Fluvial floods in coastal areas are affected by tides and storm surges, while the impact is seldom quantified because the dynamics of seawater levels are often not represented in river routing models. This study established a model framework by coupling a surge model with a global hydrodynamic model at a higher spatiotemporal resolution than previous studies so that flood processes affected by seawater level fluctuation in small river basins can be investigated. Model implementation in Zhejiang Province, China, shows that the integration of dynamic seawater levels increases the stress of flooding along the Zhejiang coasts. The ocean effect varies in space, as it is much stronger in northern Zhejiang because of the lower landform and strong tidal amplification, while the mountainous rivers in southern Zhejiang are dominated by river flow regimes. Typhoon Lekima resulted in compound flood events (i.e., rainfall-induced riverine flood, tides, and surges), during which the maximum water level at the outlet of Qiantang River was 0.80 m in the default model settings with a constant downstream seawater level (i.e., 0 m), while it increased to 2.34 m (or 2.48 m) when tides (or tides and surges) were considered. The maximum increase due to tides and surges was 2.09 m and 1.45 m, respectively, while the maximum increase did not match the time of the flood peak. This mismatching indicates the need to consider different processes in physical models rather than linearly summing up different extreme water levels (i.e., river flood, tide, and surge) found in previous studies. The model framework integrating various flow processes will help to prevent risks of compound events in coastal cities in practical and future projections under different scenarios.

Coastal evolutions are expected to have a significant impact on storm tides, disproportionately aggravating coastal flooding. In this study, we utilize a nested storm tide model to provide an integrated investigation of storm tide responses to changes in coastline and bathymetry along the Zhejiang coasts. We selected coastline and bathymetry data from 1980 and 2016, as well as data from three typical typhoon events (i.e., Winnie, Haikui, and Chan-hom) for simulating the storm surge processes. The results indicate that changes in the coastline and bathymetry from 1980 to 2016 have resulted in an increase in storm tides in the northern part and a decrease in the central part of Zhejiang. Specifically, storm tides in Hangzhou Bay have increased significantly, with an average increase of about 0.3 m in the maximum storm tides primarily attributed to coastline changes. On the contrary, in smaller basins like Sanmen Bay, while reclamation itself has reduced peak storm surges, rapid siltation has consequently exacerbated the storm surge. By decomposing storm tides into astronomical tides and storm surges, we discovered that the change in tidal levels was twice as significant as the surge change. Moreover, the nonlinear tide–surge interaction was nearly four times that of the pure surge, significantly contributing to storm surge variation. Alterations in the momentum balance reveal that the water depth-induced bottom friction and wind stress increase contributes to the local enlargement of storm tides at the bay head, while the coastline changes exaggerate nearshore storm tides through an increase in the advection term.

Sea level rise (SLR) due to climate change is expected to alter tidal processes and energy transport, disproportionately affecting coastal communities. Utilizing a nested hydrodynamics model, we provided an integrated investigation of tidal responses to SLR in the Hangzhou Bay (HZB). The scenarios of SLR in the next hundred years count for both non-uniform trends based on historical altimetry data and uniform trends from the latest IPCC projections. In a comparison of model results under different SLR scenarios, we found that the tidal range is amplified by SLR in HZB with stronger amplification at the shallow southern coast. Tidal range change generally increases with the SLR scale; however, neglecting the heterogeneities in the spatial distribution of SLR tends to overestimate the SLR effects. The harmonic analysis illustrates that SLR exaggerates the dominated semidiurnal tides (M₂ and S₂) but dampens their overtides and compound tides (M₄, M₆, and MS₄), of which M₂ amplitude amplification explains 71.2–90.0% of tidal range change. SLR tends to promote tidal energy entering HZB through the Zhoushan Archipelago (ZA) compared to the prototype, while dampened sea-bed roughness and reduced tidal velocity come with a less dissipative environment in HZB, resulting in 6–18% more tidal energy exported upstream. Numerical experiments indicate ZA has significant effects on tidal responses and energy flux generation, therefore, its quantitative influences and physical mechanism are also discussed in this paper.

Sandy-Muddy Transitional beaches (SMT-beaches), which consist of a narrow high-tide sandy beach and wide low-tide mudflat, are well developed in the embayment worldwide (George and Hill, 2008; Zhao et al., 2020). These SMT-beaches provide a large tourism potential, especially along the muddy coasts of East China where are an extreme lack of excellent sandy beaches.  Because of the differences in the physical, geological and biological processes on the two sides of the beach, morphological variations in an SMT-beach are expected to coincide with the Sand-Mud Transition (SMT) boundary migrations (George et al., 2007). The movements of the SMT are closely related to the hydrodynamics forcing, sediment supply and morphology based on previous observations (Guo et al., 2021; Zhao et al., 2020), however, numerical modeling studies unraveling the underlying mechanisms remain rare.     With an artificial sandy beach of Dongtou island, China as a reference site, this study explores the roles of tidal ranges and wave climate on SMT dynamics and sediment sorting using a morphodynamic model that couples hydrodynamics, sediment transport and morphological change. A fair comparison between model simulation and sediment cores is first conducted to validate the performance of morphodynamic model. Then horizontal and vertical sedimentation against multiple hydrodynamic variables is evaluated to reveal the mechanism of the SMT dynamics. This study will gain a more in-depth mechanistic understanding of the morphodynamic processes of SMT-beaches and provide a scientific facet to make sustainable management strategies for potential future nourishment on the muddy coast.   Reference George, D.A., Hill, P.S., Milligan, T.G., 2007. Flocculation, heavy metals (Cu, Pb, Zn) and the sand-mud transition on the Adriatic continental shelf, Italy. Continental Shelf Research 27,475–488. George, D.A., Hill, P.S., 2008. Wave climate, sediment supply and the depth of the sand–mud transition: A global survey. Marine Geology 254, 121-128. Guo, J., Shi, L., Chen, S., Bruno, C, Chang, Y., Cheng, W., 2021. Sand-mud transition dynamics at embayed beaches during a typhoon season in eastern China. Marine Geology. 441, 106633. Zhao, S., Cai, F., Qi, H., Zhu, J., Zhou, X., Lei, G., Zheng, J., 2020. Contrasting sand-mud transition migrations in estuarine and bay beaches and their potential morphological responses. Geomorphology 365, 107243.