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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. [ABSTRACT FROM AUTHOR]

Two field campaigns in Dafeng and Jianggang were organized to compare spatial variations of hydrodynamic characteristics and sediment transport patterns on intertidal flats of different types with distinct human interferences along middle Jiangsu coast, China. The major contributors to the different patterns of sediment dynamics between the two tidal flats were offshore tidal current field and human interference. Offshore tidal force provide the basic setup of tidal current patterns on tidal flats, which is then modified by local morphology. Seawalls parallel to coast reduce tidal flat width, forcing tidal energy to dissipate within a shorter distance and thus influencing tidal flat morphology. Seawalls vertical to coast and major tidal current significantly reduce tidal current speed, which favors sediment deposition on tidal flats. Two seawalls built on both sides of the observational tidal flat profile caused much reduced current speed at Dafeng tidal flat, comparing to the offshore station. Being exposed to offshore radial tidal currents, hydrodynamics at Jianggang was much stronger than that at Dafeng. Residual currents at both areas showed net landward transport at the lower flat and net seaward transport at the upper flat, in favor to sediment accumulation at the middle flat. Sediment flux over tidal cycles showed net landward sediment transport at Dafeng, and net seaward transport at Jianggang, consistent with the convex-up accretion-dominated profile observed at Dafenge, and concave-up erosion-dominated profile observed at Jianggang. The instantaneons sediment flux changed significantly due to variations in velocity and sediment concentrations, but these terms counteracted with each other within tidal cycles, leading to the dominant role of Eulerian flux in determining net sediment flux over tidal cycles. [ABSTRACT FROM AUTHOR]

Many of the world's mega‐river deltas are currently degrading due to a shortage of sediment supply feeding their construction. Here, using bathymetric and seismic data, the geomorphological/sedimentary evolution on and near a recently abandoned (1976 CE) Huanghe delta lobe is investigated. The delta front (DF) exhibits a lobe switching‐controlled pattern of progradation followed by mass wasting on the upper front, and redeposition at the seaward toe of the front; the latter process appears to be associated with sediment failure and contributes to delta degradation. During degradation, mass wasting‐driven deepening of the upper DF was initially up to ∼0.86 m/yr from 1978 to 1985 CE, slowed to ∼0.12 m/yr from 1985 to 2000 CE, and has been negligible since. In either prograding or degrading, gravity‐driven sedimentation results in a downlapping seismic unit superimposed on the bottomset of an earlier converging clinoform, with numerous buried/exposed submarine channels at its seaward terminus. These units exhibit a shore‐parallel elongated and continuous geometry, suggesting lateral spreading, which we attribute to alongshore hydrodynamic (especially tidal) remolding. Our findings underscore the significance of sediment gravity flows in the evolution of a degrading delta front, particularly as a principal mechanism of downslope mass transport that gentled (halving the gradient) and reshaped the DF over time. Gravity‐driven accumulations sourced from upper DF collapse can account for ∼1/3 of the previous DF and laterally extend over an unexpectedly large area (∼60‐km long) through hydrodynamic redistribution. Both processes should be considered for modeling other subaqueous deltas. Plain Language Summary: At underwater river deltas, gravity can cause sediment to flow downhill, which alters geological, chemical and biological processes and poses hazards to human infrastructures. Due to changes in the river on land, delta lobes can switch from one part of the coast to another, which alters the conditions that cause gravity flows. Our understanding of how the gravity flows respond to channel switching is very limited. This paper reports on seven repeated bathymetric surveys and a seismic survey collected at a recently abandoned Huanghe (Yellow River) delta lobe over four decades. We found that the gravity flows take place both when the delta lobe is active and after abandonment. Gravity flows make the underwater delta slope gentler by taking away sediment from the upper slope and moving it to the lower slope. This happened rapidly within the first decade after abandonment and slowed down thereafter. The redeposited sediments were spread out parallel to the shoreline by ocean currents. Our study highlights the significance of gravity flows in reshaping the underwater delta slope after abandonment, which may provide implications for enabling better societal response to the global deltaic degradation caused by reductions in river sediments transported to oceans. Key Points: Sediment gravity flows reshape and gentle the abandoned delta front (DF) slope over timeGravity flows dominated delta degradation via mass wasting on the upper DF and accumulating on the lower frontGravity‐driven sedimentation laterally spread over decadal span likely due to hydrodynamic redistribution [ABSTRACT FROM AUTHOR]

Heavy metal pollutants in sediment greatly impact the estuarine environment and ecosystems, increasingly influenced by anthropogenic perturbations. Here, we examined the surface sediments of the Modaomen estuary in 2003, 2015, and 2021 to understand how human-induced changes influence the fate of heavy metals in the estuary's sediments. The potential ecological risk index (RI) suggests Cd should be the priority pollutant for environmental pollution control due to its high toxicity coefficient. In each sampling period, two main sources were identified through normalized heavy metals and PCA-MLR: natural and mixed anthropogenic sources (agricultural, industrial, and traffic activities), reflecting an increase in heavy metals pollution, later mitigated by successful environmental protection measures. Moreover, anthropogenic activities have not only impacted the sources discharge of heavy metals but have also influenced their spatial and temporal distribution through factors such as land reclamation, leading to sediment coarsening and reduced heavy metal content in specific areas. • Spatial and temporal variation of heavy metals in sediment of the Modaomen estuary were investigated. • The potential ecological risk index (RI) suggests Cd should be the priority pollutant for environmental management. • Two main sources: background values and human-related activities (agricultural, industrial, and traffic) are identified. • Localized areas of sediment coarsening and increased heavy metal content are closely related to human activities. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In many estuaries, current velocity, suspended sediment concentration (SSC), and water depth have notable flood-ebb asymmetries in magnitude or duration. Such asymmetries can make marked impact on estuarine sediment transport and require in-depth analysis of the underlying mechanism. Based on field measurement over a neap-spring cycle in the Changjiang Estuary in January 2015, net suspended sediment transport modulated by multiple asymmetries were analyzed. The results showed that there were notable flood-ebb asymmetries in the current velocity, current duration, SSC, and water depth. The former two were ebb-dominated, while the latter two were flood-dominated. The net suspended sediment transport was landward and exhibited remarkable tidal fluctuation in magnitude. Ebb-dominated velocity and duration asymmetries favored seaward transport, but flood-dominated SSC and water depth asymmetries promoted landward transport. The relative contribution of these asymmetries to the net sediment transport was different and closely related to their asymmetry intensities. The magnitude of net sediment transport was highly modulated by the combined effects of these asymmetries. Further analyses revealed that the ebb-dominated asymmetries in velocity and duration were controlled by river flow, tidal wave deformation, and compensation flow of Stokes drift. The flood-dominated water depth asymmetry was related to the progressive tidal wave. The flood-dominated SSC asymmetry was mainly attributed to the lower SSCs in early ebbs, notable sediment supply limitation in late ebbs, wind wave height asymmetry, and gravitational circulation. Additionally, it is worth noting that the progressive tidal wave can yield strong current asymmetry, notable water depth asymmetry, and obvious wind wave height asymmetry during strong wind periods. • Multiple important flood-ebb asymmetries coexist in a tidal cycle in natural estuaries. • Net sediment transports are highly modulated by the combination of multiple asymmetries. • Flood- and ebb-dominated asymmetries play opposite roles in net sediment transport. • Compensation flow of Stokes drift has important influence on current asymmetry. • Variation in sediment availability for resuspension can yield notable SSC asymmetry. [ABSTRACT FROM AUTHOR]

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 10 3 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 210 Pb and 137 Cs radioisotope geochronologies, and this leads to accumulation rates of 10 0 –10 1 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. [ABSTRACT FROM AUTHOR]

• It is essential to utilize diverse sources of observational data for the validation of long-term morphological simulations. • The multiple underlayer settings of the Delft3D model enable its acquisition of decadal bed layer stratigraphy. • Wind and waves sort sediment based on grain size, smoothing out morphological changes induced by tides. • Sea level rise significantly influences regions characterized by intricate morphological features. • Radial sand ridges form under a radial regime with low tidal ellipticity, abundant sand, and minor cohesive sediment. 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. [ABSTRACT FROM AUTHOR]

Abstract: 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 kgm−1 on the upper intertidal flat to 103–4 kgm−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. [Copyright &y& Elsevier]

The pattern of sediment exchange between the Yangtze River Estuary and the southern Yellow Sea constitutes a crucial yet contentious line of scientific inquiry. This conundrum hampers our comprehensive understanding to predict the future morphological evolution of the radial sand ridges and Jiangsu tidal flats. This study investigates various processes, including tides, wind and waves, to identify the dominant factor controlling sediment exchange between the southern Yellow Sea and the Yangtze River Estuary under storm conditions, using a validated numerical model. Our results show that tide is the dominant force controlling hydrodynamics and sediment transport between the two systems. Tidally induced residual currents and sediment fluxes exhibit a consistent northwesterly trajectory, from the Yangtze River Estuary to the southern Yellow Sea. Conversely, the contributions of wind and wave-induced currents under normal wind conditions (Beaufort Scale 3, 3.4 m s−1) adjust net sediment flux by less than 10 and 46–68%, respectively. In particular, the influence of wind and waves on sediment transport varies significantly with wind direction. While southerly or southeasterly winds amplify the tide-induced northwestward sediment transport, northerly or northeasterly winds curtail it. Under strong winter storm conditions, however, the perturbation caused by northerly winds and waves supersedes the influence of tides in controlling sediment transport, leading to a net residual current and sediment flux oriented southeastwards. • Tide is the dominant force controlling residual currents and sediment transport. • Normally, sediment transport from the Yangtze River Estuary to the southern Yellow Sea. • Northern strong winds and waves revert sediment transport southeastward. • Sediment transports in winter storms are more active nearshore than offshore. [ABSTRACT FROM AUTHOR]

Marginal seas are major sinks of fluvial sediments. Identifying the sources and quantifying the contributions of different sources are essential to studying the transport and distribution of terrestrial sediments on the shelves of marginal seas. The South Yellow Sea (SYS) to the east of the Asian continent receives sediment discharged by the Huanghe (Yellow) and Changjiang (Yangtze) rivers, and it is a favorable location for studying the provenance of marginal sea deposits. In this study, we tested the geochemistry of amphibole in different grain-size fractions (very fine sand and coarse silt) of the SYS sediments, and compared this geochemistry with those of fractions in the Huanghe River (HH) and the Changjiang River (CJ) to quantify the terrestrial provenance of the sea. The results show that for the very fine sand fraction, Zr, Be, Ga, Ge, Nb, Cd, and Hf in amphibole can be used as indicators to discriminate between the HH and CJ sediments. For the coarse silt fraction, the trace elements of Ba, Zr, Li, Rb, and Hf in amphibole can be used to distinguish the sediment of the two rivers. The mean contribution of the HH sediment to the very fine sand and coarse silt fractions of SYS is 45.0% ± 6.7% and 57.6% ± 5.9%, respectively, compared with 54.8% ± 6.7% and 42.4% ± 5.9% for the CJ, respectively. Provenance analysis of amphibole reveals spatial variation in the supply of terrigenous sediment to different parts of the SYS: the nearshore area of the Shandong Peninsula and the sea areas adjacent to the abandoned Huanghe River (FHH) estuary are supplied mainly by the HH, whereas the South and eastern parts of the SYS are supplied predominantly by the CJ. In the SYS sediment of very fine sand, a line extending from 34°N, 121°E to 35°N, 123°E demarcates the boundary between areas supplied by the HH (north) and the CJ (south), while coarse silt has a longer transport distance and higher mixing degree compared with very fine sand. This study demonstrates that quantitative analysis of the trace-element geochemistry of detrital amphibole can discriminate terrigenous sediment provenance in the mixed area of the marginal sea. • Provenance analysis new strategy: the geochemical elements of amphibole • The tracer indexes are selected to distinguish the Yellow and Yangtze Rivers • Quantification and transport diffusion of fluvial materials in marginal sea [ABSTRACT FROM AUTHOR]

Internal waves are widespread in oceans and play an important role in mixing. In this study, we observed some oscillations of pycnoclines that are thought to be caused by internal waves by analyzing the vertical and temporal variations of current speed and density during ebbs of the neap tides in the south channel of the Yangtze River (Changjiang) Estuary. These oscillations have an amplitude of 1–2 m and a duration of 2–3 h. To explore the mechanism of this phenomenon, topographic features of the seabed were recorded, and a huge sandbar was observed at the place where the oscillations occurred. Therefore, we infer the oscillations in the south channel were caused by the process that the stratified water flowed over the sandbar which induced internal hydraulics and led to the excitation of internal waves. Froude number was calculated according to the internal-hydraulics equations and the results verified our hypothesis that it was the interactions between the stratified water and rapidly changing topography that triggered internal hydraulics. Internal waves caused upward-directed water movement, which influenced the vertical transport of sediment and the vertical distribution of the suspended sediment. Internal waves also increased the mass diffusivity coefficient (Kz) at the interface of internal waves. • Internal waves were found at the south channel of the Yangtze River estuary. • The internal waves were triggered by stratified water over the sandbars. • The internal waves increased the seabed sediment entrainment and resuspension. [ABSTRACT FROM AUTHOR]