**6. Conclusions**

To quantitatively clarify the problem of sediment spillover in Yangtze Estuary, a 2D numerical model using a high-resolution channel-refined unstructured grid (200,000 cells) was developed for the entire Yangtze Estuary from Datong to river mouths (620 km) and part of the East Sea. The developed model achieves a good description of the river-coast-ocean coupling, irregular boundaries, and local river regimes in the Yangtze Estuary. The model parameters are calibrated using field data, for which the sensitivity studies are done. In validation tests, the simulated histories of the tidal level, depth-averaged velocity, and sediment concentration agree well with the field data. In e fficiency tests, it takes the model 12.2 h (using 16 cores) to complete a simulation of a 1-year unsteady flow, sediment transport, and riverbed evolution in the Yangtze Estuary.

The sediment spillover in the Yangtze Estuary was studied using the conditions of a spring and neap tide in dry seasons. The fluxes of water and sediment at cross-sections are respectively calculated using the simulated histories of flow rates and sediment transport rates. For a representative cross-section in the upper reach of the North Branch (QLG), the di fference of the cross-sectional water flux (*CSWF*) between the floodtide and the ebbtide durations is 0.6–0.47 × 10<sup>8</sup> m<sup>3</sup>/day, accounting for 24.2–28.8% of the landward floodtide *CSWF*. The di fference of the cross-sectional sediment flux (*CSSF*) between the floodtide and the ebbtide durations is 43.85–11.26 × 10<sup>4</sup> t/day, accounting for 37.5–34.9% of the landward floodtide *CSSF*.

The mechanics of sediment spillover in the Yangtze Estuary are summarized as a successive process comprising the source, transport, and drainage of the sediment of the spillover.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2077-1312/7/11/390/s1, Figure S1: Channel-refined computational grid for numerical model of Yangtze Estuary. Video S1: Process of the spillover of sediment from the North to the South Branches during a full spring tide in Yangtze Estuary.

**Author Contributions:** Conceptualization: D.H. and Z.J.; methodology: D.H. and Z.J.; software: D.H. and M.W.; validation: D.H., M.W. and Z.J.; formal analysis: D.H., M.W. and Z.J.; investigation: D.H. and Z.J.; resources: S.Y. and Z.J.; data curation: S.Y. and D.H.; writing—original draft preparation: D.H. and Z.J.; writing—review and editing: M.W. and D.H.; visualization: M.W.; supervision: S.Y. and Z.J.; project administration: S.Y. and Z.J.; funding acquisition: D.H., S.Y. and Z.J.

**Funding:** Financial support from Science and Technology Program of Guizhou Province, China (Grant No. 2017-3005-4), the Fundamental Research Funds for the Central Universities (2017KFYXJJ197), China's National Natural Science Foundation (51339001, 51779015 and 51479009) are acknowledged.

**Conflicts of Interest:** The authors declare no conflict of interest.
