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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.

The riverine nutrient inputs to the ocean reflects land-use changes and can affect the health of coastal environments over time, especially for a highly-anthropogenically influenced river-estuary-shelf system. To investigate the impact of riverine inputs on the Changjiang Estuary ecosystem at a multi-decadal time scale where long-term observations are limited, we built a three-dimensional physics-biogeochemistry-coupled model system based on the Finite-Volume Community Ocean Model (FVCOM) and the European Regional Shelf Ecosystem Model (ERSEM). Our model successfully simulated the temporal and spatial nutrient variabilities in the river-estuary-shelf con7tinuum from 1960 to 2018. The results showed increasing trends of nitrate and phosphate and fluctuating silicate variability, thereby leading to rising nitrogen (N) to phosphorus (P) ratios and decreasing silicon (Si) to N and P ratios. Such changes in the stoichiometric relationship of nutrient species also alter the community structure of the primary producers in estuaries. Our model showed a general increase of diatoms over the 59 years, corresponding to decreased proportions of micro-phytoplankton and pico- phytoplankton. With different backgrounds of light and nutrient limitations in the river and inner shelf, our model suggests that the trend of the diatom proportion in the light-limited river mouth is more associated with silicate variability, with decreased diatom proportions occurring in the 2000s. Our model relates the hydroclimate, nutrient load, and biogeochemical cycling, reproducing estuarine ecosystem variability and clarifying issues such as the causality of the ecosystem interactions.

Phytoplankton, the dominant marine primary producers, are considered highly sensitive indicators of ecosystem conditions and changes. The East China Sea (ECS) includes a variety of oceanic and coastal domains that collectively challenge our understanding of phytoplankton dynamics and controls. This study evaluates the seasonal and interannual variability of phytoplankton in the ECS and the underlying environmental determinants based on 22-year satellite chlorophyll (Chl-a) data and concurrent environmental variables. A seasonal spring bloom was found in the ECS, classically driven by increased stratification, which is associated with increases in sea surface temperature (SST), photosynthetically available radiation (PAR), net heat flux (NHF), and reduced wind mixing. The most significant Chl-a interannual variability was present in a triangular area surrounded by three SST fronts in the southern ECS during springtime. Anomalously high Chl-a (~30% increase) occurred with increased SST and NHF and enhanced wind mixing during negative Pacific Decadal Oscillation (PDO) and El Niño–Southern Oscillation (ENSO) modes. This seems to be contrary to the stratification control model, which fits the seasonal spring bloom observed in this region. More front activities during the negative PDO and ENSO could be associated with Chl-a increase in this triangular area. Contrary to this mixing control scenario, a significant Chl-a increase (~36% increase) also developed during the positive PDO and ENSO modes after 2014 under conditions of higher SST, NHF, and weaker wind mixing following the stratification control scenario. This study used biologically relevant objective regionalization of a heterogeneous area to elucidate phytoplankton bloom dynamics and controls. Our analyses highlight the triangular area in the ECS for its region-specific linkages between Chl-a and multiple climate-sensitive environmental drivers, as well as the potential structural and functional variability in this region.

Estuarine plume frontal zones typically form a vertical two-layer structure with low-salinity and a high-temperature plume during the summertime. However, two field surveys in the Changjiang River Estuary and its adjacent shelf waters identified a significant surface cold-water zone (CWZ) formation in the summers of 2014 and 2015. The sea surface temperature of the CWZ was 4°C lower than the multi-year summer average. Satellite images showed that the CWZ mainly appeared in the Yangtze Shoal during the periods of July 1–17, 2014, and July 3–19, 2015. A three-dimensional physical-biogeochemical coupled model was used to explore the formation mechanism of the CWZ. Our investigation revealed that an uncharacteristic northerly wind during the southerly monsoon resulted in a significant onshore retreat of the plume front. Vertical tidal mixing is stronger than the decreased stratification in the former plume-covered region, which resulted in the formation of the CWZ. This process was accompanied by relatively lower net heat flux, which also promoted CWZ formation. The formation of CWZ had a strong ecological impact; enhanced vertical mixing transported nutrients from the lower layer to the surface column, relaxing the CWZ’s phosphate limitation. CWZ formation also increased the depth of the mixed layer and turbidity level in the water column, forming a temporary light limitation in the center. At the margin of the CWZ, it formed a patch with a high concentration of chlorophyll a. The underwater light was sufficient once the plume was restored and the CWZ was stratified again, and the phytoplankton grew rapidly in the center of the CWZ.

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.

Corals rely on a symbiotic relationship with algae (zooxanthellae), which reside in the host tissue and play a critical role for host metabolism through photosynthesis, respiration, carbon translocation, and calcification. These processes affect coral reefs on different scales from cellular to organismal and ecosystem levels. A process-based dynamic model was developed and coupled with a one-dimensional (1-D) biogeochemical model to describe coral photosynthesis, respiration, and carbon translocation at the cellular level, calcification and ion transport in different coral polyp components (i.e., coelenteron, calcifying fluid) at the organismal level; and the exchange of material between corals and the ambient seawater at the ecosystem level. Major processes controlling the carbon budget in internal symbiosis were identified. For the symbiont, photosynthesis is the primary carbon source and translocation to the host is the major sink. For the host, most of the carbon translocated from the symbiont is lost through mucus leakage. In the host dissolved inorganic carbon (DIC) pool, most of the carbon is obtained from the surrounding seawater through uptake; photosynthesis and calcification are the major sinks of DIC. Based on a series of scenario studies, the model produced increase of photosynthesis rate with decline of calcification rate under higher air pCO2 and associated carbonate chemistry variabilities in different polyp components. The model results support the hypothesis that elevated pCO2 stimulates photosynthesis, resulting in a reduced supply of DIC to calcification. Such coupled models allow the exploration of process-based mechanisms, complementing laboratory and field studies.

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.

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.

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.

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.

Zhao, C.; Ge, J., and Ding, P., 2014. Impact of sea level rise on storm surges around the Changjiang Estuary. The potential impacts of sea level rise (SLR) on storm surge around the Changjiang Estuary and the Hangzhou Bay are investigated using a shallow-water circulation model ADCIRC coupled with a spectral wave model SWAN. The validated model is applied to two typical typhoons under three scenarios: 1.0 m SLR, 0.483 m SLR and present sea-level condition. In consideration of interactions of tide, waves and surge, the impact exerted by SLR on tide and waves are also discussed. The migration of the amphidromes generated by SLR causes the co-phase lines to defect relatively counterclockwise near the Changjiang Estuary. The amplitude of tide increases slightly at the inner mouth of the Changjiang River, and more notable increases are presented at the northern part of the Changjiang River Mouth. The amplitude of tide decreases in adjacent areas of the Hangzhou Bay. The wave heights respond to the sea...

Range Doppler velocities derived from the Envisat advanced synthetic aperture radar (ASAR) wide swath images are analyzed and assessed against the numerically simulated surface current fields derived from the finite volume coastal ocean model (FVCOM) for the Changjiang Estuary. Comparisons with the FVCOM simulations show that the European Space Agency (ESA) Envisat ASAR based Doppler shift anomaly retrievals have the capability to capture quantitative information of the surface currents in the Changjiang Estuary. The uncertainty analysis of the ASAR range Doppler velocity estimates are discussed with regard to the azimuthal and range bias corrections, radar incidence angles, inaccuracy in the wind field corrections and the presence of rain cells. The corrected range Doppler velocities for the Changjiang Estuary area are highly valuable as they exhibit quantitative expressions related to the multiscale upper layer dynamics and surface current variability around the East China Sea, including the Changjiang Estuary.

A high-resolution numerical model system is essential to resolve multi-scale coastal ocean dynamics. So a multi-scale unstructured grid-based finite-volume coastal ocean model (FVCOM) system has been established for the East China Sea and Changjiang Estuary (ECS–CE) with the aim at resolving coastal ocean dynamics and understanding different physical processes. The modeling system consists of a three-domain-nested weather research and forecasting model, FVCOM model with the inclusion of FVCOM surface wave model in order to understand the wave–current interactions. The ECS–CE system contains three different scale models: a shelf-scale model for the East China Sea, an estuarine-scale model for the Changjiang Estuary and adjacent region, and a fine-scale model for the deep waterway regions. These three FVCOM-based models guarantee the conservation of mass and momentum transferring from outer domain to inner domain using the one-way common-grid nesting procedure. The model system has been validated using data from various observation data, including surface wind, tides, currents, salinity, and wave to accurately reveal the multi-scale dynamics of the East China Sea and Changjiang Estuary. This modeling system has been demonstrated via application to the seasonal variations of Changjiang diluted water and the bottom saltwater intrusion in the North Passage, and it shows strong potential for estuarine and coastal ocean dynamics and operational forecasting.

PAN, L.; DING, P., and GE, J., 2012. Impacts of Deep Waterway Project on morphological changes within the North Passage of the Changjiang Estuary, China. Over the past decade, three phases of the Deep Waterway Project (DWP) have been carried out to deepen the shipping channel of the Changjiang Estuary, where a significant sandbar previously dominated. This project has produced significant hydrodynamic and morphological changes within the North Passage of the Changjiang Estuary. High-resolution and continuous seasonal bathymetric data from 1998 to 2008 provide good insight into sedimentation processes. These data reveal several significant characteristics: (1) During phase I, strong erosion occurred around the upper region, and the entrance, middle, and lower regions are under moderate deposition. The groin-blocked region had strong depositions. (2) The main-channel region shifted to erosion-dominant conditions during phase II. Erosion also occurred in the northern areas of the upper and middle re...

An effort was made to couple FVCOM (a three-dimensional (3D), unstructured grid, Finite Volume Coastal Ocean Model) and FVCOM-SWAVE (an unstructured grid, finite-volume surface wave model) for the study of nearshore ocean processes such as tides, circulation, storm surge, waves, sediment transport, and morphological evolution. The coupling between FVCOM and FVCOM-SWAVE was achieved through incorporating 3D radiation stress, wave-current-sediment-related bottom boundary layer, sea surface stress parameterizations, and morphology process. FVCOM also includes a 3D sediment transport module. With accurate fitting of irregular coastlines, the model provides a unique tool to study sediment dynamics in coastal ocean, estuaries, and wetlands where local geometries are characterized by inlets, islands, and intertidal marsh zones. The model was validated by two standard benchmark tests: 1) spectral waves approaching a mild sloping beach and 2) morphological changes of seabed in an idealized tidal inlet. In Test 1, model results were compared with both analytical solutions and laboratory experiments. A further comparison was also made with the structured grid Regional Ocean Model System (ROMS), which provides an insight into the performance of the two models with the same open boundary forcing.

Climate changewith rising sea levels and possible changes in surge levels and wave climate will have a large impact on how we protect our coastal areas and cities. Here the focus is on estuarine locations not only affected by tide and surge propagation, but also potentially influenced by freshwater discharge. Mitigation measures might be diverse ranging from pure hard ‘engineering’ solutions all theway to significant realignment. The variation in the type/origin and extent of the flood sources greatly influences subsequent risk management measures. At the same time, society is increasingly demanding that we take a holistic view on risk management, embracing and balancing safety, ecological and socio-economic aspects. This requires that all these diverse factors need to be considered together and integrated. In this context, the Source–Pathway–Receptor (SPR) approach offers a powerful holistic tool to investigate changing risk connected to extreme events. The traditional SPR approachwith a consecutive treatment of the flood, pathway and receptor is well understood and is widely used in coastal flood risk analysis. Here an enhanced 2D conceptual version of the SPR method is used to better describe the systemand to allow flexibility in considering multiple scales, flood sources and pathways. The newapproach is demonstrated by three estuarine case studies inwestern Europe: the Gironde estuary, France; the Dendermonde region in the Scheldt estuary, Belgium; and HafenCity (Hamburg) in the Elbe estuary, Germany. They differ considerably in the surface area considered, in the type of flood sources, and hence also in the SPR configuration. After a brief introduction of the typical characteristics of the three study sites including some lessons learned from past flood protection measures, the differences in application and results of the SPR approach are discussed. Emphasis is on the specific aspects for each study site, but embedded in a generic SPR framework. The resulting generic lessons learned about the flood sources and how this shapes subsequent analysis are transferable to numerous important estuaries worldwide. ispartof: Coastal Engineering vol:87 pages:32-49 status: published

[1] A massive bloom of the green macroalgae Ulva prolifera (previously known as Enteromorpha prolifera) occurred in June 2008 in the Yellow Sea (YS), resulting in perhaps the largest “green tide” event in history. Using a novel index (Floating Algae Index) and multiresolution remote sensing data from MODIS and Landsat, we show that U. prolifera patches appeared nearly every year between April and July 2000–2009 in the YS and/or East China Sea (ECS), which all originated from the nearshore Subei Bank. A finite volume numerical circulation model, driven by realistic forcing and boundary conditions, confirmed this finding. Analysis of meteorological/environmental data and information related to local aquaculture activities strongly supports the hypothesis that the recurrent U. prolifera in the YS and ECS resulted from aquaculture of the seaweed Porphyra yezoensis (or nori) conducted along the 200 km shoreline of the Subei Bank north of the Changjiang (Yangtze) River mouth. Given the continuous growth in aquaculture efforts in the region, similar macroalgae bloom events, such as the summer 2008 event, are likely to occur in the future, particularly between May and July. This was confirmed by the 2009 bloom event in the same regions and the same period. The profit of the local P. yezoensis aquaculture industry (∼16,000 Ha in 2007) is estimated as U.S. $53 million, yet the cost to manage the impact of the summer 2008 U. prolifera bloom exceeded U.S. $100 million. Therefore, better strategies are required to balance the economic benefit of seaweed aquaculture and the costs of environmental impacts.

Changjiang Estuary is characterized by complex branches and outlets (North Branch, North Channel, North Passage and South Passage). The relatively shallow bathymetry around the mouth limits the navigational potentials (Fig.1). The Deep Waterway Project (DWP) was carried out in the North Passage of South Channel for the purpose of deepening the navigational channel from 7.0m in July 1998 to 8.5m in July 2001 (Phase I), 10.0m in May 2005 (Phase II) and 12.5m in May 2010 (Liu et al. 2004). In addition to dredging, two dykes were constructed to reduce the sediment transport from Jiuduansha and Hengsha Shoals into North Passage. A set of groins was built with connection to dykes for the purpose of strengthening the currents in the middle of channel, which helps the maintenance of the navigational capability. The construction has significantly changed the local bathymetry in the Changjiang Estuary and within the channel. The high-resolution bathymetry measurements were made over seasons. An evolution trend of the morphological change is revealed, which is discussed in this paper.