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• Newly eroded sediment leads to decrease in resuspension age. • In areas where advection dominates, sediment suspension persists for a longer duration. • The vertical distribution of resuspension age is shaped by stratification. The concept of resuspension age was employed to assess the timescales of vertical sediment transport in the Changjiang Estuary (CJE) and its adjoining areas. Resuspension age refers to the duration elapsed since particles last made contact with the seabed. Energetic tidal current in the study area results in a significant amount of sediment being eroded into the water during spring tides, causing the resuspension age to consistently remain lower during spring tides compared to neap tides. Spatially, consistently low resuspension ages occur in the North Branch and turbidity maximum because suspended sediment in these areas primarily originates from local erosion. While advective sediment dominates in the HZB, leading to highest resuspension age there. During the wet season, the South Branch experiences an average resuspension age surpassing 10 days, primarily due to particles predominantly transported from the river. Sediments deposited on the seabed during the wet season become the primary source of suspended sediment after being eroded into the water column during the dry season, leading to a reduction in resuspension age. Likewise, the decrease in resuspension age in the Subei Coast during the dry season is closely associated with the appearance of sediment mass advected by the northern branch plume. When stratification occurs, the upward diffusion of sediment is impeded, constraining newly eroded sediment close to the bed. As a result, these sediments exhibit lower ages than those in the upper water column, where sediment transport is primarily dominated by horizontal advection. [ABSTRACT FROM AUTHOR]

The morphodynamics of inlets and ebb-tidal deltas often exhibit cyclic behaviour, controlled by wave-tide interactions and the influence of these on sediment-bypassing processes. Variation in the form and timing of inlet dynamics can occur in response to changes in the key sediment transport processes and sediment supply, and particularly the relative dominance of differential wave direction driving alongshore sediment transport. This paper investigates the morphodynamics of the Deben ebb-tidal delta through multivariate analysis of multi-annual bathymetry and metocean parameters covering the period 1991 to 2023. Results show that sediment bypassing and associated morphological change in this ebb-tidal delta follows variable behaviour mid-cycle, leading to change in the morphology and configuration of the ebb-delta from cycle to cycle. In the most recent cycle (since 2003), delta volume followed a stepwise growth driven by sediment influx periods, evident primarily in the updrift shoal, followed by cross-shore redistribution. Interannual variability in wave climate was clustered into four groups that capture the relative balance between the dominant NE and SW forcing. Sediment influx periods are mostly associated with subdued SW and increased NE forcing, but the summer and winter NE wave climate play different roles on the up- and downdrift, and upper and lower components of the ebb-delta system. Tide level has increased in the last 33 years, which is strongly correlated with ebb-delta volume increase. Wave and tide conditions have discrete and independent effects on the ebb-tidal delta, whereas wave-tide interactions do not significantly change these underlying influences. • Differential within-cycle behaviour was found in multidecadal bypassing cycles. • Ebb-tidal delta volume has increased via a stepwise growth. • Sediment influx is related to reduced SW and increased NE wave forcing. • Wave and tide conditions have independent effects on the ebb-tidal delta. [ABSTRACT FROM AUTHOR]

• Fine particles control the exchangeable silica mobilization in the river. • Current sediment in the estuary contain less reactive silica than historical level. • Increased fine particle fraction affect silica sediment with forms variation in estuary. Changes in riverine sediment transport are an important part of land-sea geochemical cycling and further impact geochemical element fluxes in turbid rivers. However, as a vital nutrient element supporting primary productivity, silica mobilization from drainage in turbid rivers is overlooked. The turbid Yellow River has a strong ability to adsorb reactive silica, thereby exerting a substantial impact on the estuarine deposition of silica. Through an integration of monitoring databases, field sampling and historical hydrological data, we concluded that riverine fine particles control the exchangeable silica in the river and its estuary under soil erosion. Indoor simulation further revealed that the adsorbed content of exchangeable silica (ex-Si) in fine sediment constituted 35 % of total sediment matter. In addition, the transport of phosphorus and ex-Si was jointly regulated by fine sediment in global fluvial sediment transport, thereby exerting additional influence on the trophic structure of estuarine ecosystems. Against the backdrop of sediment budget deficit in the estuary, the heightened content of fine particles is depleting the silica storage from estuarine sediments. [Display omitted] [ABSTRACT FROM AUTHOR]

Estuarine dams are constructed in estuaries for reasons such as freshwater, flood control, and navigation. By changing tidal and river properties, estuarine dams can change the circulation and sediment transport in estuaries. Previous studies have investigated the along-channel changes in flow and sediment transport, however across-channel changes due to an estuarine dam are not well understood. To increase our understanding, this study analyzed an idealized estuary using the COAWST numerical modeling system. Models of strongly stratified, partially mixed, periodically stratified, and well-mixed estuaries were run for one year. Then, the models were subject to the construction of an estuarine dam and run for another year. For each estuarine type, scenarios with an estuarine dam at x = 20, 55, and 90 km from the mouth and freshwater discharge intervals of Δt = 0.5, 3, and 7 days were investigated, and the scenarios were compared. The results indicated that the river-dominated and tide-dominated estuaries behaved differently. In river-dominated estuaries, the residual circulation tended to be inflow in the channel and outflow over the shoals due to the estuarine exchange flow, and the secondary circulation was bottom divergent due to differential advection. The exchange flow and secondary circulation were found to weaken with longer discharge interval. The sediment fluxes in the channel were dominant, and, despite the weaker exchange flow, were found to be greater and directed seaward due to strong dam discharge for long discharge intervals. In tide-dominated estuaries, the residual circulation tended to be inflow over the shoals and outflow in the channel due to the tide-induced circulation, and the secondary flow was bottom convergent due to differential advection resulting from Stokes return flow. As the estuarine dam was located nearer to the mouth, the tide-induced circulation was replaced with exchange flow, and the secondary flow became bottom divergent. The sediment fluxes in the channel were dominant in this case as well, and were determined by tidal asymmetry from the interaction of the tides with the residual current, becoming directed landward as the seaward tide-induced circulation decreased with the estuarine dam near the mouth. With respect to bed level change, gradients in along-channel sediment fluxes were found to contribute most to bed level change near the mouth and estuarine dam, whereas gradients in across-channel sediment fluxes contributed most to deposition on the estuarine shoals. This study demonstrates that estuarine dam location and discharge interval can alter estuarine transverse variability and improves our understanding compared to previous analyses based solely on along-channel processes. • River-dominated estuaries exhibited inflow in channel, outflow over shoals. • Tide-dominated estuaries exhibited inflow over shoals, outflow in channel. • Estuarine dams near mouth with large discharge interval reduced these circulations. • Tide-dominated estuaries switched from tide-induced circulation to exchange flow. • Lateral sediment fluxes contributed to shoal deposition. [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]

• (234U/238U) were analyzed for the Changjiang estuary sediments. • Catchment erosion was revealed by sediment comminution age. • Climate changes play a leading role in catchment erosion. • Human activities changed the natural erosion processes since the late Holocene. Catchment erosion not only plays an important role on landscape evolution, but also determines the exposure of rock debris in the regolith profile, and therefore impacts the chemical weathering and associated CO 2 absorption and ultimately global climate. But climate also modulates erosion and complex interaction loops, especially at large scale can hamper a clear understanding of these processes. Here we mainly use Uranium-series Comminution Age (t com) of core sediments from Changjiang estuary to reconstruct the catchment erosion, and the relationship with climate changes and human activities during the last 14 kyrs. Since 14 ka, the sea level has been rising continuously until current highstand system tract in the East China Sea where the Changjiang estuary is located. The sediments in the Changjiang estuary mainly come from the upper reaches erosion zone during this period, but the sediment supply from the middle and lower reaches has gradually increased since the Late Holocene. The period can be divided into two modes. The first mode happens from the Bølling-Allerød period to the Mid-Holocene when the sea level rose rapidly, and the t com of sediment recorded the residence time in the upper reaches erosion zone, with a periodic pattern of t com in a good agreement with climate changes. During the cold and dry periods, the longer t com (average 460 kyrs) is consistent with less precipitation resulting in shallower erosion with transported material mainly coming from the soil with longer residence time. In contrast, during the warm and wet periods, the t com is shorter (average 290 kyrs), with the more abundant precipitations that contribute to deeper erosion in the upper catchment, and more fresh material transported to the estuary through stronger hydrodynamic forces. The second mode is more chaotic and corresponds to 0 to 4 ka, when climate was warm and humid and stable sea level, but t com fluctuated from 200 kyrs to 650 kyrs. The increasing river flooding caused by abundant precipitation leads to lateral migration of the river channel in the middle and lower reaches, and the sediments deposited in the floodplain were eroded and mobilized again. In addition, the rapid development of human activities since 2 ka has accelerated deforestation and agricultural cultivation, leading to localized erosion of the floodplain. These remobilized sediments were transported to the estuary and deposited, resulting in a larger t com value. This study overall indicates that, in the millennial scale, the t com of fine detrital fraction shows a sensitive and quick response to catchment erosion, and provides new insights into quantifying the time scale of sediment source to sink processes and inferring the paleoenvironment accurately climate changes. [ABSTRACT FROM AUTHOR]

Water samples were analyzed with an optical laser device (LISST 200×) to determine the properties of suspended sediments in the saline front of the Magdalena River mouth (Colombia, South America), a micro-tidal delta with high fluvial discharge of regional influence. The study aimed to determine variability of textural properties, suspended sediment concentration, effective density, and settling velocity under non-stationary, stratified, and turbid conditions. The suspended sediment consisted of 84.3% silt-size material (3.9–62.5 μm), with the remaining 15.7% a sand-size fraction (62.5–177 μm). Nevertheless, most of these fractions consisted of flocs, up to 995 μm in size, composed of primary particles that ranged in size from 9 to 35 μm. Most sediments were very poorly sorted, near symmetrical and platykurtic, with mean volumetric suspended sediment concentration, settling velocity, and effective density values of 253.1 μl l−1, 5.8 × 10−3 mm s−1, and 0.96 kg m−3, respectively. These properties exhibited significant depth-related differences regardless of tidal phase. Data dispersion also increased with depth. Our findings illustrate the influence of stratification on suspended sediment properties, with marked vertical variability over short time scales. Textural properties highlighted different multi-modal features of grain-size distribution. The study also revealed spatial and temporal transitions associated with contrasting sediment transport conditions. Textural analysis enabled comprehensive interpretation of large datasets obtained through laser diffraction measurements. • Variability of suspended sediment properties in saline front of a highly stratified estuary. • Primary particles (9–35 μm) form flocs with average settling velocity of 5.8 × 10−3 mm s−1. • Suspended sediments were very poorly sorted, near symmetrical, and platykurtic. • Suspended sediment properties exhibited depth-driven significant changes regardless of tide oscillation. [ABSTRACT FROM AUTHOR]

• Combined numerical model and GOCI to improve the ability of sediment flux (SF) estimation. • Multi-timescale SFs at typical cross-sections in YRE are estimated in 2013 and 2015. • Locally morphological changes are further estimated, showing erosions in 2013 and 2015. Sediment flux (SF) in the estuary is vital to the coastal and estuarine environment, especially the morphodynamical and ecological processes. However, its quantitative estimation with high accuracy is difficult because it is controlled by complex mechanisms and multiple processes. This study corrects the seasonal variations of the simulated suspended sediment concentration (SSC) by using GOCI-derived surface SSC and calculates the variations of SFs at the main cross-sections in and out of the Yangtze River Estuary (YRE). The results show that in 2013, 159 Mt and 143 Mt of sediments passed through Xuliujing hydrological station in YRE and estuarine mouth section, respectively. In the inner estuary, the significant seasonal variations of sediment transport are noted that the most seaward transport happens in summer (43.8%) and the least occurs in winter (7.3%). In the outer estuary, the southward transport towards Hangzhou Bay is the most critical pathway, accounting for 109.0% of total transport at mouth section, and is prevalent in autumn and winter. With considerations of sand mining and land reclamation, obviously erosions appear in the whole estuary during both 2013 and 2015. With stronger wind conditions in 2013, severer erosion (161 Mt) happens in outer estuary than that in 2015 (86 Mt). By combining the GOCI-derived surface SSC and the numerical model results, this study can better represent high-frequency hydro- and sediment-dynamical processes to calculate the annual, seasonal, and vertical SFs with improved accuracy. Hence this method may provide a viable way to infer locally averaged morphological changes. [ABSTRACT FROM AUTHOR]

To investigate the tidal hydrodynamics, sediment transport and morphology variation in the Hooghly Estuary, India, a two-dimensional model was developed in the CMS Flow platform. Application of numerical modeling techniques to sediment transport problems in rivers require detailed study of the river reach characteristics and development of the appropriate mathematical model. The characteristics of the river vary considerably starting from its origin to its mouth where it joins the sea, the flow and sediment transport processes also vary accordingly. This makes modeling of sediment transport for the whole reach very complex. Flow and sediment transport processes in estuaries are more complicated because of factors like two-way tidal flow, and widely varying morphology. Numerical models for estuaries are usually extensive and require high computational time because of the complexities in the solution algorithms and large time-space solution domain of the problem which also require a wide variety of data. It becomes very time-consuming to perform these simulations repeatedly, for analyzing changes in sediment transport for variations in boundary conditions, local changes in bathymetry, presence of structures or for long-term simulations. Artificial Neural Networks models when used in combination with numerical models, to simulate flow and sediment transport reduces the computational time considerably. In this work a combined ANN-numerical model is developed for simulation sediment transport and bathymetry change in the Hooghly estuary. [ABSTRACT FROM AUTHOR]

The estuaries and coastal zones of the Vietnamese Mekong Delta located at the end of the Mekong Basin are susceptible to a large variety of natural threats and human interventions such as upstream dam construction, climate change and sea level rise. In this study, we investigated sediment dynamics and morphodynamic changes in the Mekong estuaries and coastal zone, using a well-calibrated Delft3D-4 model for a 10-years period simulation. For the first time, the impacts of different drivers were distinguished to guide future adaptation strategies and policies. Our investigations pointed out that the influences of upstream sediment reduction and large-scale sand extraction would cause substantial modifications in the subaqueous delta region. Besides human-induced impacts, modeling results also showed that the sediment volume and spatial distribution changed through the simulated period according to monsoonal variation. The influence of each driver, however, not only varied in space but also time. In the flood season, the Mekong and Bassac Rivers provided a large amount of sediment (more than 90%) that was deposited on the delta front due to coastal processes. The sediment quantity transported along the coastline changed with the monsoon and was in a dominantly south-west direction due to the northeast monsoon, especially in November, December and January. As a natural process, in the estuarine region, erosion and deposition occurred alternately, but the south-west coast region (belonging to the Ca Mau peninsula) was dominated by erosion. This is because of the formation of sand bars in front of the Bassac estuaries, which influences the capacity of sediment transportation to the south-west. Consequently, navigation capacity will reduce significantly in the estuaries and coastline erosion in the Ca Mau peninsula leads to substantial loss of houses and assets of local people. Urgent actions such as stopping sand mining or modifying dam designs are needed to sustain the delta coastline. • A well-calibrated Delft3D model against observed and remote sensing data. • Sediment transport is strongly driven by both river discharges and monsoons. • Long-term sand bar formation diverted the direction of current sediment flows. • Human activities will lead to the severe recession of the delta. [ABSTRACT FROM AUTHOR]

In the Geum River, a representative altered estuary, in-situ mooring and erosion experiments were conducted to reveal the effects of human-controlled discharge on benthic sediment disturbances. The strong discharge through the estuarine dam increased the current velocities within the benthic boundary layer (BBL) up to 79% and 153% during spring and neap tides, respectively. During the discharge period, the suspended sediment concentration in BBL was five times higher than that during the non-discharge period. During the flood phase, a critical shear stress for erosion (τ ce) was in the range of 0.1278 to 0.1391 Pa. Immediately after the ebb phase with strong discharge, τ ce increased to 0.1848 Pa, and the erosion rate (E) decreased from 33.73 to 14.81 mg m−2 s−1. The repetition of human-controlled discharge removed the erodible sediments within BBL, exposing the underlying consolidated sediment bed with low E and high τ ce. The results suggest that an altered estuary is vulnerable to benthic sediment erosion under combined natural and anthropogenic forcings. [Display omitted] • Human-controlled discharge disturbed benthic sediment transport and velocity structures. • Altered estuary is vulnerable to sediment erosion under anthropogenic forcings. • Discharge removed sediments, exposing consolidated sediment with low erodibility. • Erosion experiments present direct evidence on benthic sediment resuspension. [ABSTRACT FROM AUTHOR]

• We constructed a coupling model system of suitable habitats. • We simulated changes in nearshore vegetation biomass and habitat. • We considered the impact of natural and human activities on vegetation. • The growth of Zostera japonica was severely affected. Estuarine ecosystems are influenced by the combination of freshwater runoff and ocean tides, and the natural environment is intricate and complex, with special natural environmental conditions and biological habitat characteristics. The Yellow River is characterized by high sediment-bearing water flow, and the estuary forms a unique clear and turbid confluence. The Water and Sand Regulation (WSRS) from the Xiaolangdi Reservoir to the estuary and nearshore has had significant impacts on the environmental conditions of water, sediment and nutrients, further affecting the growth and development of vegetation in the nearshore zone. In recent years, more studies have focused on the effects of climate change and ocean tides on vegetation in the Yellow River Estuary; however, few studies have been conducted on the effects of the WSRS on klysmoeionion. In this study, we investigated the influence of water flow, sediment and nutrients on klysmoeionion based on long-term field monitoring and indoor control experiments; constructed a three-dimensional hydrodynamic water quality model to simulate the hydrodynamic characteristics, sediment transport patterns and nutrient salt distribution in the Yellow River Estuary under the combined influence of tides and riverine water and sediment; and further combined the growth competition model of typical vegetation (including the native species Zostera japonica and invasive species Spartina alterniflora) to construct a suitable habitat simulation model for typical vegetation communities in the Yellow River Estuary. Habitat and biomass changes in Z. japonica and S. alterniflora in the Yellow River Estuary were simulated under the influence of the WSRS and climate change. The results showed that the distribution of suitable habitats for Z. japonica and S. alterniflora in the Yellow River Estuary and offshore was characterized by a zonal distribution along both sides of the estuary, with an increasing trend of suitable habitat area from the estuary to both sides of the estuary; due to environmental conditions and inter- and intra-species competition, the biomass of Z. japonica and S. alterniflora was mainly distributed in the estuary and near the northern and southern shores, with the southern shore have the most distribution. In contrast, S. alterniflora was mainly distributed in the northern shore away from the estuary and in Laizhou Bay. During the WSRS period, the suitable habitat area of Z. japonica decreased by 25%, and the biomass decreased by 50%, while the suitable habitat and biomass of S. alterniflora were not significantly affected. The growth of Z. japonica was severely affected by the dual effects of global warming and the WSRS, while the biomass of S. alterniflora showed an increasing trend. The WSRS can further consider the growth pattern of vegetation to protect the resources of Z. japonica and to curb the expansion of the invasive S. alterniflora. [ABSTRACT FROM AUTHOR]

Saltmarshes in most estuaries in the UK and elsewhere are heavily exploited for numerous purposes including farming, fishing, and recreation. In this study, a computational model was used to investigate the impact of saltmarsh vegetation on tidal dynamics and residual currents in three distinctly different estuaries in Wales, UK, in order to understand the impacts of marsh vegetation on wider estuarine hydrodynamics. The three estuaries, Mawddach, Taf and Loughor, vary in size, tidal range, exposure, and saltmarsh coverage. Tidal constituents and residual currents were calculated using a year-long simulation of tidal dynamics. Tidal dynamics are discussed in terms of five important primary tidal constituents (M2, S2, N2, K1, O1) and two shallow water constituents (M4, MS4). The results reveal that saltmarsh vegetation reduces the amplitude of both primary and shallow water tidal constituents not only on and at the proximity of marsh platforms but also in the wider estuary, mostly confined to tidal channels and surrounding intertidal areas. Most notable changes were observed in the middle and upper estuary. Notable changes to residual current velocities were observed on marsh flat areas and in tidal channels and saltmarsh creeks which indicates that changes to marsh vegetation have the potential to alter sediment transport and hence wider estuary hydrodynamics. Our results will be useful when making decisions to restore, reclaim and realign existing saltmarshes for environmental, conservation and socioeconomic purposes, or integrate them in nature-based solutions for estuarine flood and erosion management. [ABSTRACT FROM AUTHOR]

Abstract Accurately characterizing the spatiotemporal dynamics of residual and tidal flows is essential for understanding transport processes in tidal rivers and estuaries, as they directly affect salinity intrusion, water renewal capacity, sediment transport, and morphological evolution. This paper aims at characterizing the longitudinal and temporal variability of tidal discharges and currents in the St. Lawrence fluvial estuary (SLFE), under a wide range of river flow and tidal conditions. Tidal discharges and currents are reconstructed by cubature from nonstationary tidal analyses of water levels for a highly variable year, in terms of observed river flows, at 10 river cross-sections of the SLFE, distributed from the fluvial estuary entrance to the head of the tide. An analysis of the spatiotemporal variability in residual and tidal flows is hereby provided, with an emphasis on ebb-flood characteristics, residual currents, tidal constituent properties and tidal wave propagation characteristics at the intratidal, neap-spring and seasonal scales along the system. The main drivers and mechanisms responsible for this variability include (1) the system's hydrology and its associated watershed and meteorological characteristics, which dictate river flow intensity and seasonality, (2) the landward-travelling tides originating from the ocean, responsible for intratidal and tidal monthly variability in tides and currents, (3) nonlinear river-tide interactions, dissipating tidal energy, modulating tidal properties and inducing a surface level gradient, and (4) channel convergence and floodplain morphology, contributing to tidal asymmetry in horizontal and vertical tides. These factors ultimately influence tidal wave propagation and damping properties within estuaries, both longitudinally and at the neap-spring and seasonal scales. Highlights • Tidal discharges and currents are derived from nonstationary tidal analyses of water levels through continuity. • Drivers of flow variability include hydrology, ocean tides, river-tide interactions, and channel and floodplain morphology. • Flood velocities reduce in presence of tidal flats and respond the most strongly to changes in river flow during neap tides. • Flow duration asymmetry is temporally lagged with respect to tidal duration asymmetry due to neap-spring storage effects. • Fortnightly variations in tidal damping are driven by changes in friction closely linked to the neap-spring reversal of LW. [ABSTRACT FROM AUTHOR]

Highlights • Introduce sediment-related tidal asymmetry proxy based on skewness method. • Oceanic tides control peak current asymmetry and inter-tidal flats determine slack water asymmetry. • SLR reduces both peak current asymmetry and slack water asymmetry, suggesting a negative feedback mechanism. Abstract Tidal asymmetry in estuaries and lagoons (tidal basins) controls residual sediment transport, and quantifying tidal asymmetry is important for understanding the factors contributing to long-term morphological changes. Asymmetry in peak flood and ebb currents (Peak Current Asymmetry – PCA) controls residual transport of coarse sediment, and asymmetry in slack water duration preceding flood and ebb currents (Slack Water Asymmetry – SWA) controls residual transport of fine sediment. PCA and SWA are analyzed herein for Newport Bay, a tidal embayment in southern California, based on the skewness of tidal currents predicted for several stations by a hydrodynamic model. Use of skewness for tidal asymmetry is relatively new and offers several advantages over a traditional harmonic method including the ability to resolve variability over a wide range of time scales. Newport Bay is externally forced by mixed oceanic tides that are shown to be ebb dominant because of shorter falling tide than rising tide. Both PCA and SWA indicate ebb dominance that favors export of coarse and fine sediments, respectively, to the coastal ocean. However, we show that the ebb dominance of SWA is derived from the basin geometry and not the external forcing, while ebb dominance of PCA is linked to the external forcing and the basin geometry. We also show that tidal flats in Newport Bay play an important role in maintaining ebb dominated transport of both coarse and fine sediments. Loss of tidal flats could weaken PCA and reverse SWA to become flood dominant. Specifically, we show that sea level rise > 0.8 m that inundates tidal flats will begin to weaken ebb dominant PCA and SWA and that sea level rise > 1.0 m will reverse SWA to become flood dominant. This feedback mechanism is likely to be important for predicting long-term evolution of tidal basins under accelerating sea level rise. [ABSTRACT FROM AUTHOR]

Mercury (Hg) was discharged in the late 1960s into the Penobscot River by the Holtra-Chem chlor-alkali production facility, which was in operation from 1967 to 2000. To assess the transport and distribution of total Hg, and recovery of the river and estuary system from Hg pollution, physical and radiochemical data were assembled from sediment cores collected from 58 of 72 coring stations sampled in 2009. These stations were located throughout the lower Penobscot River, and included four principal study regions, the Penobscot River (PBR), Mendall Marsh (MM), the Orland River (OR), and the Penobscot estuary (ES). To provide the geochronology required to evaluate sedimentary total Hg profiles, 58 of 72 sediment cores were dated using the atmospheric radionuclide tracers 137 Cs, 210 Pb, and 239,240 Pu. Sediment cores were assessed for depths of mixing, and for the determination of sediment accumulation rates using both geochemical (total Hg) and radiochemical data. At most stations, evidence for significant vertical mixing, derived from profiles of 7 Be (where possible) and porosity, was restricted to the upper ~1–3 cm. Thus, historic profiles of both total Hg and radionuclides were only minimally distorted, allowing a reconstruction of their depositional history. The pulse input tracers 137 Cs and 239,240 Pu used to assess sediment accumulation rates agreed well, while the steady state tracer 210 Pb exhibited weaker agreement, likely due to irregular lateral sediment inputs. [ABSTRACT FROM AUTHOR]

The Yellow River has experienced prolific environmental changes from dam regulations that have significantly altered the downstream hydrologic regime, and transport of sediment and POC within the river-coast continuum. Here, we investigated the contrasting changes in sources and transport of POC from the lower Yellow River to the river mouth, at two different stages (e.g., Water vs. Sediment regulation) of dam operation - during the period of Water-Sediment Regulation Scheme (WSRS). Riverbed erosion downstream the Xiaolangdi Dam contributed the POC with more enriched δ13C stable isotopic values, along with those more isotopically depleted POC from riparian floodplains and the reservoir at the Water-regulation stage, when clear water was released from the reservoir. At the Sediment-regulation stage, muddy floodwaters were released, while soil-derived POC and fine-grained sediment released from the reservoir became dominant with evidently increasing POC content. The dispersal and distribution of river-laden POC at the river mouth were primarily modulated by the plume dynamics, as remotely impacted by dam regulation, in combination with the dynamic shear front. As a result, the POC distribution was confined within 10 km off the river mouth due to the rapid deposition of coarse-grained sediments. In contrast, the fine-grained sediment and more isotopically depleted POC, from modern sources (e.g., C 3 vascular plant), were delivered to the far-field plume region. POC transport along the Yellow River-Estuary continuum exhibited complex behaviors, coupled with natural and dam-regulated drivers, on a variety of spatial-temporal scales. These findings provide important insights into dynamic linkages on how coastal carbon cycling can be significantly altered, remotely from the upstream dam regulations, in a large global river system. • Dam operations and hydrological changes induced contrasting features in downstream POC delivery. • Dam-impacted plume dynamics and the shear front dominated the preferential transport of POC at river mouth. • POC transport within the river-estuary continuum exhibited complex and dynamic behaviors. [ABSTRACT FROM AUTHOR]

This study aimed at assessing trace element concentrations in two representative estuaries of Ghana (Pra and Ankobra) affected by gold-mining, relative to the levels of the unaffected Volta estuary. Surficial sediments ( n  = 16–17) were sampled at each estuary and analysed by ICP-MS for 25 elements after pseudo-total digestion. The enrichment and geoaccumulation indexes revealed a moderate to significant contamination of As, Ag and Cu in the Pra and Ankobra estuaries. Spatial maps of concentrations revealed non-localized sources. High As concentrations were attributable to runoff transport and sedimentation of gold mining-tailing particles, as suggested by results from granulometric distributions, correlation and PCA analysis. The probabilities of surpassing the probable effects level (PEL) were 77% for As, 50% for Cr and 27% for Ni in Ankobra; these values were of 13%, 23% and 10% for the Pra. Results reveal potential future implications on ecosystems and human health in these both estuaries as result of the gold-mining activity. [ABSTRACT FROM AUTHOR]

In late summer 2011 Hurricane Irene and Tropical Storm Lee impacted virtually every coastal river and estuary along the U.S. North Atlantic seaboard. We investigated the sedimentary response of the Delaware Estuary to these tropical cyclones through coordinated observations of flow and suspended-sediment transport from moored instrumentation, hydrographic surveys, and sediment coring. On 27–28 August Hurricane Irene tracked offshore of the estuary and generated strong northwesterly winds and intense, short-lived rainfall and river runoff. On 7–8 September the remnants of Tropical Storm Lee tracked inland along the drainage basin and produced weaker winds but more widely distributed and longer-lived rainfall and river runoff. An estimated 1.4 ± 0.6 × 10 6 metric tons of fluvial suspended sediment was delivered to the estuary in association with these storms. Record-setting freshwater discharges generated by Lee displaced the salt front ~ 50 km seaward of its pre-storm position, yet the front and associated estuarine turbidity maximum were not flushed beyond the mouth. Convergent gravitational and tidal circulations effectively trapped suspended sediment delivered to, and produced within, the estuary during the storm period. Normal conditions of estuarine circulation and sediment flux rebounded about five days after the passage of Irene but did not fully recover before the approach of Lee, and it was not until two weeks thereafter that the estuary recovered. Bed sedimentology and inventories of the radionuclide 7 Be (t 1/2 = 53 days) revealed that the upper estuary floor scoured in places under storm-enhanced bottom currents, whereas the lower estuary remained depositional throughout the storm period. Sediment flux and coring observations indicate that estuary bed resuspension was the main source of suspended sediment transported in the estuary during Irene, and was a major source during Lee along with fluvial sediment delivered from the drainage basin. Regardless of the source, our observations make clear that the estuary is capable of trapping suspended sediment and preventing its escape to the coastal ocean during extreme oceanic storms. [ABSTRACT FROM AUTHOR]

The kinetics of the uptake of pollutants by solids in sediments interacts with transitional eddy diffusivity in the pore fluid, leading to different depth-distribution patterns. This work aims to gain insights into the still poorly understood behaviour in the marine environment of the anthropogenic 236U, a recently postulated tracer of water masses. It is hypothesized that the transition from mobile U(VI) to highly particle-reactive U(IV) in the anoxic zone of the sediment produces a subsurface deposition pattern. A novel numerical model for kinetic reactive transport in sediments, which merges diagenetic processes for transport and box models for the uptake, is used for concept demonstration. It is applied to synthetic environments with high eddy diffusivity to obtain the singular depth-distribution patterns of pulsed inputs of tracers that mimic the anthropogenic 239,240Pu, 137Cs, and 236U. While the first is retained in the upper cm, the second shows an exponential penetration pattern over few cm, and 236U is deposited with a Gaussian-like pattern centred below the transition to the anoxic zone. These patterns are then merged into a diagenetic model to compute the depth distribution at decadal or centennial scales of dissolved and particle-bound inputs of these radiotracers. It is successfully applied to a real case using literature data for a sediment core from the Esk Estuary, UK, affected by radioactive releases from the Sellafield nuclear reprocessing plant. This work provides insight into until now poorly understood field data and provides a novel view of broad implications in the study of the behaviour of pollutants in surficial aquatic sediments. [Display omitted] • Pore-fluid eddy diffusivity and pollutant uptake kinetics interplay in sediments. • Numerical model shows distinct subsurface deposition patterns for Pu, Cs and U. • TransItion U(VI) to insoluble U(IV) leads to maximum deposition in the anoxic zone. • 239,240Pu, 137Cs and 236U profiles in a sediment from Esk Estuary were explained. [ABSTRACT FROM AUTHOR]

Sediment dynamics is essential for morphological evolution, water quality, and ecosystem health in an estuary. Winds and waves have significant effects on vertical stratification, mixing, bottom shear stress, and current circulation in an estuary, and thus exert an impact on the dynamics of sediment resuspension, advection, and deposition. Here we examine these effects in a convergent partially-mixed estuary by employing a wave-current-sediment transport coupled modeling system. Moderate down-estuary and up-estuary winds were imposed in the model, respectively, to investigate their effects on sediment dynamics in the estuary. For the wave effects, swell from the offshore and wind waves generated by local axial winds were all taken into account. Our results show that the winds and waves alter the residual flow, water mixing, and bottom stress in the estuary. Consistent with previous results, the down-estuary wind enhances estuarine circulation and salt intrusion. It thus shifts the estuarine turbidity maximum (ETM) landward and increases the suspended sediment concentration (SSC) by increasing the bottom stress. The up-estuary wind generates an opposite effect compared to that of the down-estuary wind. Moreover, the axial winds alter the lateral circulation and change the lateral distribution of SSCs in the estuary. The down-estuary wind confines the SSCs more adjacent to the channel, and the up-estuary wind has relatively less effect on modifying the lateral distribution of SSCs. When the wave effects are considered, the combined swell and wind waves generally increase the bottom stress. They also increase the stratification inside the estuary by advecting more saline water from the continental shelf into the estuary. In turn, under the scenario of down-estuary wind, the waves greatly elevate the SSC and shift the primary ETM more landward. On the contrary, under the up-estuary wind, the waves primarily increase the SSC in the lower estuary. The convergence of lateral sediment transport help to form high SSCs along the side slopes below the salt intrusion limit, which can be referred to as lateral ETMs in estuaries. This study helps to gain more insight into the sediment dynamics under the combined effects of winds and waves in partially mixed estuaries with lateral bathymetric variations. • The impacts of winds and waves are examined in an idealized convergent partially mixed estuary. • Down-estuary wind concentrates the clay and silt closer to the middle channel and pushes the turbidity maximum landward. • Up-estuary wind has less effect on modifying the lateral distribution of suspended sediment. • The effects of the axial winds and waves are sensitive to the magnitude of wind speed. [ABSTRACT FROM AUTHOR]

Estuarine turbidity maxima (ETMs) are of sedimentological, geomorphological and ecological significance. In this study, a well-validated and process-based sediment transport model was used to investigate the response of ETM dynamics in the Yangtze Estuary (YE) to construction of the Three Gorges Dam (TGD) and morphological evolution of the YE. The model results demonstrated that the ETM dynamics change in the upper estuary was controlled by the TGD construction. After the TGD construction, the ETM area decreased by 22% (from 1116 to 874 km2) and the ETM center location moved landward 7 km during the flood season. The estuarine morphological evolution played a key role in the ETM dynamics change of the mid-lower estuary. The ETM area decreased by 55% (from 1478 to 658 km2) and 15% (from 658 to 562 km2) due to the estuarine morphological evolution from 1998 to 2010 and from 2010 to 2018, respectively. Meanwhile, the ETM center location moved landward 38 km and 7 km during the periods 1998–2010 and 2010–2018, respectively. Overall, both the TGD construction and estuarine morphological evolution induced decreased suspended sediment concentrations in the estuary. Furthermore, the effect of TGD construction was dominant in the upper estuary, whereas the effect of estuarine morphological evolution became dominant in the mid-lower estuary. These findings provide implications for management of the YE and other similar estuaries. • Estuarine turbidity maximum (ETM) decreased by 22% and moved landward 7 km after Three Gorges Dam (TGD) construction. • Estuarine morphological evolution induced ETM to decrease by 15–55% and move landward 7–38 km. • TGD construction and morphological evolution dominated ETM changes in upper and mid-lower estuaries, respectively. [ABSTRACT FROM AUTHOR]

Bar-built or barrier estuaries (here referred to as Small tidal inlets, or STIs), which are commonly found along wave-dominated, microtidal mainland coasts, are highly likely to be affected by climate change (CC). Due to their pre-dominance in tropical and sub-tropical regions of the world, many STIs are located in developing countries, where STI related activities contribute significantly to the national GDPs while community resilience to coastal changes is low, with the corollary that CC impacts on STIs may lead to very serious socio-economic consequences. While assessing CC impacts on tidal inlets is in general difficult due to inherent limitations of contemporary numerical models where long term morphodynamic simulations are concerned, these difficulties are further exacerbated due to the lack of sufficient model input/verification data in often data poor developing country STI environs. As a solution to this problem, Duong et al. (2016) proposed two different process based snap-shot modelling approaches for data poor and data rich environments. This article demonstrates the application of Duong et al.’s (2016) snap-shot modelling approach for data poor environments to 3 case study sites representing the 3 main STI types; Permanently open, locationally stable inlets (Type 1), Permanently open, alongshore migrating inlets (Type 2) and Seasonally/Intermittently open, locationally stable inlets (Type 3). Results show that Type 1 and Type 3 inlets will not change Type even under the most extreme CC driven variations in system forcing considered here. Type 2 inlets may change into Type 1 when CC results in a reduction in annual longshore sediment transport. Apart from Type changes, CC will affect the level of inlet stability and some key behavioural characteristics (e.g. inlet migration distances, inlet closure times). In general, CC driven variations in annual longshore sediment transport rates appear to be more relevant for future changes in inlet stability and behaviour, rather than sea level rise as commonly believed. Based on model results, an inlet classification scheme which, for the first time, links inlet Type with the Bruun inlet stability criteria is presented. [ABSTRACT FROM AUTHOR]

Ports are often located in naturally sheltered areas such as estuaries. The strong tidal currents that occur in many of these areas drive a dynamic morphodynamic regime, with the result that the approach channels to these ports are gradually infilled. Periodic dredging is therefore necessary to maintain the operativity of the port. A case in point is Ribadeo (NW Spain), an important port for the economy of the area. In this work, the sediment transport patterns of the estuary (Ria de Ribadeo) are investigated through high-resolution numerical modelling and field measurements covering a 4-year period. On this basis, a decision-aid tool is developed that enables to predict the time evolution of the approach channel and thus contributes to the planning of dredging operations and, more generally, the maintenance of adequate operativity levels in a cost-effective way. [ABSTRACT FROM AUTHOR]

Tide-dominated coastal plain estuaries have typically up-estuary convergent tidal channels. Analysis of estuarine characteristics indicates a dependence of the convergence length on relative tidal amplitude, relative intertidal area and river flow velocity. In order to explain these relationships we investigate a condition for continuity of net sediment transport throughout the estuary, corresponding to morphodynamic equilibrium. We show, by using an analytical solution of the tidal equations, that this condition is equivalent to a condition on the convergence length. This condition is evaluated for 21 estuaries in different regions of the world. It appears that the convergence length determined in this way can explain observed convergence lengths for the considered set of estuaries. The dependence of the convergence length on different estuarine characteristics is analysed by solving the fully coupled hydro-morphodynamic equations. We show that this dependence limits the range of variation of the tidal velocity amplitude. The analysis provides insight in the morphological response of estuaries to human interventions. The condition can easily be evaluated to yield an estimate of this response. [ABSTRACT FROM AUTHOR]

Macro-tidal estuaries with mountain streams are typical estuarine systems in southeastern China. Suspended sediment transport and turbidity maxima are sensitive to changes in the tidal and fluvial regimes of estuaries. To improve our understanding of river-ocean interactions and thus improve estuarine forecasts for biogeochemical processes, we developed a high-resolution hydrodynamic and sediment transport coupled model to investigate the suspended sediment transport and estuarine turbidity maximum (ETM) in the Oujiang Estuary. This estuary is characterized as a macro-tidal estuarine system with a large tidal range and drastic fluctuations in freshwater and sediment discharge. The results led to the following conclusions: (1) the ETM occurred during spring tides due to strong tidal resuspension. In contrast, it was not formed during the neap tide because of weak bottom shear stress. The ETM moves upstream to Yanmen during low river discharge and migrates downstream to the river mouth during high river discharge, causing sediment deposition in the nearshore region. (2) Owing to the characteristics of mountain streams and upstream anthropogenic reservoirs, the ETM is strengthened during low river discharge (∼100 m3 s−1) even during the wet season, which supplements the fact that the ETM is more prevalent during the dry season and withers during the wet season. The suspended sediment forming the ETM mainly originates from shallow offshore shoals owing to tidal resuspension. Owing to the weak vertical exchange flow near the river mouth, the ETM is related to the exchange flow during the low water flow. (3) The ETM excursion shows semi-diurnal variability with different transport distance for high and low river discharges. Tidal pumping dominates the suspended sediment flux in the core of the ETM, which is mainly maintained by two components of the Euler and Stokes fluxes as the tidal energy depletes during the flood tide. • Estuarine modeling was conducted to study suspended sediment dynamics. • ETM excursion occurs for high and low river. • Decomposing sediment fluxes reveals ETM mechanisms. • River discharge and tidal pumping play roles on ETM formations. [ABSTRACT FROM AUTHOR]

Understanding suspended sediment processes in the mouth-bar region during typhoons is vital for estuarine geomorphic prediction and engineering design. An observation system was applied to explore suspended sediment dynamics during Typhoon Higos in the mouth-bar region of the river/wave-dominated Modaomen Estuary within the Pearl River Delta. The results showed that suspended sediment concentration (SSC) during typhoons could be around 30 times larger than that under normal conditions, primarily attributable to strong bed sediment resuspension induced by typhoon waves. The SSC captured during typhoon exhibited fluctuations of different amplitudes due to the movement of the high concentrated sediment in the mouth-bar region forced by the dominant flow. When the dominant flows were unified within the mouth-bar region in high velocities, including the wind-driven flow near the typhoon landfall and the ebb surge transport combined with the river flooding after landfall, the high concentrated sediment can move in a large distance along the dominant direction of the flow and induced dramatic decrease of SSC. When the dominant flows were not unified within the mouth-bar region and the velocities were relatively lower, the high concentrated sediment was dominated by the tidal process as under normal conditions and the SSC fluctuated in a smaller amplitude, including tidal modulation, flood dilution and ebb replenishment. Compared with the other estuary types, the processes controlling the storm-induced suspended sediment dynamics, including resuspension, wind-driven transportation and tidal transportation, all happened with high intensities in our surveyed river/wave-dominated estuary; and a conceptual model has been proposed to schematize these processes and their combined impacts. • Sediment during typhoon was 30 times higher than that under normal conditions due to resuspension caused by typhoon waves. • The SSC fluctuated along with the migration of a high turbidity sediment pool during typhoons. • Wind driven flow transportation when typhoon landed induced a SSC decrease in the typhoon-period. • The SSC was also decreased when intense ebb tide flow transported the sediment away from the mouth-bar region. • A schematized model of typhoon-induced suspended sediment dynamics in the mouth-bar region was proposed. [ABSTRACT FROM AUTHOR]

Several works have discussed the morphological evolution in the Changjiang Estuary (CJE) in recent years. The erosion of its subaqueous delta in recent decades has been ascribed to a decline in fluvial sediment input. However, the interaction between the reduction of riverine sediment load and human activities in the estuary that could have caused morphological change has not been considered. In this work we provide evidence on the morphological evolution around the delta front zone since 1986 and use a numerical model to explore the correlation between the change in hydrodynamics and the evolution pattern. Bathymetric data analysis suggests a decrease of net accretion rate from 16.7 mm/year (1986–1997) to 9.1 mm/year (1997–2010) in the study area. Spatially, the tidal flats accreted whereas the subaqueous delta switched from deposition between 1986 and 1997 to erosion between 1997 and 2010. We used two indicators, tidal energy dissipation and erosion rate, to quantify the change in hydrodynamics and found that the erosion of the subaqueous delta in recent decades can readily be explained by the alteration of the hydrodynamics. The newly built navigation training works in the North Passage had a significant effect on the estuarine hydrodynamics, resulting in a local morphological adjustment. This erosion generated a new source of sediments to maintain the high suspended sediment concentration and tidal flat progradation. The erosion of the subaqueous delta may continue and gradually slow down until the altered hydrodynamics and morphology reach an equilibrium state in the future. [ABSTRACT FROM AUTHOR]

The interplay between suspended-sediment transport and plume hydrodynamics in a surface-advected estuarine plume is studied using a three-dimensional numerical model. Our analysis focuses on the formation of a sediment-rich alongshore current and on the effect of sediments on the structure of the recirculating freshwater bulge. We introduce the ratio Y between the traveling time of sediment along the bulge edge and the settling timescale. When Y <1, suspended sediments enter the alongshore coastal current. When Y >1 the sediments are deposited within the bulge. We find that a critical range of settling velocities exist above which no transport in the costal current is allowed. Critical settling-velocity values increase with river discharge. Therefore, low magnitude and long-lasting floods promote sediment sorting in the continental shelf. We further find that, for a given flood duration, intermediate flood magnitudes at the limit between subcritical and supercritical flow maximize the alongshore sediment transport. Similarly, for a fixed input of water and sediments, intermediate discharge durations maximize alongshore sediment transport. [ABSTRACT FROM AUTHOR]

Summary River flow and sediment transport in estuaries influence morphological development over decadal and century time scales, but hydrological and sedimentological records are typically too short to adequately characterize long-term trends. In this study, we recover archival records and apply a rating curve approach to develop the first instrumental estimates of daily delta inflow and sediment loads to San Francisco Bay (1849–1929). The total sediment load is constrained using sedimentation/erosion estimated from bathymetric survey data to produce continuous daily sediment transport estimates from 1849 to 1955, the time period prior to sediment load measurements. We estimate that ∼55% (45–75%) of the ∼1500 ± 400 million tons (Mt) of sediment delivered to the estuary between 1849 and 2011 was the result of anthropogenic alteration in the watershed that increased sediment supply. Also, the seasonal timing of sediment flux events has shifted because significant spring-melt floods have decreased, causing estimated springtime transport (April 1st to June 30th) to decrease from ∼25% to ∼15% of the annual total. By contrast, wintertime sediment loads (December 1st to March 31st) have increased from ∼70% to ∼80%. A ∼35% reduction of annual flow since the 19th century along with decreased sediment supply has resulted in a ∼50% reduction in annual sediment delivery. The methods developed in this study can be applied to other systems for which unanalyzed historic data exist. [ABSTRACT FROM AUTHOR]

Deep understanding of the nutrient exchange between a mangrove estuary and the surrounding coastal region is important for preserving nearshore biodiversity and sustaining appropriate human ecotourism. This study investigated the effect of tidal changes on the water quality and nutrient fluxes of a small mangrove estuary in Japan. Temporal changes in the water temperature, salinity, dissolved oxygen, turbidity, and nutrient concentrations (NO 3 , NH 4 , and TN) were measured. Nutrient fluxes were calculated using data collected from the Fukido Estuary in 2016 and compared against data collected in 2007. The results showed that NO 3 , NH 4 , and TN exhibited a negative correlation with salinity in 2016, but not in 2007. The water and TN net fluxes changed with rainfall. The findings of this study provide useful insights into the nutrient fluxes in a mangrove estuary and facilitate the evaluation of tidal fluctuations and weather-induced effects on nutrient fluxes. [Display omitted] • Changes in water quality and nutrients in a mangrove estuary are studied. • Salinity and DO values fluctuated with tide. • Rainfall increased TN and water net fluxes. • Measurements obtained in 2016 and 2007 are studied and compared. • NO 3 , NH 4 , and TN showed a negative correlation with salinity in 2016, not in 2007. [ABSTRACT FROM AUTHOR]

This study presents an incorporation and application of a three-dimensional, unstructured-grid hydrodynamic model with a suspended sediment transport module in the Danshui-River estuarine system, Taiwan, and its adjacent coastal sea. The model is verified with data observed in 2010: water surface elevations, longitudinal velocities, salinities and suspended sediment concentration (SSC). The simulations agreed quantitatively with the observations. The validated model was applied to numerical experiments to investigate the formation of the estuarine turbidity maximum (ETM) and the influence of vertical mixing, different freshwater-discharge conditions and bathymetric change on suspended sediment dynamics in the main Danshui-River estuary. The model results reveal that the suspended sediment may be trapped in the deeper section of the river. Higher vertical diffusivity slightly increased SSC because of greater bottom stress during the ebb tide. Under extreme freshwater-discharge conditions, SSC increased considerably at both the surface and bottom layers of the water column, and ETM shifted its location upstream. When we change the bathymetry near the Guandu Bridge in our model, ETM extends its range a further 5.0 km from the river estuary because of higher velocity. [ABSTRACT FROM AUTHOR]

Tidal asymmetry is a phenomenon that characterises estuarine hydrodynamics and has a strong impact on sediment dynamics. Extensive research has been dedicated to studying tidal dynamics in semidiurnal macrotidal estuaries, highlighting several general principles. The ratio of flood to ebb peak velocities and differences in ebb and flood durations are often used to characterise the asymmetry encountered in estuaries. In the Charente estuary (French Atlantic coast), water surface elevation data obtained using an ADCP (Acoustic Doppler Current Profiler) and a tide gauge show that the duration asymmetry undergoes inversions during the spring-neap tidal cycle. A two-dimensional hydrodynamics model is used to investigate the connection between spring-neap inversions of the tidal asymmetry and the harmonic composition of the tide. Different constituents (M2, S2, M4 and MS4) are considered at the open boundary. The results show that M4 and MS4 play a key role in the occurrence of these inversions. The influence of the morphology is also discussed and modifications of the bathymetry are performed to evaluate its impact. In the Charente estuary, the existence of both externally and internally generated overtides thus results in a spatially and fortnightly variable tidal asymmetry. The modelled barotropic tidal currents are used to estimate the possible impact on sediment dynamics. The results suggest that asymmetry inversions tend to create sediment accumulation in an intermediate zone between the river mouth and Rochefort, located approximately 20 km upstream. [ABSTRACT FROM AUTHOR]

Rivers supply sediment to estuaries and alter estuarine tidal wave regimes. However, the long-term (> century) impact of seasonally varying river discharges on estuarine morphodynamics is not well understood. Process-based models are useful tools to analyze these morphodynamic processes. This study aims to explore modeling techniques to efficiently include the impact of discharge seasonality on long-term estuarine morphodynamic development by a process-based morphodynamic model. Inspired by the dimensions and hydraulic forcing of the Yangtze River estuary, we evaluate three methodologies to schematize a river discharge hydrograph in a 1-D morphodynamic model. These are a representative constant discharge, a non-compressed hydrograph and a compressed hydrograph. To shorten computational time, the latter methodology decreases the timescale of seasonal river flow variations compared to the tidal timescale. Sensitivity analysis suggests that the yearly averaged sediment transport controls long-term morphodynamic development. Morphodynamic equilibrium is reached after millennia when deposition during a high river flow period is balanced by erosion during a low river flow period. An important finding is that a yearly hydrograph can be squeezed in time up to a factor of 20, without leading to unrealistic river–tide interactions. Applying a constant river discharge may not produce equilibrium bed profiles that are similar to that under hydrograph forcing due to the non-linearity in sediment transport. The long characteristic timescales in defining morphodynamic equilibrium indicate that a long-term view is needed to interpret large scale estuarine morphodynamics. Though simple and highly schematized, the modeling experiments in this study provide conceptual understanding of morphodynamics in river-influenced estuaries. [ABSTRACT FROM AUTHOR]

The vertical distribution of suspended sediment in continental shelf waters has significant implications for water quality, aquatic ecology, and sediment transport. Nevertheless, there have been relatively few systematic field studies aimed at determining whether a general vertical trend in suspended sediment concentration (SSC) exists, or how any such vertical trend can be expressed mathematically. In this study, based on 402 individual SSC profiles measured hourly during 16 dual-tide voyages at 8 stations in the outer Yangtze Estuary, we found that SSC followed an average depth profile that was smoothly concave-up. This average profile, and the relationship between SSC and height above the seabed, was exponential ( R 2 >0.99). In comparison, the traditional Rouse equation based on hydrodynamics describes the measured average SSC profile poorly, in that the greater the distance from the near-bed height (where the reference SSC is taken), the greater the deviation of the simulated SSC from the measured SSC. However, in this study, a new approach was developed to overcome this flaw in the Rouse equation, which divides the equation into two parts. One part uses a reference SSC from a near-bed level, and an upwards decreasing coefficient, in the same way as the conventional Rouse equation. Conversely, the other part uses a reference SSC from at the water surface, and a downwards decreasing coefficient. Our modified hydrodynamics-based equation expresses the measured SSC well, with an R 2 between the simulated and measured SSCs exceeding 0.99. According to the average SSC profiles, the near-bed SSC was 2.8 times greater than the SSC at the water surface (close to the value of 3 suggested for modeling studies in the absence of empirical data), and the depth-averaged SSC was 1.8 times greater than the SSC at the water surface. We also found that the average SSC profile showed a uniform (from water surface to seabed) decline in SSC (25%) in response to the dramatic decline in the suspended sediment load supplied by the Yangtze River over the past three decades. However, the shape and vertical ratio of the average SSC profile have changed little, which suggests that the general form of the SSC profile is determined mainly by local hydrodynamics and sediment properties, and not by fluvial sediment supply. [ABSTRACT FROM AUTHOR]

Low-accommodation foreland basins have been of recent interest because their stratigraphic architectures provide insight into the interaction of tectonic subsidence and eustasy. Aptian-aged strata in Alberta, Canada (McMurray Formation and Wabiskaw Member) were deposited under the influence of a continental-scale river that longitudinally transported and debouched sediment into the southward-transgressing Boreal Sea in the distal, low-accommodation Western Canada Foreland Basin (WCFB). In this study, we present a high-resolution stratigraphic framework for the McMurray-Wabiskaw interval, which was deposited during a long-term transgression. Using wireline logs from >20,000 wells and more than 500 cores, regional mapping focused on resolving stratigraphic relationships and the distribution of depositional environments across approximately 60,000 km2 in the Athabasca Oil Sands Region of northeastern Alberta. The studied stratigraphic interval is part of a third-order sequence: the McMurray Formation developed during a period of relative sea-level fall, lowstand, and early transgression, while the overlying Wabiskaw Member was deposited during late transgression. Within the McMurray Formation, superimposed fourth-order sea-level fluctuations in this low accommodation setting created a complex amalgam of deltaic strata vertically and laterally juxtaposed with valley-fill deposits. Relative sea-level rises resulted in rapid transgressions in which the shorelines migrated >400 km landward. Thin (5–15 m thick), widespread deltaic parasequence sets were deposited during subsequent sea-level highstands. During relative sea-level falls, fluvial meandering channel belts developed in downcutting valleys, with associated shorelines dramatically prograding basinward. In the Wabiskaw Member, continued sea-level rise coupled with potentially decreasing sediment supply resulted in the development of tide-influenced/dominated channels and bars in estuaries. The final phase of the third-order transgression was marked by a regional transgressive ravinement event and several southwestward backstepping wave-dominated shorefaces/strandplains. This low-accommodation foreland basin succession demonstrates a transition from deposition in continental fluvial and marginal-marine settings subject to high-frequency eustatic changes, to open-marine conditions over a long-term transgression. • The McMurray-Wabiskaw interval demonstrates a high-resolution stratigraphic framework in a distal, low-accommodation foreland basin • The stratigraphy illustrates a transition from deposition in continental fluvial and marginal-marine settings to open-marine conditions over a 3rd-order transgression. • 4th-order sea-level fluctuations led to the deposition of repeated packages of meandering channel belt strata and deltaic deposits in the basin • The juxtaposition of fluvial meandering channel belts and deltas was more influenced by eustatic changes given its low accommodation and topographic relief • Later stage of the overall transgression was marked by tide-influenced channels and bars in estuaries and backstepping wave-dominated shorefaces/strandplains [ABSTRACT FROM AUTHOR]

• Spring tide amplifies more than neap tide in Qiantang Estuary. • Tidal dynamics is strongly influenced by morphological evolution. • A conceptual model of river-tide-morphology interaction is proposed. Understanding tidal dynamics in estuaries is essential for tidal predictions and assessments of sediment transport and associated morphological changes. Most studies on river-tide interaction ignored the influences of morphological evolutions under natural conditions such as the seasonal and interannual variations of river discharge. This study analyzes the multiple-timescale tidal dynamics in the Qiantang Estuary, a macro-tidal estuary in China with an extremely active morphological evolution. A large dataset including water levels at representative stations, river discharges and bathymetries since 1980 has been collected. The results of the analysis show that within a spring-neap cycle, the tidal amplification in the upper estuary is stronger during spring tide than during neap tide. This unexpected behavior is due to the high sediment concentration and the unique longitudinal profile of the estuary. On the seasonal and interannual timescales, the low water levels in the upper estuary depend on the local bathymetrical conditions. Tidal ranges in the upper estuary are larger in the high flow season and years, than in the low flow season and years, due to the erosion at high flow, in contrast to estuaries with less active morphological changes. During low flow season and years, the bed is gradually recovered, the low waters are elevated, and the tidal ranges decrease accordingly. A good relationship exists between the tidal ranges and the depth of the upper estuary. In the lower estuary, the flood dominance increases continuously due to embankment. In the upper estuary, the flood dominance is increased during the high flow periods, explaining the fast sediment input and bed recovery in the post high flow periods. A conceptual model of river-tide-morphology interaction of the estuary is proposed, which is also applicable for other shallow systems. [ABSTRACT FROM AUTHOR]

Although many studies have been carried out on the mud wedge in the inner shelf of the East China Sea (ECS), the sediment sources and seismic textures of the mud wedge are still unclear and remain controversial. Therefore, this study utilizes grain size and clay mineral analyses of surficial sediment samples and high-resolution sub-bottom seismic profile interpretation to identify the provenance and texture of these mud wedge sediments. The results show that sediment sources on the inner shelf of the East China Sea are mixed by the Yangtze River, small mountainous rivers along the Zhejiang-Fujian, and western Taiwan coasts. The sediments derived from the Ou and Min rivers are mainly concentrated in the shallowest shelf area within a water depth of 20 m which is located north of the Min River Estuary. Their contribution to the estuaries and coastal areas reflects the different sediment transport patterns of the two small mountainous rivers. The thickness of the Holocene mud wedge between Hangzhou Bay and Taizhou Bay is greater than 20 m at a water depth of approximately 10 m. The cause of this phenomenon is possibly related to the presence of the Zhoushan Islands, which are distributed along the Zhejiang coast and appear to effectively trap sediments from the Yangtze River, facilitating sediment accumulation. The findings of this study will help improve our understanding of the sediment transport and deposition mechanism of deposition on the inner shelf of the East China Sea. • We produced grain size and clay mineral analyses and high-resolution sub-bottom seismic profile interpretation. • Sediments derived from small mountainous rivers are mainly concentrated in the shallowest shelf area. • The thickness of the Holocene mud wedge is locally greater than 20 m in the shallowest shelf area. [ABSTRACT FROM AUTHOR]

The Pentland Firth between mainland Scotland and Orkney is an area of the UK continental shelf that experiences extreme tidal flows (>3 m/s). This study presents a time series of bathymetric data coupled with a tidal flow model to examine hydrodynamics and bedform response at the eastern approaches to the Pentland Firth. These observations were additionally augmented by sediment grain type and ADCP data to validate the numerical flow model. Tidal flows of the Pentland Firth result from a tidal phase difference between the east and west approaches to the channel. A resulting barotropic pressure gradient leads to flow accelerations that locally exceed 5 m/s. The extreme tidal setting of the Pentland Firth's eastern approach was found to promote distinct bedforms that are spatially varied in geomorphology with distance from the Pentland Firth. Sediment analysis also showed a decreasing grain size trend also with distance from the Pentland Firth. The modelled residual tidal current shows strong agreement with the sediment transport pathways, supported by the bedform migration direction. The energetic tidal flows of the Pentland Firth's eastern approach interacts with the highly irregular coastline, generating residual tidal counter currents leeward of flow obstructions. These counter currents (i.e. residual tidal current vortices) were reflected in bedform migrations and thence the sediment transport pathways. As the Pentland Firth is considered as a bedload parting site, the residual tidal current vortices are expected to influence the rate of erosion at the bedload parting site, by recirculating sediment back upstream as a counter current inshore of the main flow. The modification of sediment transport pathways by residual tidal current vortices may affect the development of nearshore and offshore engineering, and should be considered in any initial site assessment. • Bedform nature reflects complex flow dynamics approaching a bedload parting site. • Bedform dynamics align with complex residual tidal current. • Recirculation of sediment towards bedload parting site observed. • Sediment follows complex pathway departing from site of seabed erosion. [ABSTRACT FROM AUTHOR]

Many tropical river systems have altered water quality due to human land use, impacting the biodiversity of freshwater and coastal ecosystems. Long-term, catchment-scale monitoring is needed to understand pollutant sources, controls, and trends. This 12-year study monitored baseflow and flood event nutrient and sediment concentrations, and estimated sediment loads across the Normanby Basin in northern Australia. Suspended sediment concentrations and yields were highest in upper catchment areas where cattle grazing occurred on erosion-prone sodic soils. Mid- and lower catchment rivers and floodplains were a sink for sediments and nutrients, trapping around 75% of suspended sediments during events. Clays (<4 μm) were preferentially transported to the estuary, with an estimated 46% sediment delivery ratio. In the estuary, suspended sediment concentrations were influenced by tidal resuspension processes and there were significant sources of DIN. These findings can help prioritise land management investments for the protection of Great Barrier Reef and freshwater ecosystems. [ABSTRACT FROM AUTHOR]

This study investigates the sediment resuspension and the resultant morphological changes over the subaqueous delta front in the Changjiang Estuary (CJE). An in-situ observation platform equipped with multi-sensor instruments was deployed in the CJE to measure the currents, waves, and suspended sediment concentrations (SSC) when a typical remote tropical cyclone, Sanba, passed through the waters over 540 km away from the CJE from September 15 to 17 in 2012. In addition, a current-wave-coupled numerical model was employed to simulate the spatial and temporal variations of the waves, currents, and sediment transport. The numerical simulation accurately captures the sharp increases in wave-induced bed shear stress and associated SSC, as indicated in the field measurements during the tropical cyclone period. Remarkable erosional patterns are observed over the subaqueous delta, which highlight the role of the remote tropical cyclone in sediment suspension and short-term morphological changes in the CJE. Moreover, the simulated erosional pattern agrees well with the sediment coarsening area observed between the 2 and 10 m isobaths. Analyses of the wave spectrum suggest that the bed shear stresses induced by the swells prevail over those induced by the wind waves during the tropical cyclone period. In addition, two simulation runs with ideal wind fields were performed to clarify the contribution of the swells propagated from the outer sea to the sediment transport. Overall, this study suggests that the erosion trends of the subaqueous deltas observed in the CJE are largely determined by the waves, and the swells produced by tropical cyclone events can significantly facilitate such a trend. • The sediment resuspension and the associated morphological changes during a remote tropical cyclone event are simulated. • The erosion tendency observed in the subaqueous delta is attributed to waves rather than tides. • Swells contribute the most to sediment resuspension, while the effects of locally generated wind waves are limited. [ABSTRACT FROM AUTHOR]

Channel deepening often triggers positive feedback between tidal deformation, sediment import and drag reduction, which leads to the regime shift in estuaries from low-turbid to hyper-turbid state. In this study, a transition in profiles of suspended sediment concentration (SSC) is hypothesised by including a positive feedback loop of vertical mixing and settling. Such a hypothesis is validated by the historical observations in the North Passage of Changjiang (Yangtze River) Estuary, with decreasing SSC in mid-lower layers and increasing SSC near the bed after the deepening. A mobile pool of concentrated benthic suspensions (CBS) develops in the North Passage, with a tidally averaged length of ~20 km and a mean thickness of ~4 m. The width of the CBS pool is limited (<1 km) as the CBS is concentrated in the Deepwater Navigational Channel. The movements of the CBS pool, combined with tidal asymmetry (e.g., slack-water asymmetry and lateral flow asymmetry), results in sediment trapping in the middle reaches and on the south flank of the channel. Observations by a bottom tripod system show the response of friction/drag coefficient to sediment concentration: (1) nearly linear decrease within low SSC (<10 kg/m3); (2) constant and minimum coefficient (with drag reduction up to 60–80%) in the presence of CBS (10–80 kg/m3). An empirical relationship was derived, which can be used to predict the friction coefficient and the magnitude of drag reduction for sediment transport studies, particularly for modelling regime shifts in estuaries. • A regime shift was observed in the Changjiang Estuary, with increasing SSC near the bed and a decrease in mid-lower layers. • The Changjiang Estuary exhibits a mobile pool of concentrated benthic suspensions (CBS) after the deepening. • The friction/drag coefficient is constant and minimum in the presence of CBS, with a drag reduction of 60‐–80%. • Slack-water and lateral flow asymmetries cause sediment trapping in the middle and south flank of channel, respectively. [ABSTRACT FROM AUTHOR]

Sediment retention time in estuaries plays a significant role in the morphological evolution and estuarine ecosystem. In this study, we used a well calibrated numerical model of the Pearl River Estuary (PRE) and examined its sediment budget, transport processes, and transport timescales as regards to sediment discharge from four outlets. The sediment transport timescales are characterized by sediment age, which means the time elapsed since sediment particles enter into the estuary. The wide PRE (with a Kelvin number = 1.4–7.8) allows river plumes to develop inside the estuary, which is a controlling factor on sediment dispersion. Sediments from the estuary head are initially dispersed downstream when flushed from the outlets. However, a large portion of them are carried to the low-salinity region owing to the development of freshwater plume. These sediments therefore have a larger sediment age than the remainder, which are transported by energetic tidal currents via the high-salinity region. Most riverine sediments from the lateral outlets are constrained in the low-salinity region on the West Shoal. The sediments deposit near the mouths of the lateral outlets because of weak dynamics on the shoal. An additional time is therefore needed for the sediments from the lateral outlets to be transported outside of the estuary. Because of the sediment transport pattern in the estuary, a large portion (58%) of the total riverine sediments ultimately accumulates on the West Shoal. The sediment settling velocity greatly affects the sediment age which ranges between 10 days for sediment with a settling velocity of 0.05 mm/s to approximately 110 days for sediment with a settling velocity of 0.2 mm/s. These findings regarding the fate and transport timescales of riverine sediments have implications for ecosystem managements. • The concept of age is utilized to measure the sediment transport timescales. • The sediment age in a wide estuary is closely linked to the freshwater plume dynamics. • Age of sediment delivered in energetic areas is lower than that dispersed in quiescent regions. [ABSTRACT FROM AUTHOR]

The Nam Dinh Coast is a constituent part of the dynamic coastline, belonging to the Red River Delta. It is bounded to the north and south by the Red and Day Rivers, with Ninh Co and So Rivers located in the middle. This coast has been undergoing severe erosion during the last hundred years. Over this period, vital agricultural area along the coastline has vanished into the sea and families have been forced to abandon their houses due to the erosive action of waves. Numerous studies were conducted to identify the major causes governing the erosion problems, and several hypotheses have been proposed. In the present work, remote sensing data analysis techniques with both optical and synthetic aperture radar (SAR) were employed to examine the hypothesis that land subsidence is contributing to the erosion along the Nam Dinh Coast. The results revealed that the high lateral erosion rates observed at sandy beaches between estuaries are approximately -6 m/yr in the south and -10 m/yr in the north of the So River. Meanwhile, sediments transported from the rivers accumulate mainly around the estuaries, leading to rapid accretions of approximately +93, +4, and +46 m/yr for Day, Ha Lan (So River), and Ba Lat (Red River), respectively. Simultaneously, land is sinking and uplifting at highly uneven rates, with a few sections subsiding 10 times faster than the overall spatial average. Uplifting observed in southern areas ranges from +1.2 to +1.9 mm/yr. In contrast, subsidence was identified in the northern parts, with mean rates varying from -0.1 to -12.3 mm/yr. Based on the Bruun Rule, the combined effects of land subsidence and relative sea level rise (SLR) could explain approximately 66% of the observed rate of erosion in the northern part of Ha Lan. Findings from surface deformation data contribute to a deeper insight into coastal erosion in Nam Dinh. • The erosion mechanism in the Nam Dinh Coast is not yet fully understood. • Combined use of SAR and optical images is applied to detect subsidence and to discuss its effect on coastal erosion. • Land is sinking and uplifting at highly uneven rates, varying from -21 to +2.5 mm/yr. • Combined effects of land subsidence and relative sea level rise contribute to 66% of the erosion. [ABSTRACT FROM AUTHOR]

The Nakdong Estuary, Korea, is a dramatic example of an Anthropocene estuary. Over the last century, the construction of two dams has divided the estuary into two discharge energy regimes with the East Nakdong Estuary receiving discharge and the West Nakdong Estuary receiving no discharge. The mean-flow sediment flux gradient during high discharge was the main mechanism for deposition in the East Nakdong Estuary. Because of limited observations, the sediment transport mechanisms for the entire estuary remained unclear. To complement previous observations that were limited to a high discharge event of the East Nakdong Estuary, this study aimed to better understand the spatiotemporal variation of sediment transport and the mechanisms of deposition throughout the Nakdong Estuary. Sediment fluxes and bed level changes were estimated by utilizing the Coupled-Ocean-Atmosphere-Wave-Sediment Transport (COAWST). Results confirmed that the East and West Nakdong Estuaries had different depositional patterns. The East Nakdong Estuary was characterized as a river-dominated environment with a high influence of river discharge that experiences rapid deposition. The West Nakdong Estuary was characterized as an environment mainly influenced by tides and waves that contribute to gradual deposition throughout the year. The mean-flow sediment flux by river discharge was a main mechanism of deposition in the East Nakdong Estuary after the construction of dam. The continuous landward mean-flow flux by tides and waves through the inlets was the dominant mechanism for deposition in the West Nakdong Estuary. These model results suggest that the Nakdong Estuary may serve as an analogue for a microtidal estuary altered by an estuarine dam, particularly those for which there is a monsoonal climate or anthropogenically reduced river discharge. • The spatiotemporal variation of sediment transport in the Anthropocene Nakdong Estuary was explored by numerical modeling. • The East Nakdong Estuary was characterized as a river-dominated environment with a high influence of river discharge. • The mean-flow flux convergence occurred in the East Nakdong Estuary during high discharge, causing rapid deposition. • The West Nakdong Estuary was mainly influenced by tides and waves on sediment transport throughout a year. • The continuous landward mean-flow flux occurred in the West Nakdong Estuary over a year, contributing gradual deposition. [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]

Morphological variations in a sandy-muddy transitional beach (SMT-beach) are expected to coincide with sand-mud transition (SMT) boundary migrations. SMT migrations of SMT-beaches should also differ in river estuaries and semi-enclosed bays due to their contrasting dynamic sedimentary environments. However, the anticipated diversity of SMT migrations of estuarine and bay beaches and their potential morphodynamic responses are still poorly understood. Thus, four measurements of SMT spatial distributions and typical profiles of 6 estuarine beaches and 6 bay beaches located along the macrotidal coasts of the Taiwan Strait, East Asia are presented, and these sites were studied in April and August of 2018 and 2019, respectively. The results show that the SMT boundary of the estuarine beaches continuously migrates landward with increased elevations. Simultaneously, the SMT boundary of bay beaches synchronously remains relatively stable with minor fluctuations and relatively stable elevations. The SMT migration amplitudes present a significantly positive linear correlation with SMT elevation changes, which are strongly controlled by the sediment supply. Following the SMT migration processes, both estuarine and bay beaches display clear erosion during the summer monsoon period that is mainly associated with strong river discharge and monsoon rainfall, respectively. In contrast, all beaches present distinct accretion and recovery during the winter monsoon period that are mainly due to the decreased monsoon rainfall and river discharge and/or the strengthened onshore sediment transport. Despite limitations, the results of this study have clarified the diverse SMT migrations of estuarine and bay beaches and aid in a better understanding of their possible morphodynamic responses. Unlabelled Image • SMT migration of estuarine beach has a significant difference from that of bay beach. • A landward SMT migration is linearly correlated with an increased SMT elevation. • Sediment supply drives the contrasting SMT migrations of estuarine and bay beaches. • All SMT-beaches present dominate erosion in summer and clear accretion in winter. [ABSTRACT FROM AUTHOR]