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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The tidal asymmetry-induced siltation below tidal barriers is a worldwide problem that restricts regional socio-economic and environmental development. The hydrodynamic processes of the small mud estuary also feature a high uncertainty after estuary restoration measures. In this study, a hydrodynamic model based on the MIKE21 is used to quantify the responses of tidal asymmetry to a two-phase restoration project in Shuanglong Estuary, Bohai Bay, China. According to the numerical modeling results, the tidal flat removal in the upper estuary (first-phase restoration) induces the flood asymmetry switching to the ebb asymmetry in unrestored reach but enhances flood asymmetry in widening restored reach. Although the tidal asymmetry reverts to flood-dominated pattern after full restoration over the estuary, the imbalance between flood and ebb velocities is relieved. A possible net sediment transport pattern based on a comparison of dominant asymmetric current and actual sediment transport period shows net sediments in the upper estuary and inlet transport seaward and landward, respectively, in the first-phase restoration, whereas landward net sediment transport occurs in the whole estuary under the second-phase restoration scenario. Given these results, we assume that a switch from the flood-dominated estuary to ebb-dominated estuary can be caused by redesigning the cross-sectional profile. The quantitative comparison of Lagrangian residual currents also implies that a channel–shoal structure rather than a flat bathymetry can promote the mass transport. Therefore, reshaping the channel–tidal flat system in restoration projects can prevent the sedimentation of the estuary and improve the water environment. [ABSTRACT FROM AUTHOR]

Understanding the influence of reclamation projects on a channel-shoal system is of vital importance regarding the management and sustainable development of coastal regions. This study addresses the issue by applying a processes-based morphological model to investigate hydro-morphodynamics of Caofeidian channel-shoal in five phases of large-scale reclamation project from year 2003 to 2020. Forced by the spring-neap tidal cycles and representative waves, the model considers multi-fraction sediment dynamics regarding strong spatial variations of mud-sand mixtures in the study area. The numerical results demonstrate that sequential large-scale land reclamation induces continuous loss of sediment volume of tidal flat by reducing its sediment storage capability. The morphodynamics of channels is complex in space and perturbation of reclamation location increases this complexity. For the channels in the western uninterrupted coast, reclamation affects their morphological evolution by damping the long-shore sediment transport. The tidal asymmetry is a key factor in the geomorphology of channels in the eastern inlet-interrupted coast (Laolonggou lagoon-inlet system), which is impacted by the reclamation location. Particularly, reclamation constricting the inlet dampens the flood tidal asymmetry and triggers the deepening of inlet, while tidal flats reclamation in lagoon enhances flood tidal asymmetry which induces the siltation of inlet. The model also reveals different behaviors of multi-fraction sediments, of which the silt with larger settling velocity is subject to the hydrodynamic change induced by reclamation. Although this study is site-specific, the finding provides valuable information for sustainable management of channel-shoal systems in muddy-silt coast under intermittent or sequential land reclamation. [ABSTRACT FROM AUTHOR]

The impacts of large-scale coastal structures on tidal dynamics have been studied extensively over the past decades, but the relevant mechanisms are not fully understood. In this study, a two-dimensional Delft3D barotropic tidal model is applied to investigate the tidal dynamic changes induced by the large-scale sequential constructions of Huanghua-Binzhou Harbor in the northeastern China during year 2003–2016. It was found that the predominant M 2 tide is increased on the stoss side and decreased on the lee side of the harbor by 2–6 cm due to the harbor constructions. This feature is due to 1) the increased and decreased potential tidal energy on the stoss and lee side, respectively, by the harbor structures; 2) the structure-induced changes in momentum flux is balanced by pressure gradient, and thus leads to a larger tidal range on the stoss side and a smaller tidal range on the lee side. The sensitivity studies on nonlinear M 4 tide demonstrate that it is generated locally by M 2 self-self interaction, which is not only related to M 2 tidal energy but also nonlinearity. Consequently, M 4 tide increases towards the shore on the tidal flat with natural slope due to enhanced nonlinearity by decreasing water depth, but is proportional to M 2 tide on the tidal flat with a constant water depth, where local nonlinearity is nearly the same in space. In addition, the energy transfer from M 2 to M 4 is affected by the structure-induced changes in nonlinearity, resulting in different responses of M 2 and M 4 tide to the coastal structure. • Numerically investigate the tidal dynamics changes induced by the large-scale sequential constructions. • Predominant M 2 tide increased on the stoss side and decreased on the lee side of the harbor jetties. • Structure-induced changes in current acceleration is balanced by the changes in pressure gradient. • M 4 tide is generated locally by M 2 self-self interaction, which is relative to M 2 tidal energy and local nonlinearity. • Three prominent separated bands of tidal energy density appear in the third harbor construction phase. [ABSTRACT FROM AUTHOR]

The Yangtze River Estuary (YRE) is vulnerable to the accelerated sea level rise (SLR). In this study, a three-dimensional hydrodynamic and salinity transport model, with a high resolution unstructured mesh and spatially varying bottom roughness, is applied to quantify the influences of SLR on saline water intrusion in the YRE. The model has well been validated through observation data of tidal level, flow velocity and direction, and salinity. The performances of four scenarios of the present sea level (PSL) as well as 0.5, 1 and 2 m SLR reveal that (1) the isohaline shifts upstream nonlinearly in the YRE with SLR, and non-uniform in the spatial distribution under the same SLR. The isohalines of 1, 2 and 5 psu near the north bank of the South Branch intrude farther upstream than those near the south bank, and the upstream advance distance of 0.45 psu isohaline is much longer than those of 1, 2 and 5 psu isohalines under the same SLR. (2) The spillover of saline water from the North Branch into the South Branch intensifies with SLR, while the salinity in the lower reach of the North Branch is reduced due to increased low salinity flow from the South Branch and the North Channel caused by SLR. (3) The durations of undrinkable water increase at four reservoirs with SLR, and four reservoirs cannot intake raw drinking-water in the dry season in the 2 m SLR scenario. (4) A regression analysis obtains a quantitative quadratic expression between salinity and SLR. Finally, the effects of the operation of the Three Gorges Reservoir and more SLR scenarios on the salinity, and two different approaches for SLR simulations are also discussed. [ABSTRACT FROM AUTHOR]

The southwestern part of Bohai Bay in northeastern China is a shallow-water coast with a long fetch for water to pile up to produce large storm surge under strong wind action during the extreme weather condition. In addition, the coastline along the bay has experienced significant environmental impacts from large scale harbor constructions in the past two decades. Consequently, there is a pressing need to better understand the effects of two local large scale sequential harbor constructions on storm surge and coastal vulnerability in the bay area. The extreme cold front weather on 10-13th October 2003 caused a 1.31 billion Yuan damage. A two-way coupled tide-surge-wave Delft3D model is used to investigate the impact of the harbor constructions from 2003 to 2016 on the interaction of tide, storm surge and wave during a storm like that. The wind field by the newly-released global reanalysis wind product ERA5 of European Centre for Medium-Range Weather Forecasts (ECMWF) has been improved by assimilation with field observations at a higher resolution than previous version and is used to drive the tide, surge and wave modelling framework. The model results indicate that the storm surge in the study area is dominated by the wind-induced surge before and after the constructions whereas the maximum contribution from wave-induced surge (wave setup) is increased from 5%-15% to 8%–20% by the harbor constructions between 2003 and 2016. It is found that semi-diurnal modulation of storm surge is generated by nonlinear tide-surge interaction, which is mainly attributed to local tidal acceleration in the offshore zone and bottom stress in the nearshore zone respectively. The harbor constructions affect the coastal storm surge mainly through nonlinear tide-surge-wave interaction. In addition, the quarter-diurnal variation of storm surge induced by the harbor constructions is due to the influences of the harbor constructions on the local quarter-diurnal tidal constituents and their modulation of wave momentum transfer to coastal circulation. • Newly-released ERA5 wind is modified by observation using linear regression. • Semidiurnal surge is evident in mildly-sloping SW Bohai Bay in a cold front event. • Tide-surge interaction in offshore (nearshore) is due to acceleration (bottom friction). • Wave-induced surge contributes 5%–15% (8%–20%) to storm surge in layout 2003 (2016). • Surge change due to harbor relates with nonlinear tide-surge and wave interactions. [ABSTRACT FROM AUTHOR]