Suspended sediment regulates coastal ecosystem function and resilience of deltas and wetlands, yet satellite estimates of suspended-sediment concentration (SSC) are drawn largely from fair-weather observations. Using continuous in-situ time series together with multi-decadal satellite data and reanalysis products, we quantify a systematic fair-weather bias that misrepresents coastal SSC in both magnitude and sign. In storm-dominated regions, clouds obscure energetic resuspension, causing satellites to underestimate long-term means by up to ~20%. Conversely, upwelling coasts exhibit positive biases where clear skies coincide with elevated wave energy. The direction and magnitude of bias follow regional couplings between cloud cover, wave energy and sediment mobilization, providing a basis for prediction and correction. Projections from climate models indicate only modest changes in cloudiness this century, implying that the bias will persist without methodological adjustment. Ignoring fair-weather limitations fundamentally distorts sediment budgets and biogeochemical fluxes, underscoring the need for bias-aware algorithms in global coastal assessments.

Tropical cyclones (TCs) are posing growing threat to coastal populations and property, thereby necessitating more millennial paleoclimatic reconstructions. This study presents a 3100-year record from Marou Lagoon, Emau Island, Vanuatu. Using coarse anomalies as the main proxy, 36 intense tropical cyclones are identified (similar to 1.2/century). Active TC phases occurred during 1550-1750 CE, 350-750 CE, and the 20th century, contrasting with a pre-Common Era quiescent period. Comparative analysis with five paleotempestological records across the tropical South Pacific reveals multidecadal-to-centennial TC variability is dominantly governed by ENSO-driven shifts in the South Pacific Convergence Zone (SPCZ). La Ni & ntilde;a-like periods cause southwestward SPCZ displacement and expansion, causing basin-wide enhanced cyclogenesis, while persistent strong El Ni & ntilde;o conditions collapse the SPCZ into a zonal structure near French Polynesia, concentrating activity at eastern sites and suppressing western ones. Weak-to-moderate El Ni & ntilde;o states exhibit stochastic dominance. Critically, sedimentological analysis reveals the most prominent event bed represents the cataclysmic Kuwae eruption in mid-15th. This deposit exhibits a five-stage sequence reflecting eruption progression, providing unprecedented resolution of eruption dynamics and tsunami impacts that surpass subaerial records. Typical diagnostic features of tsunami deposits (e.g., hummocky cross-stratification) are absent due to lagoon barrier/reef sheltering, and the heterogeneous density and shape of dominant pyroclastic materials (especially pumice) invalidate standard hydraulic grain-size models. This study demonstrates that sheltered lagoons preserve high-fidelity records of both extreme TCs and volcanogenic tsunamis. Such archives offer critical insights into regional climatic drivers and geohazard mechanisms but require environment-specific sedimentological frameworks, particularly where low-density pyroclastics dominate coarse fractions.

Abstract Satellite estimates of suspended‐sediment concentration (SSC) are widely used to understand ecosystem functions and resilience of coastal environments. Yet, they rely largely on fair‐weather observations. Combining continuous in situ time series with multi‐decadal satellite records and reanalysis products, we quantify a systematic fair‐weather bias that misrepresents coastal SSC in both magnitude and sign. Negative biases dominate where cloudy conditions coincide with energetic sediment mobilization, causing satellites to miss high‐SSC events and underestimate long‐term means by up to ∼20%. In contrast, positive biases arise where clear‐sky conditions align with elevated wave energy. Bias magnitude and direction are driven by local couplings among cloud cover, wave energy and sediment mobilization, enabling prediction and correction. Climate‐model projections indicate only modest changes in cloudiness over this century, suggesting the bias will persist in the future. Ignoring fair‐weather limitations distorts sediment budgets and biogeochemical fluxes, underscoring the need for bias‐aware algorithms in global coastal assessments.

Having been discovered in both the atmosphere-ocean and ocean-seabed boundary for decades, drag reduction in the bottom boundary layer (BBL) has aroused widely attention in both academic and engineering area and plays an essential role in the physical processes such as turbulent mixing and transport of mass, momentum and heat. Yet, no consistence has been reached regarding to its mechanism owing to the complexities and difficulties involved to test the proposed theoretical hypothesis practically. An explicit relationship has been put forward in this study that the bottom drag coefficient Cd increases with current velocity in the BBL until a tipping point, over which Cd decreases inversely with the increasing velocity and the associated incrementing suspended sediment concentration (SSC). The in situ observations over an intertidal flat approved such a relationship, particularly indicating that drag reduction occurred with turbulent diffusivity tending towards a constant as the current velocity reached 0.4–0.5 m s− 1 with a high SSC of 600–800 mg L− 1 under normal weather conditions with mild winds. This suggested that turbulence was damped as a result of the high SSC in the BBL, which serves as an isolated layer to damp/block the drag/turbulence. In contrast, the turbulence or mixing being induced by tough waves under windy conditions turned out to be so intense as to penetrate the whole water column to reach the seabed. More supports of the formation of the high-SSC-resultant drag-reduction layer came from the observations that drag reductions occurred commonly in a series of environments with various sediment dynamic conditions at varied critical current velocities, though. The causal linkage of “high velocity – high SSC – weakened turbulence on the bottom – drag reduction” provides a promising and tangible start point, from which the mechanism of drag reduction and thus turbulence weakening might be revealed hopefully.

The global debate on managing invasive Spartina alterniflora—eradication versus coexistence—remains unresolved. China’s nationwide eradication campaign risks ignoring regional ecological variations and potential benefits like sediment stabilization and carbon sequestration. We advocate adaptive, site-specific strategies that balance ecological risks with functional contributions, replacing one-size-fits-all eradication with evidence-based management.

Understanding sediment transport processes within tidal flats is crucial for developing effective land-ocean interaction management strategies. The cross-shore sediment transport on tidal flats induced by episodic events, such as wind-driven flow reversal (WDFR) and fluid mud (FM), is not sufficiently understood. This study focuses on the central Jiangsu tidal flat, where two field campaigns were conducted in the winter of 2021 and the summer of 2022. During the winter campaign, WDFR events were identified. During WDFR, the wind reversed the tide flow direction, resulting in significant cross-shore sediment fluxes. In summer, FM occurred frequently during tidal slack periods when current-induced bottom stress was low. The settling of sediment from the overlying fluid into the bottom layer plays a pivotal role in initiating FM events. These events resulted in substantial cross-shore sediment fluxes, exceeding the long-shore component. This study highlights the need to appropriately address the contributions of WDFR and FM to cross-shore sediment transport in similar coastal environments.

Suspended sediment concentration (SSC) significantly impacts the water quality and morphodynamics of coastal water bodies. The highly turbid zone coverage (HTZC), i.e., the area covered by high SSC, varies on different timescales in response to dynamic factors. However, quantitative analyses concerning periodic HTZC variation and its response to dynamic forcing remain insufficiently understood. Based on SSC data collected by the Geostationary Ocean Color Imager during 2012–2020, this study quantifies spatio-temporal HTZC variations for coastal and shelf waters of the Jiangsu coast, located in the southern Yellow Sea, China. The 100 mg/L SSC contour’s offshore distance (SCOD) is computed as an index to represent HTZC. The Lomb-Scargle periodogram detects strong seasonal and fortnightly cycles in the SCOD dataset. The phase-folding analysis reveals that the seasonal cycle dominates the HTZC variations and is modulated by significant wave height. Seasonally, the cross-shore expansion of HTZC is 41–65 km. During the fortnightly tidal cycle, the interaction between the two principal semi-diurnal tidal constituents, M2 and S2, leads to a periodic change in the tidal range. This fortnightly change causes variations in the HTZC, with the cross-shore expansion fluctuating between 11 and 20 km. Along the Jiangsu coast, the cross-shore expansion range of HTZC increases from the north to the south, suggesting complex 2D sediment dynamics. Moreover, a time lag exists between the HTZC variation and dynamic factors, with an order of 1–2 months and less than one day on seasonal and fortnightly timescales, respectively. Our findings highlight features in SSC variation that are commonly ignored in traditional visual interpretation and the advantage of the proposed methodology in revealing mechanisms for suspended sediment variation.

Fluid mud (FM), a near-bed layer of high suspended sediment concentration, can be transported offshore in the form of gravity flow under the support of waves and currents, playing a critical role in sediment transport and geomorphological evolution. Wave-supported FM has been extensively studied, whereas research on the transport processes of tide-dominated FM remains insufficient. Here, we conducted four field campaigns on the central Jiangsu tidal flats, revealing that FM events occur frequently during winter, spring, and summer. The FM can be transported offshore as a gravity flow, with gravity-driven velocities ranging from 0.01 to 0.06 m s-1. In situ measurements combined with theoretical modeling indicate that FM generation and transport in the central Jiangsu tidal flats are tide-dominated. On a tidal cycle scale, the tide-dominated FM pattern was divided into four stages. Stage I: during flood tides, fine sediment is transported landward from offshore waters to the tidal flat; Stage II: sediment settles from the overlying water column to the bed during flood slack tide, leading to the formation of FM; Stage III: ebb currents facilitate the downslope transport of FM as a gravity flow; Stage IV: ebb currents disperse the sediment within the FM layer, ultimately causing its dissipation. The tide-dominated FM presented in this study significantly differs from wave-supported FM, advancing the understanding of sediment dynamics on tidal flats and underscoring the importance of observing tide-dominated FM in similar coastal environments globally.

Tropical cyclones (TCs) are among the most devastating hazards, causing damage and fatalities in coastal communities. Our understanding of the climatic factors that modulate tropical cyclone activity is impeded by the short historical records and the scarcity of paleoclimatic reconstructions, with a notable dearth of data in the Southern Pacific region. In this paper, we present a sedimentary record from a coastal karst basin in Bay of Islands, Vanua Balavu, Fiji to provide insight into the regional intense TC activity over the past two millennia. A total of 53 intense storm events captured by this site are identified using coarse fraction (>63 mu m) anomalies in sediment core retrieved from the basin, yielding an overall average event frequency of 2.6 events/century. Multiple centennial-scale quiescent periods (from 200 to 300 CE and 1000 to 1150 CE) and active periods (namely from 350 to 750 CE, 900 to 1000 CE, 1150 to 1250 CE, 1400 to 1500 CE, and 1650 to 2017 CE) are found in the reconstruction, and the most active interval spans from 1650 to 1800 CE at 4.5 events/century. A comparison between existing paleostorm records and climate forcing indices suggests that the southward displacement of the South Pacific Convergence Zone (SPCZ) during the Little Ice Age with more La Nina events is responsible for the basin-wide increasing of tropical cyclone activity in the South Pacific. Decline of TC occurrence in the western SP during the Medieval Climate Anomaly is attributed to the northward movement of SPCZ. However, event frequency peaks of the latitudinally aligned sites in the South Pacific exhibit a certain degree of asynchrony, necessitating the acquisition of more detailed high-resolution paleostorm reconstructions within these basins and corroborative evidence from global climate models.

Anthropogenic climate warming is predicted to increase the intensity of global tropical cyclones (TCs) on decadal timescales, known as the ‘temperature-TC intensity’ paradigm. However, no proxy is currently available to directly quantify TC intensity in the northwestern Pacific region over centennial to millennial timescales. Here, we reconstruct the intensity of past TCs inferred from event-beds detected in two sedimentary systems in eastern China spanning approximately 1910 to 645 yr BP using an instrumental-calibrated technique, thereby encompassing a sufficiently wide range of temperatures to test the paradigm in the time domain. Intriguingly, our two intensity indices, based on flooding depth and wind speed, provide the initial quantitative evidence that TC intensity in eastern China has been anomalously weak since around 1485 ± 45 yr BP, with a reduction of approximately 30 ± 8% in intensity, despite no concurrent temperature shift. This reduction appears to have been pre-conditioned by a combined influence of a weaker El Niño-Southern Oscillation, a stronger Atlantic Meridional Overturning Circulation, and an increased level of Saharan dust. We suggest that the magnitudes of these factors may have crossed a tipping point and have not reverted to their pre-shift levels since that time, resulting in their impact on TC intensity exceeding that of temperature by triggering changes in the oceanic and atmospheric state within the tropical Pacific region where TCs originate.

Deltas are threatened by erosion due to climate change and reduced sediment supply, but their response to these changes remains poorly quantified. We investigate the abandoned Yellow River delta that has transitioned from rapid growth to ongoing deterioration due to a river avulsion removing the sediment supply. Integrating bathymetric data, process observations, and sediment transport modeling, we find that while the subaerial delta was stabilized by engineering measures, the subaqueous delta continued to erode due to intensified storms, losing 39% of its mass deposited before the avulsion. Long-term observations show that winter storms initiate scouring of the subaqueous delta, contributing up to 70% of seabed erosion. We then analyze 108 global deltas to assess subaqueous delta erosion risks and identify 17 deltas facing similar situations of sediment decline and storm intensification during the past 40 years. Our findings suggest that subaqueous delta erosion must be integrated into delta sustainability evaluations.

Unvegetated tidal mudflats are typically dissected by networks of channels that arguably exert a prominent role in the ecomorphodynamic evolution of these environments. However, the intricate processes of tidal flows propagating through tidal channels and across mudflat platforms remain inadequately understood, particularly concerning the localized hydrodynamics occurring at the interface between channels and mudflats. In this study, we report the results of in-situ hydroacoustic measurements carried out in the macrotidal Yangkou tidal flat (Jiangsu coast, China), where we synchronously assessed the three-dimensional flow fields and suspended sediment concentrations at three distinct sites—namely, the channel thalweg, channel bank, and distant mudflat platform—over the course of 8 semidiurnal tidal cycles. Whereas mudflat areas typically exhibit an overall flood dominance, tidal channels feature ebb-dominated flows and higher concentrations of suspended sediments. In contrast to vegetated tidal settings, where in-channel flow velocities are systematically higher than across the frictionally dominated intertidal plains, hydrodynamics of bare mudflat systems is controlled by flow inertia and sheet-flow conditions whenever water levels exceed the channel bankfull threshold. At bankfull, pronounced changes in tidal flow orientation are observed, which critically enhance both cross-sectional velocities within the channel and bottom shear stress near channel banks. Our results support earlier suggestions that the hydrodynamics of bare mudflat systems are strongly influenced by the interactions between in-channel and overbank flows. Therefore, we propose that future modeling works related to accurately reproducing morphodynamic processes occurring in unvegetated coastal environments should take into account the localized three-dimensional flow interactions near the channel-flat interface.

Coastal areas dominated by major rivers are one of the largest carbon sinks worldwide. However, the factors controlling the generation of primary microbial gas therein are still poorly understood. Here, the geochemical characteristics of natural gas and organic-rich muds (organofacies) from the upper Quaternary delta-shelf-estuary system that links the large Changjiang and the adjacent smaller Qiantang River are investigated to understand the key factors controlling the preservation of organic matter and its gas-generation potential. Muds from the floodplain, salt marshes of the paleoestuary, and distal delta front and prodelta of the paleodelta act as efficient gas-generation organofacies. Organic matter in them consists mainly of terrestrial higher land plants and is now undergoing methanogenesis. These organofacies were deposited during transgression accompanied by an intensified Asian summer monsoon that resulted in a greater delivery of organic matter and a higher preservation efficiency and reactivity of organic carbon compared to the subsequent regressive organofacies. Notably, the factors influencing the gas-generation potential in this system varied in a proximal-distal direction. Energetic physical processes acting during the accumulation of the distal delta-front and prodeltaic muds resulted in a longer oxygen exposure time, repetitive redox oscillations, and replenishment of labile marine organic matter, which accelerated microbial degradation before methanogenesis began. Our findings thus suggest that the most efficient gas-generation organofacies in a large river-dominated delta-shelf-estuary system are those deposited in a proximal environment, especially under conditions of rapid aggradation during times of sea-level rise and increased runoff associated with a warm climate.

Waves and currents are responsible for sediment movement around and off coral reefs, affecting reef growth at both geological and modern timescales. Given the wide distribution and limited hydrodynamic information of reefs in the South China Sea, we carried out observations on tidal-cycle hydrodynamics in the Nansha Islands with tripod stationary instruments on the seafloor in order to fill the gap in our understanding of these processes. It was found that the magnitudes of near-bed orbital velocity were comparable with that of the mean tidal current, despite generally calm wave conditions. Waves dominated the combined wave-current skin-friction shear velocities acting on reef sediment, which were significantly higher than those generated by currents alone. Due to the large physical roughness of reef, drag coefficient and hydrodynamic roughness length estimated from logarithmic velocity profiles were two orders of magnitude higher than that in macro-tidal-estuary or inner shelf areas covered with siliceous muds or sands. The combined sinusoidal wave and asymmetric tidal current, along with the physical reef roughness, shaped velocity profile structures in the bottom boundary layer, which exhibited a logarithmic profile during the flood tide and a potential flow during the ebb. In absence of wave breaking, strong turbulence dissipation was observed across the rough reef, promoting strong mixing of water, which is crucial for delivering nutrients for coral growth. These findings imply the need to consider the unique characteristics of rough reef structure and combined effects of waves and currents to model the hydrodynamics in reef environment correctly. This understanding is critical for predicting energy and material transport in reef environments, which is essential for maintaining healthy coral ecosystems, and opens new paths for managing and preserving coral reefs in the face of environmental change.

A better understanding of how the vertical profile of suspended sediment concentration (SSC) is influenced by multiple factors is required to better understand the hydrodynamics and sediment dynamics of estuaries. Based on in situ measurements obtained during a spring-neap cycle in the estuarine turbidity maximum (ETM) of the Changjiang Estuary in January 2015, we investigate the intratidal evolution and controlling mechanisms of SSC profiles. The results show that during the dry season the ETM was characterized by periodic salinity stratification, low SSC, suspended sediments with reduced grain size, and low sediment availability for resuspension. Three types of SSC profiles were identified, i.e., profiles with two-layer, linear and quasi-uniform structures. The shapes of the SSC profiles varied with time and generally deviated from the equilibrium profile due to the highly variable hydrodynamic and sediment conditions. During ebbs of spring and intermediate tides, the SSC profiles generally evolved from a two-layer to a quasi-uniform structure via a linear structure. Analysis shows that the linear and quasi-uniform profiles occurred under the combination of vertically well-mixed salinity, negligible resuspension, low settling velocity associated with very fine grains, and shallow water depth. Such an evolution pattern can be well reproduced using a one-dimensional numerical model, with the diffusion coefficient being parabolically distributed in the water column and the four factors mentioned above. The findings reported here imply a high variability of SSC profiles in estuarine environments.

The evolution of tidal flat wetland is closely related to the ecological security of the Yangtze estuary. Based on the method of multi-sourced data analysis, this paper aims at revealing the evolution process of the tidal flat of Chongming Island on a centennial scale and the phases of reclamation intensity change over the last thousand years, and discussing the driving human and environmental factors. The results of this study show that the landscape transformation from natural wetland to reclamation region of Chongming Island has experienced four stages: early development during the seventh to eighteenth centuries, slow reclamation from the mid-eighteenth to the mid-twentieth centuries, rapid reclamation in the 1960–1990s, and limited reclamation in the last 20 years. The proportion of natural tidal flat above 0 m in elevation (Wusong Datum) outside the sea dikes in total land area of this island dropped from about 50

Green sea dykes, also known as ecosystem-based sea dykes, represent a novel type of coastal defense consisting of both traditional structural engineering and coastal ecosystems, designed to cope with the future trends of sea level rise and intensified storms. Here we focus on the mid-latitude mud coasts (eastern China in particular), which face the most prominent risks of storm surge, storm-induced giant waves, and shoreline erosion, and summarizes the scientific basis of green sea dykes and the current status of engineering practices. We show that the basic mechanisms of nearshore wave energy dissipation include bottom friction, sediment transport, and form drag. These explain the wave damping capacity of oyster reefs and salt marshes on mud coasts. In tidal flat environments, oyster growth increases frictional resistance and even causes wave breaking; the resuspension and transport of fine-grained sediments on salt marsh beds and the movement or resistance to hydrodynamic forcing of salt marsh vegetation stems effectively dissipate wave kinetic energy, and their efficiency increases with the elevation of the bed surface. Based on the wave damping capacity of oyster reefs and salt marshes on mud coasts, ecosystem-based sea dykes are being built in combination with traditional structured sea dykes. By utilizing natural tidal flats outside the dykes or implementing artificial modification projects, a certain scale of salt marshes and/or oyster reefs can be maintained, which serve to protect the sea dykes and enhance their wave resistance functions. From the perspective of system optimization, it is necessary to further improve the efficiency and sustainability of green sea dykes under constraints such as regional environment characteristics, ecosystem health, investment capacity, and ecological resilience. Related scientific issues include the theorization of the wave damping process of salt marshes, the niche and scale control of oyster reef and salt marsh ecosystems, the establishment of engineering standards and the design of the optimal form of sea dykes.

The tidal sand ridges in the southwestern Yellow Sea are unique in the world characterized by radial morphology, significantly higher elevation above low water level, and fine-grained sediment deposits. Using a numerical model, we examined the morphological evolution of the radial sand ridge system from 1979 to 2059. The model was calibrated and validated using hydrodynamic and sediment transport data, seabed topography, and grain size distributions from sediment cores. Our findings reveal that tidal force is the primary driver of the ridges' evolution, and they will continue to expand in response to sediment supply from the abandoned Yellow River delta and the Yangtze delta, with the latter contributing more. The ridges are mainly composed of fine sand and mud, with finer sediment dominating in the north and coarser sediment dominating in the south. Wind and waves sort sediment, depositing sand on ridges and fine sediment in channels, leading to erosion of higher elevated lands and filling of deep channels. Sea-level rise increases regional erosion, which has a large impact on areas with complex morphology. The 500-year morphological evolution of sand ridge systems, simulated through an idealized model, illustrated that the formation of radial sand ridges is dependent upon specific conditions such as the radial tidal regime, sufficient sediment sand, and the presence of cohesive sediment. This study deepens our understanding of the long-term prediction of coastal morphology and interpretation of seabed stratigraphy, and offers insights for effective coastal management.

There are concerns that intense tropical cyclones (TCs) are expected to become more frequent and powerful in warming climates. However, the long-term trend of TC activities, the spatiotemporal variability of such trends across different latitudes of the Western North Pacific (WNP), and the drives of the variability remain unclear. Here, we present a reconstruction of a 1600-year paleo-TCs activity using a sediment core taken in Li’an Lagoon located in southeastern Hainan Island, South China Sea. We used muti-dating methods (210Pb and AMS 14C dating) for age control and muti-proxy analysis (XRF geochemical element scanning and grain size analysis) for identifying periods of frequent intense TC activities. Based on an updated compilation of basin-wide paleo-TC records, we confirm that there exists a seesaw pattern of intense TC frequency between low and middle latitude in the WNP. Comparing with global and regional paleoclimate proxies, we propose that the basin-wide latitudinal TC activity variation in the WNP can be linked to the migration of Western Pacific Subtropical High (WPSH) and its associated high-latitude forcings (e.g., NAO) and low-latitude internal variability forcings (i.e., El Niño Southern Oscillation, sea surface temperature (SST)). More intense TC will occur at low latitudes in the future, though with less frequency.

AbstractAmidst escalating global changes and heightened human activities, tidal flats worldwide are facing a transition from accretion to erosion. In order to quantify the growth pattern of tidal flats and its response to changes in multiple external driving factors, here we established a geometric model, in combination with field surveys, to study the historical behavior (1127–1990) and future trends (2100) of a typical tidal flat system on the Jiangsu coast, China. The results indicate that sediment supply and relative sea level change determine the accretion‐erosion status and the eventual morphological pattern of the tidal flat. If these two factors remain stable and sediment supply can compensate for the increase in sediment accommodation space caused by sea level change, the tidal flat will continue to accrete until its growth limit is reached. Otherwise, tidal flats will face erosion risk, which can be further enhanced by localized land subsidence. Furthermore, changes in the coastal profile show a trend of progressive retreating, with the lower part of intertidal zone being eroded more rapidly than its upper part. This demonstrates that the sediment supply has been reduced to such a level that the present “in‐transition” evolution pattern of central Jiangsu coast will soon be replaced by severe inundation and erosion. Hence, we advocate for a comprehensive, multi‐tiered coastal protection strategy focusing on wave suppression, enhancement of sediment retention, and increased storm resistance. This strategy underscores fundamental underlying mechanisms of the accretion‐erosion transition, which is crucial for the future safeguarding of coastal wetlands globally.