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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]

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]

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]

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]

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]

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]

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]

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]

The Rouse profile has been traditionally used to represent the vertical distribution of suspended sand in a marine benthic boundary layer. Yet it is one of the biggest unknowns in estuarine morphodynamics, largely due to uncertainties of the ratio of the sediment fall velocity to bed friction on which the Rouse exponent (R=w s/βku ⁎) is based. A field campaign was carried out at three different locations in the Oka estuary, northern Spain, in order to examine these uncertainties. Each location differed in grain size and flow condition thus offering a wide range of settings. The first survey was inside the estuary (wave sheltered, flood tide dominated and relatively broad estuary section), the second was at the distal ebb delta (ebb tide dominated and narrow estuary section), and the third was over the wave exposed proximal ebb delta (wave/flood tidal current combined flows and open sea). The aim of this study is to evaluate the applicability of the Rouse (1937) theory for the distribution of sand in suspension throughout a turbulent benthic boundary layer. A modified version of a Helley–Smith sampler was used to trap sand and measure the vertical distribution of sand in the water column. As well, a 1200 ADCP was used to measure flow velocity and backscatter together with an ADV (turbulence). The sand traps were found to have a sampling efficiency of 44%. The grain size at all stations was finer near the surface and coarser near the bed. The sand transport inside the estuary (Station 1) is inwards dominant. By contrast, the sand concentration during the ebb tide was ten times higher than during the flood tide at Station 2 and even higher at Station 3, which suggests that the sand transport over the ebb delta is seawards. The average Rouse parameters for Stations 1, 2, and 3 are 0.48±0.035, 0.78±0.23, and 0.46±0.06 respectively, which correspond to a coefficient of proportionality of the movability number, (χ) of 4 (Van Rijn, 1993). These differ from previous findings of Villatoro et al. (2010) and Amos et al. (2010b). [ABSTRACT FROM AUTHOR]

Abstract: Huangmaohai Estuary is a micro-tidal funnel-shaped estuary, located along the southwestern side of the Pearl River Delta complex. Variations of sediment transport patterns under different conditions of river discharge and tidal mixing are investigated by using field measurements and data analysis during both dry and wet seasons, respectively. The intratidal variation of sediment dynamics is largely controlled by the tidal asymmetry. The typical pattern of 25-hour mean sediment transport during the dry season is that the transport is landward in the channel and seaward on the shoals. A bifurcation pathway of sediment transport shows that sediments are imported from the East Opening and exported through the Middle Opening. However, this pattern can be altered by mixing processes and river discharge. Enhanced mixing or increased discharge can result in a predominantly seaward transport. Conversely, weak mixing can result in an emphatic landward transport. In general, the sediment transport is closely associated with the morphological evolution in the estuary. [Copyright &y& Elsevier]

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]

Abstract: Zooplankton time series collected with different temporal resolution and coverage were examined to characterize seasonal and diel patterns in the abundance of Munida gregaria larvae and juveniles in the Baker river estuary. Zoeae were more abundant in late winter and spring, coinciding with the season of lower sediment transport and higher primary production in the region. The occurrence of juveniles was exclusively in summer. There was a significant correlation between the abundance of zoeae and high-frequency temperature variability near the pycnocline over periods of 7–20 and 26–30 days prior to each plankton sampling. These time scales of correlation suggest that internal motions may be a proximal environmental cue for lunar rhythms in larval hatching, rather than directly causing the aggregation of larvae at the sampling area. To characterize shorter-term patterns in larval abundance and vertical distribution, stratified samples were collected every 3h over a full late-spring day (November 2008) near one of the monitoring stations. Zoeae were significantly more abundant at 10–25m depth (p=0.039), and changes in depth-integrated abundance of both zoea and megalopae were strongly associated with the tidal cycle. Together, these results suggest that the spatial structure and population dynamics of M. gregaria in this region may respond to the combined forcing of seasonal changes in freshwater inputs, tidally-driven processes such as lateral transport of larvae and juveniles, and internal-wave mediated changes in local conditions. [Copyright &y& Elsevier]

Abstract: Spatial variations in grain-size parameters contain information on sediment transport patterns. Therefore, in this study, 106 surficial sediment samples taken from the Pearl River Estuary (PRE), South China, were analyzed, to better understand the net sediment transport pattern in this region. The PRE is an area of fine-grained sediment and shows muddy patches with relatively coarse sediment in the north. The sorting coefficient of surficial sediment over the whole estuary is higher than 2, indicating a very poor sorting in this region. The relationship between the sand, silt and clay contents of the sediments was analyzed by a log-ratio analysis, and it was indicated that the selective deposition is a non-linear function of the sediment mixture composition. The net sediment transport pathways indicate four distinct characteristics of sediment transport over the PRE. The sediment is transported obviously southeastward in the upper part of the estuary due to the relatively stronger fluvial dynamics and northwestward in the lower part, due to the stronger tidal dynamics. However, in the central part of the PRE, the sediment transport vectors display a clockwise rotation trend and net deposition is taking place, as indicated by the convergence of the grain size trend. The trend vectors show that some of the sedimentary material is supplied from upper and lower parts of the estuary to this depocenter. This result is consistent with the residual current circulation pattern near Neilingding Island. The patterns of sediment transport reveal directions with the main areas of deposition and possible dispersal patterns in the PRE. [Copyright &y& Elsevier]

Abstract: A two-dimensional (2-D) numerical model and a three-dimensional (3-D) numerical model of the Yangtze Estuary, including tidal flow, suspended sediment transport and bed evolution, were established based on the Delft3D-FLOW model respectively. The models were calibrated with the field data. The calibrated models were employed to predict the morphological evolution process of the Yangtze Estuary for the next 20 years, with the predicted area covering the Chongming Eastern Beach and the Nanhui Tidal Flat. According to the simulated results in the medium term, due to the progress of the Three Gorges, erosion will happen in the southeast and south region and deposition will happen in the northeast and north region of the Chongming Eastern Beach, while a deep channel will be formed gradually in the middle part. The Nanhui Tidal Flat will develop southeastwards. Five scenarios of different river discharges and sediment loads are obtained from an exponential regression equation between flow rate and suspended sediment concentration deduced from the monthly averaged flow rate and suspended sediment concentration during 2003–2009. These five scenarios were used for investigating the topographic responses of the Chongming Eastern Beach and the Nanhui Tidal Flat to different river discharge and sediment supply conditions. The predictions show that the erosion rate will decrease and the deposition rate will increase with the increase of sediment supply, i.e., less the sediment supply leads to more recession. [Copyright &y& Elsevier]

Abstract: We compute rates of sediment removal from the Columbia River, USA plume near field using a control volume technique. By using data from repeated transect passes along the plume axis, we account for variations in plume width, depth, and velocity, and we construct a framework for determining the dominant terms in the complete sediment balance. The contribution due to turbulent sediment flux is estimated using collocated measurements of turbulent shear and suspended-sediment concentration. The maximum sediment clearance rate on the landward end of the transect, 4km from the river mouth, is approximately 0.1gm−2 s−1, which corresponds to an effective removal velocity of 10mms−1. This is significantly higher than the settling velocities expected for single-grain and aggregate particles observed in the estuary. The plume clearance rate decreases seaward, with values of 0.01gm−2 s−1 6.5km from the river mouth. We investigate several potential mechanisms to explain the along-axis variability in sediment removal, including gravitational settling of several particle-size distributions and removal by turbulent mixing. A simple model that accounts for the settling of the distribution of particles observed in the estuary with the addition of a very fine sand settling class best explains the observed magnitude and the seaward decrease in the sediment removal rate. Although it is unlikely that fine sands are retained in the plume from the estuary due to their high settling speeds, we hypothesize that they may be locally resuspended landward of the transect. The turbulent flux of sediment out of the plume is almost an order of magnitude smaller than the total removal rates observed on the landward end of the transect. However, the turbulent flux generally is greater than the removal rate predicted for unflocculated fine silt, indicating that it may be important for smaller particle size distributions or farther from shore. [Copyright &y& Elsevier]

Abstract: In coastal and estuarine waters, depth-averaged suspended sediment concentration (DASSC) at a fixed observation site has two sources: local resuspension and advection along a horizontal gradient. The empirical decomposition method can separate the horizontal residual suspended sediment flux (HRF) into several terms, e.g., Eulerian flux, Stokes'' drift and tidal pumping. A simple depth-averaged 1D model is solved analytically for an observational site over one semi-diurnal tidal cycle to explore the determinants of the forcing factors comprising residual, M2 and M4 current velocities, mean water depth, M2 tidal amplitude and the DASSC gradient. The analytical solutions provide clear physical explanations of tidal DASSC variations and HRF. The solutions are applied to fit and explain the observations at a fixed station in a macro-tidal channel located along the Jiangsu Coast, China. The results suggest a dependence of the M4 DASSC variation on resuspension induced by the M2 velocity, and the dependence of the M2 DASSC variation on the advection and resuspension induced by the interaction between the M2 and M4 velocities. The horizontal DASSC gradient results in large seaward tidal pumping, which is offset by the landward Lagrangian flux to produce a reduced landward HRF. [Copyright &y& Elsevier]

Abstract: This study fits into a wider research program from RWS RIKZ concerning the exchange of sediment between the coast and the tidal basins, Western Scheldt and Wadden Sea, which are the largest basins along the Dutch Coast, over different time-scales. For both basins, questions about the evolution of the import/export at the mouth recently arose. In case of the Western Scheldt, which is the subject of this study, mainly the uncertainty about the future developments after the change from import to export at the mouth was noticed in the 1990s, which necessitated a more detailed study of this area. To understand the sediment exchange between the Dutch coast and the tidal basin Western Scheldt, the mechanism responsible for this transport must be identified. This study focuses on only one possibly important mechanism: the interaction between the tides and bathymetry. An existing numerical, depth-averaged Delft3D model of the Western Scheldt has been applied. The bathymetry of the years 1970, 1983 and 2002 was selected based on previous sand balance studies. Thus three different situations at the mouth are represented: strong import, import and export of sediments. Each situation was forced by the same morphological tide, where a tide is selected in such a way that during ebb and during flood the computed sediment transport through a cross-section near Vlissingen is similar to the average ebb and flood transport for a complete spring–neap tidal cycle. A comparison was made of the vertical tidal asymmetry in the model with field data for calibration purposes. Then, both the vertical and horizontal tides were analysed. Time series of water levels, velocities and discharges were harmonically analysed to derive the M 2, M 4 and M 6 tidal constituents. The tidal asymmetry is expressed by the amplitude ratios and the phase differences. Furthermore the tide-driven sediment transport ascribed to a given bathymetry was determined. The residual transport patterns were analysed and a sand balance was derived from the modelled transports. This led to the evolution in time of the sediment exchange between, and the erosion/sedimentation within, different macro cells () of the Western Scheldt. The human interventions, the changes in tidal asymmetry and the evolution in the sediment transport interact. The increased landward transport inside the river is reproduced in the model; however, the change from sediment import to sediment export at the mouth could not be explained by tidal asymmetries only. Other processes such as winds and waves should also be studied to determine the mechanisms, which influence most of the observed sediment exchanges. [Copyright &y& Elsevier]

Abstract: The Hangzhou Bay is a macro-tidal bay located to the south of the Changjiang estuary in China. Along its northern shore, a large-scale tidal channel system has developed, which includes a main northern tidal channel, with a length of more than 50km and a width up to 10km, and a secondary southern tidal channel. A process-based morphodynamic model, incorporating the cohesive sediment transport module of Delft3D, is used to analyze the physical processes and mechanisms underlying the formation and evolution of this tidal channel system. The results show that spatial gradients of flood dominance, caused by boundary enhancement via current convergences, is responsible for the formation of the channel system, due to a combination of the various factors such as funnel-shaped geometry hindering associated with the presence of islands, and flow deviation by the southern tidal flat and so on. The model results agree well with the real morphological features. This study also indicates that the reclamation of the southern tidal flat imposes a profound influence on the morphological evolution of the tidal channel system in the Hangzhou Bay. It is feasible to use the model to simulate long-term estuarine morphological changes with cohesive sediment settings. [Copyright &y& Elsevier]

Abstract: Modeling geomorphic evolution in estuaries is necessary to model the fate of legacy contaminants in the bed sediment and the effect of climate change, watershed alterations, sea level rise, construction projects, and restoration efforts. Coupled hydrodynamic and sediment transport models used for this purpose typically are calibrated to water level, currents, and/or suspended-sediment concentrations. However, small errors in these tidal-timescale models can accumulate to cause major errors in geomorphic evolution, which may not be obvious. Here we present an intermediate step towards simulating decadal-timescale geomorphic change: calibration to estimated sediment fluxes (mass/time) at two cross-sections within an estuary. Accurate representation of sediment fluxes gives confidence in representation of sediment supply to and from the estuary during those periods. Several years of sediment flux data are available for the landward and seaward boundaries of Suisun Bay, California, the landward-most embayment of San Francisco Bay. Sediment flux observations suggest that episodic freshwater flows export sediment from Suisun Bay, while gravitational circulation during the dry season imports sediment from seaward sources. The Regional Oceanic Modeling System (ROMS), a three-dimensional coupled hydrodynamic/sediment transport model, was adapted for Suisun Bay, for the purposes of hindcasting 19th and 20th century bathymetric change, and simulating geomorphic response to sea level rise and climatic variability in the 21st century. The sediment transport parameters were calibrated using the sediment flux data from 1997 (a relatively wet year) and 2004 (a relatively dry year). The remaining years of data (1998, 2002, 2003) were used for validation. The model represents the inter-annual and annual sediment flux variability, while net sediment import/export is accurately modeled for three of the five years. The use of sediment flux data for calibrating an estuarine geomorphic model guarantees that modeled geomorphic evolution will not exceed the actual supply of sediment from the watershed and seaward sources during the calibration period. Decadal trends in sediment supply (and therefore fluxes) can accumulate to alter decadal geomorphic change. Therefore, simulations of future geomorphic evolution are bolstered by this intermediate calibration step. [Copyright &y& Elsevier]

Abstract: A three-dimensional, hydrostatic, primitive equation numerical model with modern turbulence closures is used to explore lateral circulation and the associated transport of sediments in idealized, moderately to highly stratified estuaries. The model results suggest that boundary mixing on a sloping bottom can drive a significant amount of lateral circulation. This mechanism has received little attention to date in the estuarine literature. Good agreement with an analytical solution and similar vertical structures of lateral flows to observations from the Hudson River estuary support the importance of the boundary mixing mechanism. Boundary mixing is at least as important as differential advection for the modeled scenarios, when the two mechanisms are evaluated using the salt balance equation for model runs without rotation. Linearly superposing analytical solutions for lagged boundary mixing lateral flow and Ekman-forced lateral flow yields a good representation of the near-bottom lateral flow from the model with rotation. The 2h lag required for the boundary mixing solution is roughly equal to the vertical diffusion time scale, indicating that lateral flow adjustment depends on development of a bottom mixed layer. Sediment dynamics at cross sections seaward and landward of the salt intrusion are very different. Seaward of the salt intrusion, sediments are eroded in the channel and preferentially deposited on the right slope (looking seaward), mainly due to the combination of high sediment concentration in the channel during flood with strong up-slope transport on that side (tidal pumping). Lateral sediment re-distribution landward of the salt intrusion is negligible due to weak residual lateral circulation. [Copyright &y& Elsevier]

Abstract: Motivated by field studies of the Ems estuary which show longitudinal gradients in bottom sediment concentration as high as O(0.01kg/m4), we develop an analytical model for estuarine residual circulation based on currents from salinity gradients, turbidity gradients, and freshwater discharge. Salinity is assumed to be vertically well mixed, while the vertical concentration profile is assumed to result from a balance between a constant settling velocity and turbulent diffusive flux. Width and depth of the model estuary are held constant. Model results show that turbidity gradients enhance tidally averaged circulation upstream of the estuarine turbidity maximum (ETM), but significantly reduce residual circulation downstream, where salinity and turbidity gradients oppose each other. We apply the condition of morphodynamic equilibrium (vanishing sediment transport) and develop an analytical solution for the position of the turbidity maximum and the distribution of suspended sediment concentration (SSC) along a longitudinal axis. A sensitivity study shows great variability in the longitudinal distribution of suspended sediment with the applied salinity gradient and six model parameters: settling velocity, vertical mixing, horizontal dispersion, total sediment supply, fresh water flow, and water depth. Increasing depth and settling velocity move the ETM upstream, while increasing freshwater discharge and vertical mixing move the ETM downstream. Moreover, the longitudinal distribution of SSC is inherently asymmetric around the ETM, and depends on spatial variations in the residual current structure and the vertical profile of SSC. [Copyright &y& Elsevier]

Abstract: The study of plant-bound nutrient transport has been largely neglected in estuaries. Lately however, it has been shown that nutrients bound to macroalgae and seagrasses can constitute a major part of the nutrient transport in shallow tidal estuaries. Organic detritus in estuaries comes from various sources. This paper looks into the source of detritus from sloughing, and the transport behaviour of plant detritus under unidirectional flows. In order to determine the extent of the sloughing of macrophytes, the threshold current velocities for the traction and resuspension of the most common submersed macrophytes in Venice Lagoon (Ulva lactuca, Enteromorpha sp., Ceramium rubrum, Cladophora sp., and Chaetomorpha linum) were studied in a laboratory flume. It was found that all macrophytes subjected to flows of 1.5–3.0cms−1 move initially as bed load. The threshold for suspension of the macrophyte tissue was at current speeds >3cms−1. The exception was the filamentous macroalgae, C. linum, which moved as bed load at all current speeds. This implies that the advection of plant-bound nutrients in Venice Lagoon is widespread and takes place over virtually all stages of the tide. Initial experiments were carried out on unattached macroalgae. A second study focused on the sloughing of attached macroalgae by steady currents. The threshold current speeds at which sloughing commenced varied between different types of algae, and sloughing rate was related largely to current speed. The resuspension rates were uniform between the different macrophyte groups. Our results help explain why plant matter has been trapped in nets close to the bed of Venice Lagoon on ebbing tides. It shows that a major component of the bedload is organic in origin. The results verify that a large proportion of the net nutrient export from estuaries is bound in macrophyte tissue. These findings need to be included in future ecological models that describe the resuspension, sloughing and settling of macrophytes. [Copyright &y& Elsevier]

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]

Abstract: Sediment–turbulence interactions near the bed are still poorly understood in highly turbid estuaries, especially in the presence of fluid mud layers. This results primarily from the difficulty in measuring co-located velocity and suspended sediment concentration (SSC) at sufficiently high rate to resolve small turbulent flow scales. In this paper, we show how a set of commercial acoustic and optical backscattering systems known as ADCPs, ADVs and OBSs, can be deployed and used in a complementary way to perform large-scale profilings of tidal current and SSC combined with high-resolution velocity and SSC measurements in the highly turbid near-bed zone. The experiment was done in the Gironde estuary (France) which is well known for its turbidity maximum zone characterized by high SSC values, above 1gl−1 near the surface. A first simple inversion method is proposed to convert the backscattered acoustic intensity measured with ADV into SSC data in the highly turbid near-bed zone. Near-bed SSC data from the OBS are used to compensate for the important acoustic sediment attenuation effect at an acoustic frequency of 6MHz. No a priori knowledge of acoustic backscattering properties of mud suspensions is required with this calibration procedure. We obtain an attenuation coefficient for mud suspensions of 0.28m2/kg at 6MHz leading to a good agreement between the SSC timeseries from the three ADV receivers and the OBS over the entire tidal cycle. The obtained SSC data are then analyzed with respect to the near-bed velocity, Reynolds shear stress and turbulent kinetic energy (TKE) timeseries in order to identify the relevant sediment transport processes during the tidal cycle. Significant differences in bed shear stress and TKE levels are found between ebb and flood stages with effects on near-bed sediment dynamics. During the ebb, maximum levels of tidal current, bed shear stress and TKE are associated with a reduction of near-bed sediment concentration (from 400kgm−3 down to 100kgm−3). Bed liquefaction process is assumed to occur at this moment with the presence of highly concentrated mud layer and a possible lutocline at a distance of less than 20cm above the bed. During the first 1.5h of flood, turbulent activity remains moderate. The near-bed flood current is then inhibited very abruptly while a sudden increase in SSC occurred above the bed. Assuming that the ADV is able to estimate relevant turbulent erosion fluxes, the co-located velocity and SSC are multiplied and compared with settling flux measurements made onboard under quiescent water conditions. The mean sediment settling fluxes (averaged over 3min) increase with SSC and are in relative good coherence with fluxes in quiescent water below the hindered regime, for SSC below 15gl−1. Reducing averaging time from 3min to 30s allows to increase the range of turbulent fluxes and SSC values, up to 99gl−1. At this scale, fluxes keep increasing quasi-linearly at higher SSC, suggesting the inhibition (delay or reduction) of the hindered settling regime as previously shown by from laboratory experiments. However, the 3-min averaged concentration field remained too low to conclude definitively on the effectiveness of such a process. Further analysis conducted at higher SSC regimes and under fully verified equilibrium are necessary. [Copyright &y& Elsevier]