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The vertical density gradients of salinity and suspended sediment concentration (SSC) cause stratification in estuaries, which play a vital role in the turbulence structure, water mixing, and sediment transport. To investigate the effect of stratification, especially SSC-induced stratification, on maintaining the estuarine turbidity maximum (ETM), we conducted in situ measurements on sediment dynamics at the upper and central ETM sites in the South Passage of Changjiang Estuary in July 2018. The gradient Richardson number was estimated as a proxy for the stratification that is attributable to salinity or/and SSC. We found that salinity-induced stratification was observed mainly on the surface and in the middle layers, whereas SSC-induced stratification occurred mainly in the near-bottom layers. Furthermore, at the central ETM, the baroclinic effect was enhanced during the neap tide when the salinity-induced stratification was stronger than that during the spring tide. In the early phase of floods with minimum velocity during the neap tide, salinity-induced stratification suppressed the turbulence and vertical diffusion of sediments. Moreover, the flocculation enhanced the settling process within the water column. Consequently, high concentrations of fine-grained sediments formed near the bottom and promoted SSC-induced stratification, thereby leading to the continuous accumulation and trapping of sediments. In conclusion, the interactions among the “salinity- and SSC-induced stratification” processes served as crucial constraints of the temporal and spatial variations of the ETM in the Changjiang Estuary.

Continental shelf islands are contributors of terrestrial sediment supply to shelf regions, and the sediment flux from these islands shall be quantified. We calculated the sediment flux of continental shelf islands in the southeastern China using two empirical equations under two preconditions. The first, the sediment load/yield of the islands has the same pattern as the adjacent small, mountainous rivers along the coastline; and the second, each of the islands was treated as a single catchment. The results show that the sediment supply from these islands reached an order of magnitude of 1 Mt/a, which is comparable to the supply from the local smaller rivers. A sensitivity analysis indicates that this value represents the lower limit of estimate; if the accurate amount of sub-catchments of any island is considered, then this value will be enhanced slightly. This study demonstrates that the sediment supply from continental shelf islands to oceans is an important factor affecting the regional sedimentation and, therefore, should be paid with attention.

How climate controls tropical cyclone variability has critical implications for modern human society but is not well understood due to the short length of observational records. To probe this knowledge gap, we present a synthesis of intense typhoon activity from the northwestern Pacific over the past 2000 years, which is supported by a new, well-resolved tidal flat sedimentary record from the Jiangsu coast, eastern China. The record reveals nine intervals of typhoon frequency, indicating that the frequency of intense typhoons has varied on multi-centennial scales over the past 2000 years. Our synthesis shows strong evidence for a seesaw pattern of intense typhoon frequency between southeastern China and Japan and Korea. This pattern can be explained by the El Niño and Southern Oscillation–East Asian Monsoon–sea surface temperature hypothesis, which potentially explains the basin-wide typhoon climate in the northwestern Pacific region. A shift in typhoon activity was identified from 550–280 to 280–50 yr B.P. during the Little Ice Age, when typhoon activity changed from active to quiescent or vice versa. Centennial-scale shifts in Intertropical Convergence Zone and Western Pacific Warm Pool sea surface temperature are likely to be the primary forcing mechanisms driving this shift. Results obtained here provide links between typhoon activity and the El Niño and Southern Oscillation, the East Asian Monsoon, and the Western Pacific Warm Pool sea surface temperature, and therefore improve our ability to fully assess intense typhoon activity in future climate warming.

The southern coast of Hainan Island, China, is one of the most frequently hit areas of tropical cyclones in the Northwest Pacific regions. Although some of the extreme typhoon events were known in historical times, quantitative information on the timing and magnitude of paleo-typhoon events in this coastal area remains rare. In the present study, a large number of coral reef boulders were found on the Xiaodonghai reef platform, on the south coast of Hainan Island. Morphometric analysis of the boulders shows an exponentially fining landward trend, indicating a storm origin; a wave-induced current velocity of 2.41–5.71 m/s during the storm events is required to transport the boulders that were originally situated outside the reef edge. Based on the U/Th and 14C dating for the age-indicating samples taken from the boulders, seven major periods with intense typhoon activities were identified for the last 4,000 years, i.e., 1800–1500 BCE, 1200–900 BCE, 50–120 CE, 550–800 CE, 900–1000 CE, 1350–1900 CE, and 1910–2000 CE. A comparison with the regional typhoon records in terms of climatic parameters in the northwestern Pacific and the South China Sea regions indicates that the longitudinal variations of intense typhoon frequency were mainly controlled by El Niño-Southern Oscillation (ENSO), dominantly modulated by the Intertropical Convergence Zone. Because of the future warming climate, there will be a trend of enhanced typhoon risk for the southern Hainan Island coasts.

Reservoir interception has significantly affected the fluvial sediment budget as well as the sedimentary processes of the entire Changjiang catchment. To evaluate the impact of reservoirs, we analyze the combined effects of 1037 large and medium-sized reservoirs on the fluvial flux in general, and more specifically on the sedimentary processes in the middle and lower reaches. Results indicate that reservoir emplacement in the Changjiang catchment currently reduces the sediment load towards the East China Sea by 453 Mt y−1. Estimates at Yichang station show that the sediment discharge would exceed 555 Mt y−1, if there were no reservoirs involved. It is expected that in the near future, more dams will be constructed. The entire reach of the Changjiang River can be divided at Yichang station into two distinctly characterised reaches with regard to sedimentation, where the upper reach exhibits mostly siltation (over 589 Mt y−1 of sediment deposition), and the lower reach is affected by erosion (sediment loss, including sand extraction, exceeding 112 Mt y−1). As a consequence, the sediment flux to the sea will further decrease to 100 Mt y−1. Due to human interference, the upstream sediment load reduced and caused significant changes in the erosion/deposition pattern of the middle and lower reaches, which together altered the terrestrial sediment input to the sea. Before 2003, the upstream reaches were the dominant sediment source. After 2003, the sediment contribution of the middle and lower reaches became more important, and its sediment contribution will further increase to 78% of the total sediment load reaching the sea, after completion of the cascade reservoirs at the Jinsha Tributary. Hence, the middle and lower reaches are converting from a sediment sink to a major sediment source.

Suspended sediment concentration (SSC) has a significant impact on the estuarine environment and its morphological evolution. At any given location, the temporal variability of depth-averaged SSC is due to a combination of two processes: horizontal advection and local resuspension. In this study, we investigated the sediment dynamics at three anchored monitoring stations close to the maximum turbidity zone of the Changjiang Estuary, and developed a box model to differentiate the effects of advection and resuspension. Further, settling velocities were estimated using the ADV Reynolds flux method, excluding the advection SSC component. We found that predicted changes in advection- and resuspension-induced SSCs were consistent with the bottom shear stress and accretion/erosion observations. The combination of observed bed accretion/erosion changes and the predicted advection-induced SSCs indicates that the advective transport of suspended sediment is an important process in accelerating persistent erosion at the monitoring stations. Although SSC variations due to advection and resuspension are of similar magnitudes, our model results indicate that if resuspension dominates, the resuspension-induced component can reach up to twice the magnitude of the advection-induced component. We conclude that the box model is a valuable tool for evaluating subaqueous delta accretion/erosion in response to sediment reduction caused by upstream dam construction and climate change.

Mega-deltas are major sinks of river-borne sediments and important sources of terrigenous sediments for open shelves. Their evolution has far-reaching impacts on adjacent coastal waters, from the point of view of along-shelf morphodynamics and biogeochemistry. However, the complex budgeting patterns of input, storage, bypass, and final accumulation of sediment are still poorly understood. The Changjiang (Yangtze River) in China is among the world's largest river systems, not only in terms of water and sediment discharges but also the massive amount of sediment deposited in its subaqueous delta and distal muds. Here we discuss about the along-shelf sediment redistribution in the Changjiang Subaqueous Delta and Distal Muds (CSDDM) over the last century. For the purpose of understanding its spatial diversity in detail, we divided the study area into three spatially connected parts, namely the Changjiang subaqueous deltaic mud (CJM), the Zhejiang inner-shelf mud (ZJM), and the Fujian inner-shelf mud (FJM). The concept of sediment retention index helps understand the overall evolution of this mega-delta. Hydrological survey data from the peripheral rivers and short marine-sediment cores are used to evaluate the amount of sediment supplied to and deposited in the study area. The results show that over the last century, the rate of sediment supply to the CSDDM reached ~645 Mt year−1 on average (ranging between 535 and 725 Mt year−1), while the total sediment deposition in the CSDDM reached ~683 Mt year−1 on average (390–976 Mt year−1), with the ratio for the deposits in the CJM, ZJM and FJM being close to 3:5:2. As such, the input and output of sediment were generally in balance. The sediment retention indices are estimated to be 0.35, 0.86 and 1.00 for CJM, ZJM and FJM, respectively. This study contributes to our knowledge of marine sediment fluxes, facilitates a better understanding of the growth and development of mega-deltas under system regime shifts, and helps identify a sustainable development model for areas of high population density, heavy economic activity and rapid urbanization.

The radionuclide 210Pb is suitable for century-scale dating and has been used to calculate the sedimentation rate in a variety of environments. However, two common ways to apply 210Pb dating techniques may give misleading results. One is “prolonging of age”, i.e., using the calculated sedimentation rate to date back to 200 or 300 years. This practice must be treated with caution because the 210Pb dating techniques do not guarantee direct dating for ages much older than 100 years. Another is “loss of age”, i.e., the calculated time span between the topmost layer and the 210Pb background layer in cores is less than 100 years when an apparent sedimentation rate is used in the calculation. Here, we propose that based on the principle of 210Pb dating, the upper limit of age suitable for direct 210Pb dating is between 110 and 155 years. The “prolonging” application is acceptable only if the sedimentary environment in the past several hundred years was stable and the sedimentation rate was generally constant, and verification with independent evidence (such as historical records or biomarker methodology) is needed. Furthermore, after analyzing many published and collected data, we found four possible reasons for the “loss of age”. First, the compaction effect of sediment should be corrected in laboratory analysis or else the calculated age will be underestimated. Second, the accuracy and uncertainty of 210Pb activity measurement affect the judgment of the background. To be cautious, researchers are apt to choose a background activity with a younger age. Third, use of a slightly smaller value of supported 210Pb activity in a calculation will lead to considerable underestimation of the time span. Finally, later-stage erosion and migration are common for sedimentation, which lead to loss of sedimentary records and are often reflected as a “loss of age” in cores. We believe that proper use of 210Pb dating data may provide helpful information on our understanding of sediment records and recent environmental changes.

Growing evidence has suggested that human activities could cause significant changes in marine sedimentation processes. The spatio-temporal response of sedimentary environment to human activities, however, remains unclear. Here, we present new evidence of sedimentary changes in the East China Sea over the past three decades (i.e., 1984–2016 CE) by using the mean grain size of sensitive components and geochemical parameters (i.e., Si/Ti and Fe/Al) from two sedimentary records from the Zhejiang-Fujian mud belt (ZFMB). Comparison with a compilation of published data shows that natural climate oscillations were the dominant factors that controlled the sedimentation processes before 2005 CE. Variations in the mean grain size of sensitive components over the ZFMB were mainly controlled by the East Asian Winter Monsoon before 2005 CE. The Si/Ti and Fe/Al ratios in the northern and southern of the ZFMB before 2005 CE were controlled by the Pacific Decadal Oscillation/Kuroshio Current and the East Asian Summer Monsoon, respectively. However, our synthesis shows a significant shift in the sedimentation processes between the north and south of the ZFMB since 2005 CE, i.e., after acceleration of the human imprint on sedimentation (e.g., dam construction). The mean grain size of sensitive components and the Si/Ti and Fe/Al records in the north and south of the ZFMB since 2005 CE are mainly controlled by human activities (e.g., human-induced catchment changes in the Changjiang River) and natural climate oscillations (e.g., the East Asian Winter Monsoon and Pacific Decadal Oscillation/Kuroshio Current), respectively. Our study reveals a human-induced asynchronous sedimentary signal in the East China Sea since 2005 CE, and may be applicable to river-dominated marginal seas worldwide during the human-impacted period, providing new evidence to better characterize the Anthropocene epoch.

An extensive system of tidal channels, starting with Jinshan Trough in the east, is located along the north shore of Hangzhou Bay, China. This contribution investigates the morphological evolution of Jinshan Trough by using 17 bathymetric charts from a series covering a period of 51 years from 1960 to 2011. Three stages of evolution during this period are distinguishable based on the morphology and annual mean volume data. The first stage (1960–1987) is characterized by extension of the trough; the second stage (1987–1996) is a relatively stable period with some adjustments in the trough morphology; the third stage (1996–2011) is marked by the processes of erosion and deposition in the beginning of the period and a subsequent slow erosion process. Spatio-temporal variability of the trough was evaluated by using empirical orthogonal function (EOF) analysis. The first eigenfunction indicates that erosion is the main evolution process and there exists three stages similar to those distinguished from volume variations. The second eigenfunction mainly reflects erosion and deposition in the northwest part of the trough located in the flood tidal current shadow area of the artificial headland in Jinshan. The third eigenfunction mainly reflects annual fluctuations of erosion and deposition in the side slope at the artificial headland in Jinshan. A particularly intense erosion process occurred between 1996 and 1998. The major effects on morphological evolution in Jinshan Trough from 1960 to 2011 were investigated and tentative conclusions were presented. Continuous coastal reclamations in Jinshan had the most pronounced effect on the morphological evolution during the first and the second stages. The storm surge had a pronounced effect on the evolution at the beginning of the third stage.

In straight estuary channels, differential advection and the Coriolis force are the major driving mechanisms for lateral circulation. An analytical model was developed to explore the roles of the two mechanisms in the dynamics of tidally dominated well-mixed estuaries. The model provided a nondimensional parameter, K eh , a type of Kelvin number (considered as horizontal Kelvin number) to elucidate the relative importance of the two mechanisms. Differential advection is effective under small K eh , while the Coriolis force is effective under larger K eh . The critical value of K eh has an order of magnitude of 0.1 in well-mixed estuaries. Lateral circulations generate residual currents through the lateral advection term in the along-estuary momentum equation. When differential advection is effective, the lateral-advection-induced flow has a laterally sheared structure with the landward flow in the channel and seaward flows over shoals. When the Coriolis force is effective, it has a laterally sheared structure with the landward flow in the left part of the cross-section and the seaward flow in the right (facing ocean). When the two mechanisms are equally important, it has an asymmetric laterally sheared structure with a stronger seaward flow over the right shoal. Those lateral structures indicate that the lateral-circulation-induced flow generally reinforces the estuarine gravitational circulation.

In order to evaluate the impact of human activities (mainly dam building) on the Changjiang River sediment discharging into the sea, the spatial–temporal variations in the sediment load of different tributaries of the river were analyzed to reveal the quantity, grain size and composition patterns of the sediment entering the sea. The results show that the timing of reduction in the sediment load of the main stream of the Changjiang was different from those associated with downstream and upstream sections, indicating the influences of the sub-catchments. Four stepwise reduction periods were identified, i.e., 1956–1969, 1970–1985, 1986–2002, and 2003–2010. The proportion of the sediment load originating from the Jinsha River continuously increased before 2003; after 2003, channel erosion in the main stream provided a major source of the sediment discharging into the sea. In addition, in response to dam construction, although mean grain size of the suspended sediment entering the sea did not change greatly with these different periods, the inter-annual variability for sediment composition or the relative contributions from the various tributaries changed considerably. Before 2003, the clay, silt and sand fractions of the river load were supplied directly by the upstream parts of the Changjiang; after 2003, although the clay component may still be originating mainly from the upstream areas, the source of the silt and sand components have been shifted to a large extent to the river bed erosion of the middle reach of the river. These observations imply that the load, grain size and sediment composition deposited over the coastal and shelf water adjacent to the river mouth may have changed rapidly recently, in response to the catchment changes.

Fine-grained sediments suspended in coastal waters play an important role in submarine topography evolution and associated environment changes. The convergence of suspended sediments concentrated near the seabed results in high sediment concentration, and contributes significantly to sediment transport. In order to investigate the mechanism triggering high suspended sediment concentration (SSC), we deployed a tripod to the seabed to obtain in situ bottom boundary layer measurements of sediment dynamics and a buoy to the sea surface to collect meteorological and wave data at the northern Changjiang River mouth from December 20, 2015 to January 20, 2016. The high SSC (e.g. >3 g/L) events were observed together with fluid mud (thicknesses of 4–16 cm) near the seabed during neap tides when cold air intrusion generated winter storms and strong waves. Further, we found that the high SSC event was mainly resulted from wind waves and sediment resuspension supported by local benthic fluid mud, which was associated with three stages. At the setting up stage, the winter storm brought long duration of strong-waves (e.g. significant wave height >1.5 m) more than 15 h, resulting in a maximum wave-current combined bottom shear stress of 3.2 Pa and the increase of SSC to >1 g/L. At the reinforcement stage, the strong waves and bottom shear stress lasted for several hours, and further increased the SSC to >3 g/L. At the final decay stage, wind waves and the maximum wave-current shear stresses decreased significantly, with the disappearance of high SSC. Thus, there was no high SSC event observed even with strong wind waves during spring tides because strong wave duration was too short for the reinforcement process. The sediment source for the high SSC events was mainly from bottom fluid mud resuspension, as well as the advection transport from the adjacent subaqueous Changjiang River delta.

The definition of the threshold of sediment motion is critical for continental shelf sediment dynamics. The work by A. Shields laid the foundation for this research direction, leading to the well-known Shields curve. Here we review the most widely used threshold curves that have followed from the original Shields curve over the last 80 years, and propose that in terms of physical processes the threshold (critical Shields parameter) is a function of at least six variables, i.e. grain Reynolds number, grain size distribution, sphericity, roundness, particle cohesiveness and the scale effects of turbulence. Identifying these key factors, we paid a special attention to the role of the scale effects of turbulence. Turbulence was thought to be a random process, but the improvement of measurement techniques revealed that it has both temporal and spatial structures: the magnitude of instantaneous velocity fluctuations varies in time and in location, which can cause the deviation between in situ measurements and flume experiments. In coastal and shelf waters, in situ measurements of tidal currents and suspended sediment concentrations have revealed that resuspension takes place even though the bed shear stress is well below the Shields curve. Further process and mechanism studies are required to improve the theoretical framework regarding the turbulence structures and their interplay with sediment threshold. The scientific problems for future studies include the establishment of laboratory experiments, in situ measurements and process-based modelling under different water depths and hydrodynamic conditions to quantify the scale effects of turbulence; the development of new observation techniques for higher resolution and for extreme environments; development of new data processing methods, including big data methods to analyse turbulence structures; and the quantification of the effects of biological contributions and non-particle components on the family of Shields curves.

The link between tropical cyclone (TC) activity in the South China Sea (SCS) and global climate change is commonly debated and there is a clear need for long-term geological records of TC activity if we are to clarify this connection. Multi-millennial, high-resolution paleo-storm records from the SCS are rare in the region and this causes difficulties for those exploring potential climate drivers of TC variability over centennial to millennial timescales and reduces our ability to fully assess the risks associated with future TC activity. This paper presents an age-constrained, mud-dominated sedimentary sequence from a coastal lagoon on Hainan Island, China. Multiproxy analyses incorporating chronological, lithological, sedimentological, and geochemical evidence were used to infer storm deposits preserved within the sequence and to reconstruct a time series of storm activity in the SCS during the mid-to-late Holocene. This long sedimentary record shows that TCs were highly active over the periods 5500 to 3500 and 1700 to 0 cal yrs. BP, and these periods contrast with a relatively quiet period 3500–1700 cal yrs. BP. An apparent inverse correlation between TC reconstructions from the SCS and those from the Korean Peninsula and Japan implies an oscillating pattern across the SCS and western North Pacific (WNP) over centennial to millennial timescales. A comparison between the sedimentary and paleoclimatic records implies that the El Nino–Southern Oscillation was not the only mechanism responsible for typhoon variability over the past 7500 cal yrs. BP, suggesting that other factors such as the thermal state of the western Pacific warm pool likely also had a strong influence on both SCS and WNP TC variability.

Sediment transport within small tidal inlets is sensitive to natural processes, whilst large tidal inlets are relatively robust systems because of their large tidal prism. However, remarked morphological changes may be initiated even under the condition of low sediment supply, as illustrated by Jiaozhou Bay, a large coastal embayment on the Shandong Peninsula, eastern China. Jiaozhou Bay is characterized by its relatively slow rate of natural change, and while the embayment has a flood-dominated entrance channel and muddy seabed, the suspended sediment concentration is generally low due to the lack of abundant source material. Observations of sediment dynamics show that net suspended sediment transport is directed towards outside of the bay, with an order of magnitude of 103 t during a tidal cycle. The export of sediment associated with this flood-dominated environment implies that the net transport pattern is controlled by tidal exchange processes rather than the strength of the seabed shear stress. Sediment budget calculations show that supply of artificial sediment into the bay can account for up to 72% of the total input, which is in agreement with the 210Pb and 137Cs radioisotope geochronologies, and this leads to accumulation rates of 100–101 mm yr−1; without this, the deposition rate would be low under natural conditions. The flood tidal delta area is also influenced by the input of anthropogenic material, and acts as a depocenter with relatively high accumulation rates. Furthermore, although the inlet system has not yet reached its equilibrium state (i.e., the entrance cross-sectional area is still larger than the equilibrium cross-sectional area), land reclamation activities have resulted in a rapid reduction of the embayment area (by 37%) over the last 80 years. Our findings indicate that the rapid changes observed in the tidal basin area and seabed morphology are mainly the result of human activity rather than natural processes.

To study the fluvial interaction between Changjiang River and Poyang Lake, we analyze the observed changes of riverine flux of the mid-upstream of Changjiang River catchment, the five river systems of Poyang Lake and Poyang Lake basin. Inter-annual and seasonal variations of the water discharge and sediment exchange processes between Changjiang River and Poyang Lake are systematically explored to determine the influence of climate change as well as human impact (especially the Three Gorges Dam (TGD)). Results indicate that climate variation for the Changjiang catchment and Poyang Lake watershed is the main factor determining the changes of water exchanges between Changjiang River and Poyang Lake. However, human activities (including the emplacement of the TGD) accelerated this rate of change. Relative to previous years (1956-1989), the water discharge outflow from Poyang Lake during the dry season towards the Changjiang catchment increased by 8.98 km(3)y(-1) during 2003-2010. Evidently, the water discharge flowing into Poyang Lake during late April-late May decreased. As a consequence, water storage of Poyang Lake significantly reduced during late April-late May, resulting in frequent spring droughts after 2003. The freshwater flux of Changjiang River towards Poyang Lake is less during the flood season as well, significantly lowering the magnitude and frequency of the backflow of the Changjiang River during 2003-2010. Human activities, especially the emplacement and operation of the TGD and sand mining at Poyang Lake impose a major impact on the variation of sediment exchange between Changjiang main river and Poyang Lake. On average, sediments from Changjiang River deposited in Poyang Lake before 2000. After 2000, Changjiang River no longer supplied sediment to Poyang Lake. As a consequence, the sediment load of Changjiang River entering the sea increasingly exists of sediments from Lake Poyang during 2003-2010. As a result, Poyang Lake converted from a depositional to an erosional system, with a gross sediment loss of 120.19 Mty(-1) during 2001-2010, including sand mining.

The sources and burial patterns of organic matter in Shamei Lagoon, Hainan, China, were investigated by analyzing the collected sediment cores, in terms of the variations in total organic carbon (TOC), total nitrogen (TN), stable carbon isotopic composition (δ13C and δ15N), and the ratio of total organic carbon to total nitrogen (TOC/TN). On such a basis, the environmental evolution and primary productivity changes of the lagoon were revealed, and the impacts of both natural environmental changes and human activities were identified. Downcore variation patterns of the parameters representing sources and burial flux of organic matter were predominantly related to natural evolution of the lagoon from 1800 to 1950 and they were increasingly affected by human activities afterwards. In the period 1800–1900, the water exchange capacity of the lagoon declined substantially, but it was still suitable for the growth of marine phytoplankton, as is indicated by relatively high proportion and burial flux of organic matter originated from marine sources. During 1900–1950, the lagoon was transformed into a freshwater environment. After 1950, due to strong influence by human activities, the local freshwater phytoplankton became the major source of buried organic matter; at the same time, marine lost its niche and its burial flux was much lower than that of its freshwater counterpart. The large increment of buried freshwater organic matter also indirectly reflects the eutrophication trend since 1950.

In recent years, the increased pressure of saltwater intrusion in the Modaomen Estuary, one of the outlets in the Pearl River Delta (PRD), China, has threatened the freshwater supply for the surrounding regions. The estuary has complex geometry and bathymetry and branches into three waterways, Madaomen Waterway, Hongwan Waterway, and Hezhou Waterway, entering into the coastal sea. In this study, a three-dimensional baroclinic model (EFDC) was used to investigate salt transport among these branches and saltwater intrusion in the mainstem of the estuary. The salt transport at selected cross sections was decomposed into three components: advection, steady shear, and tidal oscillatory, following the method of Lerczak et al. (Lerczak, J.A., Geyer, W.R., Chant, R.J., 2006. Mechanisms driving the time-dependent salt flux in a partially stratified estuary. Journal of Physical Oceanography 36, 2296–2311). Results from this study indicate that the Hongwan Waterway serves as a salt source for the mainstem of the estuary, especially during spring tides, while the Hezhou Waterway mainly acts as a salt sink for the mainstem. The down-estuary wind increases the steady shear transport in the Modaomen Waterway and the advection transport in the Hongwan Waterway, which alters the saltwater intrusion in the estuary. Closure of the Hongwan and the Hezhou Waterways could result in a 20% decrease of saltwater intrusion in the estuary. These results provide scientific basis for water resource management in the region.

This paper focuses on the hydrodynamic and suspended sediment transport processes in response to sequential reclamations over the intertidal flat, Jiangsu coast, which used to be one of the widest intertidal flats in China. Investigations into the sediment transport patterns reveal that net suspended sediment fluxes increased from 102 kg m?1 on the upper intertidal flat to 103–4 kg m?1 on the lower intertidal flat, per tidal cycle. This variation was caused mainly by the phases of the current velocity, suspended sediment concentration (SSC), local bed geomorphology, inundation patterns, and extreme weather conditions. The longshore component of tidal currents intensifies seaward over the intertidal flat. Extreme weather conditions (e.g. storms) and strong winds were important factors affecting the current velocity structures, the intensity of residual currents, and the SSCs. The SSCs were characterized by several peaks throughout the tidal cycle. These peaks were associated with strong currents, combined wave-current interactions and intense turbulences due to the initial flood surge. Furthermore, the suspended sediment was finer and contained more clay materials during the flood phase of the tidal cycle, than during the ebb. Fine particles were transported within the upper water column by landward residual currents, but coarse particles were transported within the lower water column by seaward residual currents. Overall, residual currents were apt to transport fine-grained sediment landward and coarse-grained sediment seaward. This pattern indicates that the suspended sediment fluxes are highly variable in the water column, even for very shallow tidal environments. The cross-shore component accounted only for a small percentage of the total sediment flux. However, this part of sediment supply dominated accretion over the intertidal flat. The intertidal flat has become narrower and steeper, following sequential reclamations. A negative feedback process existed to maintain the convex profile shape, although the human activities (i.e. reclamation) could have modified the natural process. The surficial sediment tended to become finer on the mid-upper intertidal flat but coarser on the lower intertidal flat following reclamations, in response to a reduction in the tidal currents over the intertidal zone and enhancement of wave action over the lower intertidal flat. Accretion, with high rates, was maintained by landward transport of sediment. In such an accretional system with influences of reclamation, the width or slope of the intertidal flat is adjusted continuously. Before the cease of reclamation, the intertidal flat is unable to achieve an equilibrium shape.

Power-law relationship between tidal prism (P) and the cross-sectional area of the entrance channel (A) is applicable to assess the equilibrium conditions of a tidal inlet system. The classic method of determining P-A relationships proposed by O’Brien depends on datasets from multi-tidal inlet systems, which has shown some limitations and is unable to assess equilibrium of a single tidal inlet. This paper focuses on establishing a new P-A relationship for a single tidal inlet. Our experimental result shows that in order to maintain the status, power n should be > 1, implying that the inlet width will narrow and current speed within the entrance will increase as tidal prism becomes smaller. A possible explanation for power n

An MIDAS-400 Customised Data Acquisition System was deployed to obtain current velocity and suspended sediment concentration profiles over the intertidal flat at Wanggang, on the Jiangsu coast (China). At the same time, seawater temperature, conductivity and pressure were measured. Using a data sampling rate of 4 Hz, the high-frequency pressure signals in response to water surface fluctuations were recorded and subsequently transformed into wave parameters. The analytical results show that the apparent bed roughness length has a magnitude close to the height of sand ripples, which is much larger than the grain size diameter. The bottom shear stress associated with current-wave interaction is higher than the tidally-induced stress and intensified turbulent mixing and resuspension. Furthermore, several suspended sediment concentration peaks occurred during a tidal cycle, which can be related to strong turbulence near the bed caused by frontal tidal currents and the enhanced resuspension; advection and extreme weather conditions (e. g. storms and strong winds) also played an important role. Finally, large net suspended sediment fluxes were observed to present when the flood or ebb currents prolonged, i.e. the flow became quasi-unidirectional during a tidal cycle. In general, the MIDAS-400 is a suitable tool for high-resolution data collection for the situations of combined current-wave action over intertidal flats; the data obtained can be used to analyze the processes and mechanisms of material transport.

A MIDAS-400 customized data acquisition system has been used for the high-resolution sediment dynamic measurements over the Dafeng intertidal flats of northern Jiangsu during 6 tidal cycles from July 3 to July 10, 2003. The bed shear stress and bedload transport rates, in response to wave-current interactions, are calculated, which indicate that wind waves enhance the bottom shear stress and bedload transport rates. At the station for measurements, bedload transport was directed to seaward, with a sediment discharge of 30–150 kg·m-1 per tidal cycle. The surficial sediment samples were collected from a grid with 10 m spatial intervals over a rectangle 2 × 104 m2 experimental area, near the MIDAS-400 during the spring tide of July 4, 2003, and the neap tide of July 9, 2003. In addition, leveling survey was undertaken to obtain the detailed topography of the sampling area. Grain-size trend analysis of the sediment samples shows that the bedload transport patterns are complex, mainly controlled by the hydrodynamics and local geomorphology (e.g. tidal creeks) over the intertidal flats. Furthermore, the grain-size trends pattern during the neap tide rather than during the spring tide is consistent with the calculated bedload transport, and the grain-size trend vectors for the spring tide are larger than those for the neap tide, indicating that the sampling thickness of surficial sediment during the neap tide may contain the information on the geomorphological evolution (or sediment deposition/erosion) from the spring to the neap tide.

A small sedimentary basin (the Yuehu lagoon), located at the eastern tip of Shandong Peninsula, China, was selected to study its filling by sediment and the vertical flux of particulate organic carbon in response to natural/anthropogenic processes. Surficial and short core sediment samples were collected and analyzed to obtain data sets of grain size, organic carbon content, deposition rates and vertical fluxes of sediment and organic carbon. The analytical results show that the lagoon is covered mainly with fine-grained sediments with high deposition rate and particulate organic carbon content being found from the central part of the mud deposit. The sediment balance of the lagoon indicates high denudation rates of the catchment basin, which may be related to soil erosion in response to farming and land use pattern changes. Furthermore, preliminary analysis of the organic carbon fluxes of the Yuehu lagoon and other embayments of the region shows that these coastal systems make an important contribution to the regional shallow sea carbon burial.

In order to investigate the sediment dynamic behavior of the Yuehu, a small inlet system characterized by abundant sediment supply and rapid sediment infilling, measurements and sampling were undertaken to obtain data sets of tidal water levels, current velocities, suspended sediment concentrations, grain size parameters, deposition rates and organic carbon contents. Sediment budget and the time-velocity asymmetry patterns of the inlet system were analyzed. The results show that the deposition rates are relatively high within the tidal basin. The total sediment flux cannot be balanced by the input from the open sea, the aerosol and biological production; rather, the material from land (which has been intensified by agricultural activities over the past several decades) represents a major component for the balance. Thus, the denudation rate must be reduced to protect the Yuehu as a natural reserve. Furthermore, it is found from the present study that the Yuehu inlet system exhibits all of the four time-velocity asymmetry patterns with varied frequencies of occurrence, compared with the two asymmetry patterns identified for larger inlet systems; such phenomena are partly due to the adjustment of entrance channel geometry. This behavior may be representative of the small tidal inlets at their late stage of morphological evolution and, therefore, may be utilized to prolong the lifespan of small inlet systems. (C) 2003 Elsevier Ltd. All rights reserved.

Numerical experiments were carried out to evaluate the physical effects of upwelling/downwelling on the suspended sediment concentration (SSC) on continental shelves. The results show that (1) because the upwelling or downwelling velocity tends to be of the same order of magnitude as the settling velocity of suspended sediment, various SSC distribution patterns are possible, depending on the relative importance of the velocities; (2) the presence of upwelling can enhance the SSC; and (3) in an upwelling/downwelling system, maximum concentrations may be situated in the middle of the water column, if the settling velocity is larger than the upwelling velocity. Hence, the physical processes associated with upwelling/downwelling are relevant to the SSC distribution observed over the mud patch near Cheju Island, East China Sea. Furthermore, a qualitative analysis indicates that the presence of upwelling/downwelling in the overlying water column increases the accumulation rate at the seabed.

Liu, Y.; Wang, Y.P.; Li, Y.; Gao, J.; Jia, J.; Xia, X., and Gao, S., 2012. Coastal embayment long-term erosion/siltation associated with P-A relationships: A case study from Jiaozhou Bay, China. Journal of Coastal Research, 28(5), 1236– 1246. Coconut Creek (Florida), ISSN 0749-0208. The sedimentary environment of a large coastal embayment, Jiaozhou Bay, eastern China, in terms of erosion/siltation, has been assessed on the basis of seabed bathymetry and coastline data sets from sea charts (1936–2002) and Landsat-5 TM images (1986–2011). Nine types of sedimentary environment are identified according to the sedimentation rate. The results indicate that slow siltation was generally present from 1936 to 1963, with exceptions of some patchy, slightly eroding areas within the embayment. Transformation to a slow erosion pattern occurred in the main channels over the central embayment in 1963, and such a situation continued until 1982. Subsequently, since 1986, most of the coastlines extended toward the sea at a rate of 10 1 –10 2 my 21 ; this was mainly attributable to human activities, including reclamation over the NW tidal-flat and harbour construction along the east and south coastlines. Using the same data set, patterns of changes in tidal prism and deposition rate within the entrance channel were established. An equivalent friction coefficient (K) is proposed to evaluate the P-A relationship of the large tidal inlets in Jiaozhou Bay. The K value increases gradually over time, indicating a trend toward stable equilibrium in Jiaozhou Bay. This coefficient can be used to examine the status of morphological stability and equilibrium of coastal embayments.