtze Estuary is a large-scale shallow water system characterized by three-level bifurcations (North and South Branches, North and South Channels, North and South Passages) and has four outlets into the East Sea (see in Figures 1 and 2). Significant runoff from the Yangtze River (about 9000 × 10<sup>8</sup> m<sup>3</sup>/year) and periodical tides from the ocean meet in the estuary and interact with each other, leading to complicated hydrodynamics and sediment transport. The landward floodtide flow often spills over from the North to the South Branches, carrying a lot of sediment. The estuarine circulations of water and sediment fluxes, characterized by the spillover of water and sediment, play an important role in shaping the morphology of the Yangtze Estuary [1–5].

The spillover of water and sediment in the Yangtze Estuary is very complex because of the special morphology and the complex runoff-tide interactions. First, the spillover happens in a three-level branching estuary, where the exchanges of water and sediment are complex between different branches of the branching Yangtze Estuary. Second, the North Branch of the Yangtze Estuary is characterized by a special morphology [5]. The upper reach is narrow and almost orthogonal to the South Branch, preventing upstream inflows from entering during ebbtides. The lower and tail reaches

are trumpet-shaped with a wide outlet, in favor of accommodating a grea<sup>t</sup> deal of landward tidal flows during floodtides. Third, the river, the coast, and the ocean in the Yangtze Estuary are closely related, where the bidirectional flows inside the estuary evolve gradually into clockwise irregular rotational tidal flows in o ffshore regions (under the influence of runo ff-tide interactions, rapidly varying topographies and complex solid boundaries in coastal areas). As a result, it is di fficult to study the spillover problem of water and sediment in the Yangtze Estuary. Moreover, because of limitations of field data and methods (the details will be introduced in the following paragraphs), quantitative studies on the spillover of sediment from the North to the South Branches in the Yangtze Estuary have not been reported. The quantitative knowledge on the spillover of sediment in the Yangtze Estuary is currently quite limited.

On the other hand, surrounded by the most developed regions of China (Shanghai city and Jiangsu Province), the Yangtze Estuary has seen extensive launching of flood-defense, water-resource, reclamation, and navigation projects because of requirements for the development of cities. It is generally necessary to check the reasonability of the designed constructions before launching a project. The influences of a project on the estuarine environment (e.g., the tidal flow, sediment transport, and long-term riverbed evolutions) should also be evaluated to clarify its possible negative side and for corresponding preventions. Under the influence of the spillover of water and sediment, figuring out the aforementioned issues of a project in the branching Yangtze Estuary is challenging. As a result, it is important to have extensive knowledge of the horizontal circulations of water-sediment fluxes in the Yangtze Estuary, which will provide a guide and a support for the design of constructions in real applications.

**Figure 1.** The location of the Yangtze Estuary and the study area (a Google Map diagram showing the geographical features of the location). (**a**) Location; (**b**) Tidal reaches and estuary.

The tidal flows and sediment transport in the Yangtze Estuary are often studied by analyzing field data using the physical model or adopting the numerical models. However, the knowledge based on the analysis of topographical and hydrological data is often limited by space–time resolutions of the field data of sediment transport. Existing studies of analysis are often only carried out for local reaches or parts of cross-sections in some branches of the Yangtze Estuary, e.g., the sediment transport rate along the streamline of main-flow channels [1,2,6]. Scale models are expensive to build and operate. As an e ffective and less expensive method, many numerical models have gradually become the most widely used method in studying estuarine hydrodynamics and sediment transport due to continuous improvements in computers and numerical schemes.

Two-dimensional (2D) or three-dimensional (3D) numerical models applied to the Yangtze Estuary should meet the multiple requirements for computational accuracy and e fficiency. First, to ge<sup>t</sup> a full description of river-coast-ocean coupling, the upstream tidal reach, the entire Yangtze Estuary and part of the East Sea are included in a single model. The computational domain of the entire Yangtze Estuary (from Datong to seaward contours of −4 m) is 84.4 × 10<sup>8</sup> m2, as shown in Figure 2. Second, the computational grid should be fine enough to describe well the local river regimes in the Yangtze Estuary and simulate the estuarine mesoscale structures and transport process correctly [7,8]. Corresponding to fine grids, a small time step of 1–2 min is often required to ensure the stability and accuracy in simulating the fully unsteady flows and sediment transport. Third, simulations of long-term tidal flows, sediment transport, and riverbed evolution are often required in studies of the morphological dynamics. When the domain of the entire Yangtze Estuary is divided using a high-resolution grid, a huge computational cost is required. These requirements challenge almost all existing 2D or 3D numerical models [9]. As a result, in real applications of the Yangtze Estuary, researchers often have to use coarse grids, establish local models [10–12], or adopt simplified methods, such as the method of the morphological scale factor [13,14].

**Figure 2.** Description of the bound, the three-level bifurcations and the strong river-coast-sea coupling in Yangtze Estuary (computational domain and grid are also given).

In this paper, an efficient 2D numerical model is developed to simulate the tidal flow, sediment transport, and riverbed evolution in the Yangtze Estuary using a high-resolution channel-refined unstructured grid. The model is then applied to a quantitative study on the mechanics of the spillover of water and sediment in the Yangtze Estuary.
