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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]

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]

Kuang, C.; Liang, H.; Mao, X.; Karney, B.; Gu, J.; Huang, H.; Chen, W., and Song, H., 2017. Influence of potential future sea-level rise on tides in the China Sea. This study investigates the diurnal and semidiurnal tidal responses of the entire China Sea to a potential rise in sea level of 0.5-2 m. A modified two-dimensional tidal model based on MIKE21 is primarily configured and validated for the present situation; then, three (0.5, 1, 2 m) sea-level rise (SLR) scenarios are simulated with this model. The predicted results show that the principal lunar semidiurnal (M2) and diurnal (K1) tidal constituents respond to SLR in a spatially nonuniform manner. Generally, changes of M2 and K1 amplitudes in shallow waters are larger than those in the deep sea, and significant tidal alterations mainly occur in the Bohai and Yellow seas, Jianghua Bay, Hangzhou Bay, Taiwan Strait, Yangtze River estuary, Pearl River estuary, and Beibu Bay. Possible mechanisms further discussed for these changes mainly relate to bottom friction decreasing, amphidromic point migration, and resonant effect change. Additionally, simulated changes in M2 and K1 amplitudes in response to three SLR scenarios imply that M2 amplitude changes are proportional to the magnitude of SLR, whereas this proportionality does not hold for K1 amplitudes. Identifying the response of tides in the China Sea to SLR not only increases our knowledge of tidal systems, but also assists in setting conservation requirements and management plans in coastal areas. [ABSTRACT FROM AUTHOR]

The effects of sea level rise (SLR) are relevant to the future evolution of coasts and estuaries. This study analyses the importance of different SLR trends on the morphological evolution of a mixed-energy fine-sediment coast (the Caofeidian Sea in Bohai Bay, China) using a validated numerical model. Results show that SLR produces spatially non-uniform relative morphodynamic responses, depending on the local coastline (uninterrupted or inlet-interrupted) and wave exposure. In the Caofeidian Sea, the uninterrupted coast is exposed to waves and tidal currents, and SLR reduces the accretion of tidal flats via heightening waves. On the inlet-interrupted coast, the morphological response to SLR is dominated by changes in tidal asymmetry and current velocity. In low SLR scenarios, the enhanced flood tidal asymmetry makes the system initially resilient to SLR. For high SLR scenarios, tidal sediment transport capacity is dampened by the reduction in current velocities, which accelerates the degradation of tidal flats along the inlet-interrupted coast. Differences among results with eight theoretical SLR trends show that the magnitude of SLR-induced morphological change is influenced by the target value (0.5 and 1.0 m) and rising mode (abrupt growth, linear growth, parabolic growth, and exponential growth) of the SLR. A newly-introduced parameter (equivalent SLR, E SLR), which reflects the time-weighted average value of an SLR trend for a period of time, shows a strong linear relationship with SLR-induced morphological change. Results suggest that the morphological response to other SLR scenarios for the Caofeidian Sea is expected to be interpolated from the E SLR ratios without performing extra time-consuming simulations. • SLR triggers erosion of tidal flats on the uninterrupted coast via heightening waves rather than enhancing currents. • Morphological response of inlet-interrupted coast is subject to net sediment flux controlled by tidal asymmetry and currents. • Morphodynamic modelling that neglects the SLR trends has a limited predictive capacity of SLR effects on the coasts. [ABSTRACT FROM AUTHOR]

The storm impact on coastal environment is complex and systematic. With a validated numerical model, this study quantitatively investigates the effects of a historical storm process on the morphodynamic evolution of an unsheltered Nanpu channel-shoal system (NS) in the Bohai Sea, China. Results show that the storm event in October 2003 induced a destructive erosion in the tidal flats during the entire storm period. The channel also suffered erosion, however, it rapidly restored after this storm. Numerical estimations also confirm that wave and wind-driven current dominate the erosion of tidal flat and channel, respectively. Wind-driven current can further enhance the erosion on the tidal flat by intensifying the wave-current interaction, while surge plays a minor role in the channel-shoal evolution. The morphological evolution pattern of NS varies as a function of wind direction and wind speed, where three morphological evolution patterns appear as the wind speed growth in the investigated NE and NW wind directions. In the study area, NE strong wind usually produces flooding disasters and erosion of NS, while NW strong wind can induce severe tidal flat loss and channel accretion. Although this study is based on a site-specific model, the findings will provide valuable insights into the morphological feedbacks under destructive storm and hence assist in making efficient coastal management strategies in unsheltered coasts. • Channel experiences destructive erosion during wild wind and constructive deposition after wild wind. • Destructive erosion on the shoal, existing in the whole storm event, is subject to wave and exaggerated by wind currents. • Effects of surges, wind-driven currents and waves on the maximum shear stress and morphological evolution are clarified. • Morphodynamics of channel-shoal system varies as a function of wind and three evolution patterns are captured. [ABSTRACT FROM AUTHOR]

Storm surges are among the most devastating natural hazards occurring in coastal regions of Zhejiang Province. A nested tidal model is built to simulate the storm surge generated by typhoons with varied velocities. Two typhoons with extreme moving speeds (Mary and Kai-tak, with low and high moving speeds, respectively) between 1949 and 2020 are simulated and analyzed. To investigate the changes of storm surge and the influence of M 2 tide caused by the moving speed, three different moving speeds (2.0, 1.0 and 0.5 times) are studied based on Chan-hom. The results show that: The landing intensity of the low moving speed typhoon is weaker than that of the high moving speed typhoon, but the former has a longer-lasting impact on the storm surge. When the moving speed is slower, the wind points towards the shore for a longer time, which increases the average water level in the area from the west of the Zhoushan sea area to the Qiantang River. With the decrease of typhoon moving speed, the amplitude of the tide decreases, while the change of lag angle varies in different water areas. • 218 typhoon tracks of Zhejiang Province from 1949 to 2020 were drawn according to the typhoon intensity. • Storm surge process under typical moving typhoon was simulated. • Characteristics of wind speed and influence of storm surge process under different typhoon moving speeds were studied. • Spatial distribution of storm surge at different moving speeds and influence of moving speed on M 2 tidal component were discussed. [ABSTRACT FROM AUTHOR]

Blue carbon stored in coastal wetlands can be highly impacted by extreme events, such as typhoons. However, the sediment carbon accumulation capacity in this circumstance, remains unclear. In this study, a field site crossing the bare mudflat-saltmarsh interface was established in Southern Hangzhou Bay, to investigate the geomorphological and subsequent sediment carbon accumulation changes during the typhoon season (2 months) and the succeeding calm weather period (10 months). Surface Elevation Tables (SETs) were used, together with GPS-RTK and UAV-LiDAR surveys, to observe surface elevation changes at different spatial scales. The elevation and sediment organic carbon density data were used to estimate the gain/loss of sediment carbon pool under the impact of two successive typhoons, "In-Fa" and "Chanthu". This revealed that during the typhoon season, 6.3 cm of surface erosion occurred on the bare mudflat whilst rapid depositions of 6.1 and 12.3 cm occurred in Scirpus mariqueter and Spartina alterniflora habitats, respectively. In the gap between the two typhoon events, the bare mudflat experienced deposition of 4.2 cm, signalling recovery from erosion, but deposition within the saltmarsh front slowed. During the calm weather period, sediment deposition increased landward, with magnitudes of 13.1 cm, 16.4 cm, and 18.2 cm. Sediment carbon budget estimates indicated that during the typhoon season, the bare mudflat lost ~3.4 Mg C ha−1 but saltmarshes gained ~4.9 Mg C ha−1 in their sediment organic carbon pools. These changes in sediment carbon accumulation rates over the typhoon season accounted for >30 % of annual change in the study area, and may be associated with tidal creek dynamics. This study suggests the important role of typhoons in mediating the sediment carbon pool across the mudflat-saltmarsh interface. With future climate change, increasingly frequent catastrophic events could cause considerable changes in coastal blue carbon sequestration capacity. • Sediment carbon accumulation rates were recorded at the mudflat-saltmarsh interface. • Differing patterns of carbon accumulation were found during typhoon and calm periods. • Typhoons caused a sediment carbon loss in mudflat but a gain in saltmarsh. • Typhoons accounted for >30 % of the annual change in the sediment carbon pool. • Tidal creeks could be crucial in mediating sediment carbon pools in typhoon season. [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]

In order to characterize the vertical variation of abundance and community composition of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in sediments of a eutrophic lake, Lake Taihu, molecular techniques including real-time PCR, clone library, and sequencing were carried out in this study. Abundances of archaeal amoA gene (ranged from 2.34 × 10 to 4.43 × 10 copies [g dry sediment]) were higher than those of bacterial amoA gene (ranged from 5.02 × 10 to 6.91 × 10 copies [g dry sediment]) for all samples and both of them exhibited negative correlations with the increased depths. Diversities of archaeal and bacterial amoA gene increased with the elevated depths. There were no significant variations of AOB community structures derived from different sediment depths, whereas obvious differences were observed for the AOA community compositions. The information acquired in this study would be useful to elucidate the roles of AOA and AOB in the nitrogen cycling of freshwater ecosystems. [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]