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Understanding of erosion and accretion patterns over intertidal mudflats during storm periods is vital for the management and sustainable development of coastal areas. This study aimed to investigate the effect of the 2014 storm Fung-wong on the erosion and accretion patterns of the Nanhui intertidal mudflats in the Yangtze estuary, China, based on field measurements and Delft3D numerical modeling. Results show that prolonged easterly winds during the storm enhance the flood velocity, weaken the ebb velocity, and even change the current direction. The current velocity, wave heights, and bed-level changes increased by 1–1.43 times, 2.40–3.88 times, and 2.28–2.70 times than those of normal weather, respectively. The mudflats show a spatial pattern of overall erosion but increasing erosion magnitude from the high (landward) mudflat to the low (seaward) mudflat during the storm. The magnitude of bed-level change increases with increasing wind speed, but the spatial pattern of erosion and accretion remains the same. The main reason for this pattern is the longer submersion duration of the low mudflat compared with the high mudflat, so the hydrodynamic process is longer and stronger, leading to an enhancement in bed shear stress and sediment transport rate. Wind speed increases the hydrodynamic intensity but does not affect on the submersion duration over each part of the intertidal mudflat. This study is helpful to improve the understanding of physical processes during storms on intertidal mudflats and provides a reference for their protection, utilization, and management, as well as for research in related disciplines.

Estuarine fronts have significant effects on estuarine circulation, water quality, and productivity. However, there are limited studies on the joint ecological effects of sediment and plume fronts caused by tidal mixing and a low-salinity plume in and off the Changjiang River Estuary (CRE). Based on observational data during the summers of 1988–2016, we analyzed the spatial correlations between environmental factors and chlorophyll-a (Chl-a) concentrations along the two frontal boundaries. The water mass that is shoreward of the sediment front was characterized by the highest nutrients and total suspended matter (TSM); extensive light limitation caused by TSM consequently led to the lowest Chl-a. The water mass between the sediment and plume fronts displayed the highest Chl-a, which benefited from increased light availability due to rapid deposition of TSM across the sediment front and higher nutrients contributed by the plume and coastal upwelling. Shelf water mass beyond the plume front showed the lowest TSM and nutrients; the distinct reduction of nutrient supply resulted in a relatively low Chl-a. The spatial pattern of summer phytoplankton biomass tended to be a result of the balance of light and nutrients constructed by the two fronts and coastal upwelling in and off the CRE.

While our understanding of pH dynamics has strongly progressed for open-ocean regions, for marginal seas such as the East China Sea (ECS) shelf progress has been constrained by limited observations and complex interactions between biological, physical and chemical processes. Seawater pH is a very valuable oceanographic variable but not always measured using high-quality instrumentation and according to standard practices. In order to predict total-scale pH (pH T ) and enhance our understanding of the seasonal variability of pH T on the ECS shelf, an artificial neural network (ANN) model was developed using 11 cruise datasets from 2013 to 2017 with coincident observations of pH T , temperature ( T ), salinity ( S ), dissolved oxygen (DO), nitrate (N), phosphate (P) and silicate (Si) together with sampling position and time. The reliability of the ANN model was evaluated using independent observations from three cruises in 2018, and it showed a root mean square error accuracy of 0.04. The ANN model responded to T and DO errors in a positive way and S errors in a negative way, and the ANN model was most sensitive to S errors, followed by DO and T errors. Monthly water column pH T for the period 2000–2016 was retrieved using T , S , DO, N, P and Si from the Changjiang biology Finite-Volume Coastal Ocean Model (FVCOM). The agreement is good here in winter, while the reduced performance in summer can be attributed in large part to limitations of the Changjiang biology FVCOM in simulating summertime input variables.

The increasing vulnerability of coastal megacities to storm surge inundation means both infrastructure and populations are subject to significant threat. Planning for further urban development should include consideration of the changing circumstances in coastal cities to ensure a sustainable future. A sustainable urban plan relies on sound preparedness and prediction of future climate change and multiple natural hazards. In light of these needs for urban planning, this paper develops a general method to simulate typhoon-generated storm surge at the mesoscale (1–100 km in length). Mesoscale simulation provides a general approach with reasonable accuracy that could be implemented for planning purposes while having a relatively low computation resource requirement. The case study of Shanghai was chosen to implement this method. The mesoscale simulations of two historical typhoons not only provides realistic typhoon storm surge inundation results at the city level but is also suitable for implementing a large amount of simulations for future scenario studies. The method will be generally applicable to all coastal cities around the world to examine the effect of future climate change on typhoon-generated storm surge even when historical observation data are inadequate or not available.

Lateral flow significantly contributes to the near-bottom mass transport of salinity in a channel-shoal system. In this study, an integrated tripod system was deployed in the transition zone of a channel-shoal system of the Changjiang Estuary (CE), China, to observe the near-bottom physics with high temporal/spatial resolution, particularly focusing on the lateral-flow-induced mass transport. These in situ observations revealed a small-scale salinity fluctuation around low water slack during moderate and spring tidal conditions. A simultaneous strong lateral current was also observed, which was responsible for this small-scale fluctuation. A high-resolution unstructured-grid Finite-Volume Community Ocean Model has been applied for the CE to better understand the mechanism of this lateral flow and its impact on salinity transport. The model results indicate that a significant southward near-bed shoal-to-channel current is generated by the salinity-driven baroclinic pressure gradient. This lateral current affects the salinity transport pattern and the residual current in the cross-channel direction. Cross-channel residual current shows a two-layer structure in the vertical, especially in the intermediate tide when the lateral flow notably occurred. Both observation and model results indicate that near-bottom residual transport of water moved consistently southward (shoal to channel). Mechanisms for this intratidal salinity variation and its implications can be extended to other estuaries with similar channel-shoal features.

Massive floating macroalgal blooms in the ocean have had an array of ecological consequences; thus, tracking their drifting pattern and predicting their biomass are important for their effective management. However, a high-resolution ecological dynamics model is lacking. In this study, a physical–ecological model, Floating Macroalgal Growth and Drift Model (FMGDM v1.0), was developed to determine the dynamic growth and drift pattern of floating macroalgal, based on the tracking, replication and extinction of Lagrangian particles. The position, velocity, quantity and represented biomass of particles are updated synchronously between the tracking module and the ecological module. The former is driven by ocean flows and sea surface wind, while the latter is controlled by the temperature, salinity, and irradiation. Based on the hydrodynamic models of the Finite-Volume Community Ocean Model and parameterized using a culture experiment of Ulva prolifera, which caused the largest bloom worldwide of the green tide in the Yellow Sea, China, this model was applied to simulate the green tides around the Yellow Sea in 2014 and 2015. The simulation result, distribution and biomass of green tides, was validated using remote sensing observation data and reasonably modeled the entire process of green tide bloom and its extinction from early spring to late summer. Given the prescribed spatial initialization from remote sensing observation, the model could provide accurate short-term (7–8 d) predictions of the spatial and temporal developments of the green tide. With the support of the hydrodynamic model and biological data of macroalgae, this model can forecast floating macroalgae blooms in other regions.

Estuarine systems are important sites of eliminating reactive nitrogen (N) delivered from land to sea. Numerous studies have focused on N cycling in estuarine sediment. However, the N elimination role of suspended sediments in estuarine turbid water column, which might provide anaerobic microenvironment for N loss, is rarely considered. This study examined the community dynamics and activities of denitrifying and anaerobic ammonium oxidation (anammox) bacteria in the water column of the turbidity maximum zone (TMZ) of the Yangtze Estuary, using molecular and 15N isotope-tracing techniques. Results showed that the anammox bacterial community was dominated by Candidatus Kuenenia and Candidatus Brocadia in the TMZ water column, while the main nirS-harboring denitrifiers were affiliated with Rhodobacterales. The denitrifying nirS gene was two orders of magnitude more abundant than anammox bacterial 16S rRNA gene, ranging from 1.77 × 105 to 1.42 × 108 copies l−1 and from 7.68 × 104 to 4.27 × 106 copies l−1, respectively. Compared with anammox, denitrification, with rates of 0.88 to 20.83 μmol N l−1 d−1, overwhelmingly dominated the N removal in the TMZ water column and was significantly correlated to suspended sediment concentrations (SSC). Based on the measured N removal rates, it was estimated that about 2.5 × 105 ton N was annually removed from the TMZ water column, accounting for approximately 18.5% of the total inorganic N (TIN) discharged from the Yangtze River. Overall, this study implies the importance of estuarine turbid water column in controlling N budget, and also improves the understanding of N loss mechanisms in estuarine TMZ systems.

Lateral flows redistribute sediment and influence the morphodynamics of channel-shoal systems. However, our understanding of lateral transport of suspended sediment during high and low water slack is still fairly limited, especially in engineered estuaries. Human interventions such as dike-groyne structures influence lateral exchange mechanisms. The present study aims to unravel these mechanisms in a heavily engineered, turbid channel-shoal system in the Changjiang Estuary, using a high-resolution unstructured-grid three-dimensional model and in situ observations. Analysis of model results reveals two typical transport patterns during slack-water conditions, that is, shoal-to-channel transport during low water slack and channel-to-shoal transport during high water slack. A momentum balance analysis is carried out to explain mechanisms driving the lateral transport of suspended sediment during high water slack, revealing the importance of lateral pressure gradients, Coriolis force, and the curvature-induced term. Groyne fields play a crucial role in sediment transport, especially during low water slack. A model scenario in which one groyne is removed reveals that groyne fields strongly influence lateral sediment transport. The decomposition of the sediment transport flux reveals that the turbidity maximum is shaped by a balance between seaward advection by residual flows, and landward transport by tidal pumping and gravitational circulation. Within the turbidity maximum, sediment is laterally redistributed by lateral flows during slack-water conditions, greatly influencing estuarine channel morphology.

Coastal ecosystems are strongly influenced by terrestrial and oceanic inputs of water, sediment and nutrients. Terrestrial nutrients in freshwater discharge are particularly important for mega-river estuaries. A remarkable increase in nutrient loads transported from the Changjiang River through the estuary to the shelf has been observed from 1999 to 2016. The Finite-Volume Community Ocean Model and the European Regional Seas Ecosystem Model were coupled to assess the interannual variability of nutrients and phytoplankton under these flux dynamics. The system exhibited a rapid ecosystem response to the changing river nutrient contribution. Singular vector decomposition (SVD) analysis demonstratedthat abundant nitrate from the river was diluted by low-nitrate water transported from the oceanic domain. In contrast, phosphate exhibited local variation, suggesting the estuarine ecosystem was phosphate-limited. The SVD results showed that there were no significant correlations between the suspended sediment and nutrients, but a significant correlation between sediment and phytoplankton. The nutrient structure of the river discharge resulted in the dominance of non-diatom species in the phytoplankton bloom from spring to autumn. The ratio of diatom and dinoflagellate populations showed a rapid feedback response to the strong oscillations in river nutrient input. High diatom primary production occurred near the sediment front, whereas dinoflagellate bloom extended significantly offshore. Both diatoms and dinoflagellates had major peaks representing spring blooms from empirical orthogonal function Mode 1 and 2, and secondary peaks from Mode 2 in the autumn, which coincided with the autumn bloom.

Fluid mud (FM) is a unique sedimentary feature in high-turbidity estuaries, where it can make a rapid contribution to morphodynamics. Insufficient field measurements and fixed-point monitoring lead to deficient understandings of the formation, transport, and breakdown of the FM under extreme weather conditions. A field survey was conducted in the Changjiang Estuary during the period of turbidity maximum, just after Typhoon Haikui. The measurements captured the formation of the FM beneath the suspended layers, particularly around the lower reach of the North Passage. The thickness of the observed FM gradually decreased landward along the channel, with the maximum value reaching ~0.9 m. The major features of the observed storm-induced FM were simulated using the Finite-Volume Community Ocean Model. The results indicated that the initial appearance of the FM was the result of a typhoon-intensified, salinity-induced stratification in the outlet region. The subsequent landward propagation of the FM was driven by the combined effects of the FM-induced mud surface pressure gradient force and saltwater intrusion near the bottom. Weak mixing during the subsequent neap tidal period sustained the FM as it rapidly extended into the middle region of the North Passage. This produced a large velocity shear at the interface of the FM and upper suspension layer, increasing the entrainment from the FM to the upper suspension layer. As a result of the increased tidal mixing, the FM weakened and then finally broke down in the subsequent spring tidal period.

Total alkalinity (AT) is an important variable in the regulation of the seawater carbonate chemistry system, determining the capacity to buffer changes in pH. In the coastal oceans, carbonate system dynamics are controlled by numerous processes such as land-derived inputs, biological activity, and coastal water dynamics, and seasonal alkalinity variations can play an important role in the regional carbon cycle. However, our understanding of these variations on the East China Sea (ECS) shelf remains poor due to limited observations. In order to estimate and investigate the seasonal variability of AT on the ECS shelf, an artificial neural network (ANN) model was developed using five cruise datasets from 2008 to 2018. The model used temperature, salinity, and dissolved oxygen to estimate AT with a root-mean-square error (RMSE) of ∼7 umol kg–1, and was applied to calculate AT for eight cruises during 2013–2016. In addition, monthly water column AT for the period 2000–2016 was obtained using temperature, salinity, and dissolved oxygen from the Changjiang Biology Finite-Volume Coastal Ocean Model (FVCOM) Data. Spatial distributions, seasonal cycles and correlations of surface AT indicated that the seasonal fluctuation of the Changjiang River discharge is the major factor affecting seasonal variation of surface AT on the ECS shelf. The largest seasonal fluctuations of surface AT were found on the inner shelf near the Changjiang Estuary, which is under the influence of the Changjiang River discharge. publishedVersion

Using geological evidence to extend the history of typhoon activity beyond the instrumental period and understand the possible linkage between future climate change and increasing storm intensity is of great significance in preparing for coastal risks in a warming world. In this study, an extreme storm event that occurred on the southern Yangtze coast, East China, during the latter stage of Neolithic Liangzhu Culture were estimated based on sedimentary records and numerical modeling. We first investigated the spatial distribution of peat layers formed during the time of the Liangzhu Culture to produce a high-resolution map of the paleo-coastline of the Ningbo Plain. Inverse modeling using the grain size of storm deposits recovered at the end of Liangzhu Culture at the Neolithic Yushan site was applied to calculate the instantaneous water level including wave run-up and storm surge caused by the extreme event. Numerical simulations of multiple typhoon scenarios combined with precipitation and sea-level rise constrained by the paleo-coastline and paleo-topography were then used to estimate the storm surge and inundation on the coast of Hangzhou Bay. Through comparison of results between the inverse model and numerical simulations, we infer that the intensity of the storm recorded at the Yushan site was likely greater than any typhoon in the local instrumental records and that the storm deposits were formed by a Super Typhoon, which moved close to or made landfall directly on the southeast coast of Hangzhou Bay. Our modeling further suggests that the extreme storm event was coupled with an abrupt relative sea-level rise at ca. 4.5 cal kyr BP, which indicates a warmer west Pacific Ocean at that time.

Oceanic turbulence plays an important role in coastal flow. However, as the effect of an uneven lower boundary on the adjacent turbulence is still not well understood, we explore the mechanics of nearshore turbulence with a turbulence-resolving numerical model known as a large-eddy-simulation model for an idealized scenario in a coastal region for which the lower boundary is a solid sinusoidal wave. The numerical simulation demonstrates how the mechanical energy of the current is transferred into local turbulence mixing, and shows the changes in turbulent intensity over the continuous phase change of the lower topography. The strongest turbulent kinetic energy is concentrated above the trough of the wavy surface. The turbulence mixing is mainly generated by the shear forces; the magnitude of shear production has a local maximum over the crest of the seabed topography, and there is an asymmetry in the shear production between the leeward and windward slopes. The numerical results are consistent with results from laboratory experiments. Our analysis provides an important insight into the mechanism of turbulent kinetic energy production and development.

Tidal choking is a geometric feature caused by a narrowed channel. Construction of the Yangshan Harbour, Shanghai, China obstructed three key channels and intensively changed the local geometry and topography. In this study nine numerical experiments based on the Finite-Volume Community Ocean Model are conducted to study the project's influence on tidal characteristics. Results show that stronger tidal choking happened at the East Entrance after project, mainly due to the jet induced water-level drop forced by Bernoulli law and the longer and narrower geometry. The stronger tidal choking forces a faster flow and larger tidal energy flux at the choked channel while reducing the tidal amplitude in the Inner Harbour Area (IHA). The scouring on this channel reduces the choking effect but further enlarges tidal energy flux. Moreover, damming the channels decrease the tidal amplitude at the lee side of tidal propagating direction while increasing the amplitude on the stoss side. The dams also decrease the tidal current on both sides, and meanwhile develop two patches with stronger current aside the dam. The project induced changes in tidal characteristics are complex in space, and perturbations in bathymetry increase this complexity. Yangshan Harbour's construction induces little changes in the total tidal energy density in the IHA, but induces obvious changes in the spatial distribution of tidal energy. Although this study is site-specific, the findings may be applicable to tidal dynamics in land reclamation close to open seas, such as the dramatic reclamation of islands in the South China Sea.

Vulnerability assessment is critical for management of hazards in coastal area. However, research on erosion vulnerability assessment in delta region is still very limited. In this paper, we assessed the erosion vulnerability of 49 towns along the coast of the Yangtze River Delta by combining five physical (i.e., mean tidal range, mean wave height, suspended sediment concentration, tidal flat width, and coastal slope) and three social (i.e., embankment height, GDP per unit area, and fiscal revenue) indicators using the percentile ranking method. Population density and land use were analyzed to reveal the potential socio-economic impact of erosion. The assessment result of physical vulnerability is validated by comparison with the bathymetric change between 1980s and 2010s and previous assessment studies using different indicators and aggregation methods. Our results show that areas categorized as very high, high, moderate, low, and very low vulnerability account for 13%, 22%, 39%, 24% and 2% of the coastal towns, respectively. Vulnerable towns with very high and high socio-economic impacts are located along the coastline with narrow tidal flat and lower social coping capacities. The large variability in the physical vulnerability assessment result demonstrates that fine scale study at town level is extreme important, considering the spatial heterogeneity in geomorphological process within the fluival-tidal delta. Our method of erosion vulnerability assessment has potential use in deltaic environments elsewhere.

The East China Sea (ECS) inner shelf is highly impacted by tropical cyclones. Due to the limited availability of observations, the sediment dynamics and associated sedimentary process under the impact of typhoons are still poorly understood. In this study, a high-resolution unstructured-grid FVCOM model was employed to investigate the rapid responses of sediment dynamics on the ECS inner shelf to the passage of Typhoon Chan-hom in July 2015. The results indicated that there was an evident asymmetry in sediment dynamics around the typhoon track. During the typhoon, sediments were resuspended in nearshore area and transported downwind with increasing southwestward and seaward sediment flux. The gear-like sediment delivery presented a ratchet effect that induced an evident spatial asymmetry in responses of sediment dynamics and shelf sedimentation to the short-lived typhoon event. The coastal upwelling normally existing in summer was intermittently blocked by the passage of typhoon, and rapidly transited to downwelling, which substantially enhanced the cross-shelf sediment transport. As a result, severe erosions were mainly occurred near the typhoon track within 20-m isobaths, in contrast to newly formed sedimentary layers (~10 cm in thickness) in areas far south of the typhoon track and around the 50-m isobaths. The spatial asymmetry of sedimentary response to the typhoon passage is therefore of importance to locating sediment records that well preserved the typhoon signals. The ratchet effect we proposed provided reasonable interpretations of the spatially asymmetric responses of shelf sedimentations in the northern Gulf of Mexico and the central Great Barrier Reef shelf to tropical cyclones, respectively.

The variation in suspended sediment concentration (SSC) has an important impact on the protection of estuarine shoal wetlands and the maintenance of waterways. Typhoons (tropical cyclones), one of the most severe meteorological disasters, frequently occur along the southeastern coasts of China and substantially affect the spatial-temporal distribution of SSC in the Yangtze Estuary. In this study, the spatial-temporal distribution of SSC was observed at high frequency through hourly geostationary satellite remote sensing during four typhoon events, namely, Typhoons Damrey/Saola (August 2012), Soulik (July 2013), Matmo (July 2014) and Dujuan (September 2015). Such observation is difficult to achieve through in situ approaches due to the unsafe working conditions created by typhoon winds. Moreover, wind speed, wave height and period, and bottom shear stress (BSS) data combined with turbidity data were used to analyze the causes of typhoon-induced SSC variations. The results indicated that the average wind-driven significant wave height during typhoons was 2.1-fold higher than that in pre-typhoon periods. In the lower mouth of the Yangtze Estuary and in Hangzhou Bay, the SSC increased by 0.14–0.33 g/L, whereas in the upper mouth, the SSC was less affected by wind. The significant increase in SSC persisted for 1–2 days following typhoon transit. With the assistance of the FVCOM model, it was observed that the enhanced wave-current dynamics and increased BSS during the typhoons were the main reasons for the sharp increase in SSC. Moreover, it was inferred from the satellite images that sediment exchange occurs between channels and shoals during a typhoon, which affects the SSC distribution for up to one month.

Land-ocean interaction plays an essential role in the transport fate of terrestrial matters in the coastal and shelf regions. Flood discharge from a mega river, containing massive water, sediment, and nutrient loads, could result in substantial and complex impacts on the physical and biogeochemical dynamics of coastal systems. In this study, field campaigns were conducted in a region from the Changjiang River Estuary to the East China Sea (ECS) before and after a significant flood. The impacts of the flood on physical and biogeochemical environments were assessed. The results revealed that the fluvial flood enhanced the offshore expansion of the low-salinity river plume and associated sediment/nutrient fronts. However, the area of elevated chlorophyll-a at the river mouth did not expand noticeably. A numerical model was applied to quantify the contribution of the Three Gorges Dam (TGD) to the spatial intensity and temporal duration of fluvial flood effects on estuary–shelf continuum. The results predicted a maximum of 2° latitudinal offshore displacement of the shelf water. Salinity and nitrate exhibited conservative expansions, with a longer relaxation time (~2 months) than chlorophyll-a and phosphate. After the TGD-regulated flow event ceased, salinity and nitrate effects persisted, but phosphate and chlorophyll-a recovered rapidly. The flood decreased the dissolved oxygen (DO) concentration around the river mouth and the offshore region, but not in the nearshore transient area. In contrast, the non-TGD regulation increased the regional DO concentration, which reduced the hypoxia risk. The TGD has become a crucial anthropogenic driver of environmental changes in the Changjiang Estuary-ECS continuum.

Understanding the decadal morphodynamic evolution of estuaries and deltas and their controls is of vital importance regarding management for estuarine function and sustainable development. This work addresses this issue by applying a process-based model system (Delft3D) to hindcast and then forecast the morphodynamic evolution of the Yangtze Estuary at a decadal timescale. Forced by the river and tides, the model considers sand-mud mixture and the variations of river water discharge and sediment discharge. The morphodynamic model is validated against three periods, i.e., an accretion period (1958–1978), an erosion period (1986–1997) and a recent accretion period with human activities (2002 − 2010). Model results show good performance with respect to spatial erosion and deposition patterns, sediment volume changes, and hypsometry curves. The model reveals quite different behaviors for mud transport between the dry and wet seasons, which is subject to the prescription of river boundary conditions and bed composition. We define six scenarios to project evolution to the year 2030 under decreased river inputs and increased relative sea level. The simulations reveal that overwhelming amount of erosion will likely occur in the inner and mouth bar area of the estuary. Particularly, the mouth zone will shift from net deposition before 2010 to net erosion by 2030, mainly because of decreasing sediment supply. Changes in water discharge have minor effects on the projected trend. Net erosion will be considerable when the sediment supply is extremely low (100 Mt yr − 1 ) due to the abundance of erodible modern sediment in the Yangtze Estuary. Erosion within the mouth bar area may be unexpected, including the deepening of the tidal inlet at East Chongming mudflat and the formation of a flood channel on the seaward side of Jiuduansha Shoal. Overall, the model results provide valuable information for sustainable delta management under changing conditions for both the Yangtze system and other similar estuaries and deltas with diminishing sediment supplies.

While our understanding of pH dynamics has strongly progressed for open ocean regions, for marginal seas such as the East China Sea (ECS) progress has been constrained by limited observations and complex interactions between biological, physical, and chemical processes. Seawater pH is a very valuable oceanographic variable but not always measured using high quality instrumentation and according to standard practices. In order to predict water column total scale pH (pHT) and enhance our understanding of the seasonal variability of pHT on the ECS shelf, an artificial neural network (ANN) model was developed using 11 cruise datasets from 2013 to 2017 with coincident observations of pHT, temperature (T), salinity (S), dissolved oxygen (DO), nitrate (N), phosphate (P) and silicate (Si) together with sampling position and time. The reliability of the ANN model was evaluated using independent observations from 3 cruises in 2018, and showed a root mean square error accuracy of 0.04. A weight analysis of the ANN model variables suggested that DO, S, T were the most important predictor variables. Monthly water column pHT for the period 2000-2016 was retrieved using T, S, DO, N, P, and Si from the Changjiang Biology Finite-Volume Coastal Ocean Model (FVCOM).

Intertidal wetlands are dynamic geomorphological areas located at the land-sea interface and perform multiple ecosystem functions. Owing to increased human activities, intertidal wetlands have been subjected to dramatic changes in recent decades; therefore, high-resolution monitoring of wetland topography is critical to its management. However, satellite imagery with high spatial resolution usually demonstrates a low revisit frequency (e.g. several days to greater than ten days) and is frequently obstructed by clouds, limiting its capability to display the high-resolution time-series information of intertidal wetland terrain elevation variations. Conversely, satellite imagery with a high revisit frequency generally demonstrates a lower spatial resolution. In this study, a spatial-temporal data fusion method was utilised to generate hourly time-series images with a spatial resolution of 16 m by combining the satellite GF-1/WFV data (spatial resolution: 16 m; revisit frequency: 4 days) with geostationary satellite GOCI data (spatial resolution: 500 m; revisit frequency: 1 h). In combination with the tidal level information, digital terrain elevation (DTM) data of the intertidal wetland can be derived from fusion images. The DTM was synchronously validated by the terrain elevation data acquired on the same day utilising unmanned aerial vehicle (UAV)-borne LiDAR in the North Branch intertidal wetland of Chongming Island, Yangtze Estuary, with a root mean square error of 0.16 m. The application in Chongming-Dongtan indicates that this method is effective for monitoring high dynamic changes in intertidal wetland terrain elevations.

The general framework for identifying tidal duration asymmetry proposed by Song et al. (2011) is extended to express fortnightly variability in duration asymmetry. The extended metrics are verified and studied using observed sea-level data at 481 stations worldwide. The results reveal that fortnightly variability is universal and that duration asymmetry can be stronger during neap tide than during spring tide. The fortnightly variability in duration asymmetry is primarily induced by three types of tidal interactions: interactions within the principal tidal constituents, interactions between high-frequency and principal tidal constituents, and interactions between long-period and principal tidal constituents. Among these interactions, the first type is most important at most of the stations and is related to the form number F. The contributions of different interactions can be quantified using their frequencies, amplitudes and phases. Global patterns of the fortnightly variation are illustrated using TOPEX/Poseidon altimetry data. The findings show that remarkable fortnightly variation in the tidal duration asymmetry occurs in most open oceans and is significant around an amphidromic point. The metrics derived in this study can be used to examine any time-varying characteristics in tidal asymmetry (not limited to duration asymmetry) by selecting a suitable frequency threshold. This article is protected by copyright. All rights reserved.

The Yangtze Estuary in China has been intensively influenced by human activities including altered river and sediment discharges in its catchment and local engineering projects in the estuary over the past half century. River sediment discharge has significantly decreased since the 1980s because of upstream dam construction and water-soil conservation. We analyzed bathymetric data from the Yangtze Estuary between 1958 and 2010 and divided the entire estuary into two sections: inner estuary and mouth bar area. The deposition and erosion pattern exhibited strong temporal and spatial variations. The inner estuary and mouth bar area underwent different changes. The inner estuary was altered from sedimentation to erosion primarily at an intermediate depth (5–15 m) along with river sediment decline. In contrast, the mouth bar area showed continued accretion throughout the study period. The frequent river floods during the 1990s and simultaneously decreasing river sediment probably induced the peak erosion of the inner estuary in 1986–1997. We conclude that both sediment discharge and river flood events played important roles in the decadal morphological evolution of the Yangtze Estuary. Regarding the dredged sediment, the highest net accretion rate occurred in the North Passage where jetties and groins were constructed to regulate the navigation channel in 1997–2010. In this period, the jetties induced enhanced deposition at the East Hengsha Mudflat and the high accretion rate within the mouth bar area was maintained. The impacts of estuarine engineering projects on morphological change extended beyond their sites.

Coastal flooding is reported as one of the major causes for the interruptions of Neolithic cultures inhabiting the south Yangtze coastal plain, China during the middle Holocene. The archaeologically sterile transgressive sedimentary layers could be the results of extreme events or relative sea-level rise induced by the enlargement of the local tidal range or global/regional sea-level rise. To clarify the mechanism of the transgressive deposits at typical Neolithic sites and the termination of Neolithic Liangzhu Culture in the south Yangtze coastal plain, this study reconstructed the paleo-topography at 7.6, 6.4 and 4.5 cal ka BP on the basis of a database including 107 sediment cores and 644 radiocarbon ages and simulated the paleo-tidal regimes and the inundation areas induced by typhoon events under the paleo-topography and sea levels using a hydrodynamic model of high-resolution Changjiang Estuary Finite-Volume Community Ocean Model (CE-FVCOM). The simulation results of tidal regimes showed that the tidal amplitude increased from 1.6 m to 2.2 m in the head of Hangzhou Bay induced by the paleo-topographic change during the period 7.6–4.5 cal ka BP. By contrast, it reduced from 1.6 m to 1.0–1.2 m in the Ningbo Plain, southeast of the bay, which was unable to explain the long-lasting marine inundation there and supported our previous assumption of an event of abrupt sea-level rise at 4.5–4.4 cal ka BP. Simulation of the storm-induced inundation further suggested that coastal flooding mainly occurred along the coastline of the Taihu Plain under the combined effect of tidal amplification and typhoon events at 4.5 cal ka BP, while the inundation almost covered half of the Taihu Plain if all factors including tidal amplification, extreme event and the abrupt sea-level rise were considered. We therefore suggest that the obvious increase in flood risk under the backdrop of sea-level rise was a major cause for the abandonment of the Liangzhu Culture in the south Yangtze coastal plain at around 4.4 cal ka BP.

The increasing vulnerability of coastal mega-cities to storm surge inundation means both infrastructure and populations are subject to significant threat. Planning for further urban development should include consideration of the changing circumstances in coastal cities to ensure a sustainable future. A sustainable urban plan relies on well preparedness and prediction of future climate change and multiple natural hazards. In light of these needs for urban planning, this paper develops a general method to simulate typhoon generated storm surge at the meso-scale (1–100 km in length). Meso-scale simulation provides a general approach that could be implemented to fulfil the purpose of planning, and has relatively low requirements for computation time and data, while still providing reasonable accuracy. The case study of Shanghai was chosen to implement this method. The meso-scale simulated results of two historical typhoons not only provides a realistic result at the city level, but is also suitable for implementing a large amount of simulations for future scenario studies. The method is generally applicable to all coastal cities around the world to examine the effect of future climate change on typhoon generated storm surge, even where historical observed data are inadequate or not available.

Simulating the sediment transport in a high-turbidity region with spatially varying bed properties is challenging. A comprehensive strategy that integrates multiple methods is applied here to retrieve the critical shear stress for erosion, which plays a major role in suspended sediment dynamics in the Changjiang Estuary (CE). Time-series of sea surface suspended sediment concentration (SSC) were retrieved from the Geostationary Ocean Color Imager (GOCI) satellite data at hourly intervals (for 8 h each day) and combined with hydrodynamic modeling of high-resolution CE Finite-Volume Community Ocean Model (CE-FVCOM) to estimate the near-bed critical shear stress in the clay-dominated bed region (plasticity index > 7%). An experimental algorithm to determine the in situ critical shear stress via the plasticity index method was also used to verify the GOCI-derived critical shear stress. Implemented with this new critical shear stress, the sediment transport model significantly improved the simulation of the distribution and spatial variability of the SSC during the spring and neap tidal cycles in the CE. The results suggest that a significant lateral water exchange between channels and shoals occurred during the spring flood tide, which led to a broader high-SSC area in the CE throughout the water column.

A comprehensive acoustic retrieval algorithm to investigate suspended sediment is presented with the combined validations of Acoustic Doppler Current Profiler (ADCP) and Optical Backscattering Sensor (OBS) monitoring along seven cross-channel sections in the high-turbidity North Passage of the Changjiang Estuary, China. The realistic water conditions, horizontal and vertical salinities, and grain size of the suspended sediment are considered in the retrieval algorithm. Relations between net volume scattering of sound attenuation (Sv) due to sediments and ADCP echo intensity (E) were obtained with reasonable accuracy after applying the linear regression method. In the river mouth, an intensive vertical stratification and horizontal inhomogeneity were found, with a higher concentration of sediment in the North Passage and a lower concentration in the North Channel and South Passage. Additionally, The North Passage is characterized by higher sediment concentration in the middle region and lower concentration in the entrance and outlet areas. The maximum sediment flux rate, occurred in the middle region, could reach 6.3×105 and 1.5×105 t/h during the spring and neap tide, respectively. Retrieved sediment fluxes in the middle region are significantly larger than that in the upstream and downstream region. This strong sediment imbalance along the main channel indicates potential secondary sediment supply from southern Jiuduansha Shoals.

In the past, two physical mechanisms, baroclinic instability (BI) and strong asymmetric tidal mixing (SATM) during the spring tidal period, were proposed for the offshore detachment of the low-salinity plume over the inner shelf of the East China Sea (ECS). These two mechanisms were re-examined using both observations and a fully three-dimensional (3-D), high-resolution, unstructured-grid, free-surface, primitive-equation, Finite-Volume Community Ocean Model (FVCOM). The observed currents and salinities showed that the plume was characterized by a two-layer system, in which the upper layer is mainly driven by the river discharge-induced buoyancy flow and the lower layer is predominantly controlled by tidal mixing and rectification. The SATM mechanism was based on the model run without calibration against observed currents and salinity around the plume region, so that it should be applied with caution to a realistic condition observed on the inner shelf of the ECS. The BI mechanism was derived under a condition without consideration of tidal mixing. Although BI could still occur along the frontal zone when tides were included, it was unable to produce a single, large, detached low-salinity lens observed on the inner shelf of the ECS. The process-oriented model experiment results suggest that for a given river discharge and realistic tidal flow, the spatially non-uniform southwesterly surface wind during the southeast monsoon-dominant summer could increase frontal spatial variability and thus produce a significant offshore detachment of low-salinity water on the inner shelf of East China Sea.