*3.1. Computational Grid and Boundary Conditions*

To ge<sup>t</sup> a full description of the river-coast-ocean coupling, the upstream tidal reach, the entire Yangtze Estuary and part of the East Sea are included in a single model. Station Datong (620 km upstream of the outlets), which is regarded as the tidal limit of the estuary and has routinely collected hydrological field data, was chosen as the upstream boundary. Seaward open boundaries are extended to deep-water (>50 m) regions, where a global tide model (GTM) [29] can provide an accurate history of astronomical tides. The eastern seaward open boundary is located around 124◦ E, with southern and northern boundaries being at 28.7◦ N and 33.9◦ N, respectively.

With three-level bifurcations and tens of islands or shoals, the Yangtze Estuary has complex river regimes [30]. The common resolution of bathymetry graphs, for an accurate description of the local river regimes of the Yangtze Estuary, is listed in Table 1. Coarse computational grids only describe

an oversmoothed riverbed and are unable to correctly solve estuarine mesoscale structures [7] and transport process [8]. To ensure a good description of irregular boundaries and local river regimes, a channel-refined unstructured grid is used, whose grid scale (listed in Table 1) is approximately equal to the intervals between two neighboring survey points of the corresponding bathymetry graph. The main-flow channels in tidal reaches are covered by refined structured-like grids, with floodplains and inner islands covered by relatively coarse unstructured grids. There are 199,310 quad cells, and an example of the grid is given in Figure 2 (see Figure S1 for details).


**Table 1.** Grid scales of high-resolution unstructured grids in Yangtze Estuary.

At the upstream boundary (see Figure 2), field data of discharges and sediment concentration at Station Datong were used to set the upstream boundary conditions. At downstream boundaries, the seaward open boundary is forced by semidiurnal tides. The time series of the tidal levels at the seaward boundary are predicted by the GTM developed in reference [29]. The seaward boundary is divided into 48 segments (see Figure 2), for each of which the tidal harmonic constants are, respectively, interpolated from a constituent database on a full global grid.

In calibration and validation tests of the HDM and the STM, the computational time step (Δ*t*) is set to 90 s, and is equally divided into 9 sub time steps in the backtracking of the point-wise ELM.
