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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]

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]

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]