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Based on acoustic detections from a network of five stations along the south coast of Portugal, 25 transits of tagged meagre were identified between 2019 and 2020, mostly in summer. Comparisons with hourly current measurements from a moored Acoustic Doppler current Profiler and an High Frequency Radar system indicate that meagre move westward with favourable alongshore flows at a confidence level >90% (while eastward transits observations were too few to be significant). Several similar transits of different individuals occurred within two days suggesting a species response to environmental changes. The analysis of surface seawater temperature from ERA5 shows that meagre always swam westward towards colder water, despite their warm water affinities. As such, they reached food-rich upwelling areas when the water temperature was higher than usual (due to the westward advection of warm water), probably optimizing their feeding ability. The demonstrated alongshore meagre movements in response to temperature variations induced by the mesoscale coastal circulation is particularly relevant for the management of this high economic value species in southern Portugal. • The alongshore movements of meagre were identified using acoustic telemetry. • The movement direction was compared with measured coastal currents. • Meagre move westward with favourable alongshore flows. • This behaviour is triggered by temperature variations induced by flow advection. [ABSTRACT FROM AUTHOR]

Characterizing the spatial variability of net flow transport in a tide-dominated area like the East China Sea (ECS) is key to understanding the material and heat budget as well as their impacts on local ecosystem. However, the residual circulation in the ECS was much less confirmed by observations as it can only be derived from very sparse mooring data. Here for the first time we obtain the spatial patterns of tidal and residual currents in the inner ECS shelf from ADCP underway measurements conducted in 12 cruises using a spatial least squares (SLS) fitting technique. The SLS method is validated with the traditional harmonic analysis using numerical simulation results and then implemented with the observational data. The tidal current is dominated by two semidiurnal components M2 and S2 in this area, with the major axis of tidal ellipse along northwest-southeast direction. The ADCP-derived tide parameters are in good agreement with those from the mooring data and barotropic tide model. The annual spatial pattern of residual circulation is characterized by a southward coastal current and a northward shelf current, which are identified as the East China Sea Coastal Current (ECSCC) from Changjiang River runoff and the Taiwan Warm Current (TWC) originated from Taiwan Strait and Kuroshio. Their magnitudes are comparable but of one order smaller than tides. The vertical structure of TWC shows a subsurface-intensified feature in line with previous mooring observations. The north limit of TWC is approximately at 31.5°N around the submerged valley, beyond which the northward TWC becomes almost invisible. By a series of virtual cruise experiments, we show that the observation data length is the most crucial factor controlling the SLS fitting accuracy, while the role of other factors like sampling frequency is secondary. With a fixed-length dataset, the degree of polynomial function used for SLS fitting must be properly determined to minimize the under-fitting and over-fitting error, and there is also a trade-off in the fitting accuracy between tidal and residual currents depending on the fitting strategy. • The tidal and residual circulations in the ECS are separated using a spatial least squares fitting method with 4-year ADCP underway measurements. • The annual residual circulation is characterized by a southward alongshore current and a northward subsurface-intensified shelf current. • Data length is of utmost importance for a more accurate fitting of tidal and residual currents and other parameters should also be carefully selected. [ABSTRACT FROM AUTHOR]

Subtidal circulation patterns were studied along an elongated hypersaline system. Month-long data were collected using three acoustic Doppler current profilers. These profilers were moored at depths of around 15 m in three locations: 10 km inland from the mouth of the lagoon, within the central region, and near the head of the lagoon. The variability of temporal and spatial subtidal circulation was explored using Empirical Orthogonal Functions analysis, which explained 93% of the variability through the first three modes. The first mode was influenced by tidal advection, resulting in a tidally-driven residual circulation for elongated channels, with inflows occurring over the shoals and outflows in the deeper channel sections. During the neap-tide cycle, inverse density gradients became relevant, coinciding with the peaks of mode 2 amplitude that accounted for 15% of the total variability. The gravitational circulation exhibited its highest intensity near the head, characterized by a vertically sheared flow with inflow occurring near the surface and outflow underneath. Predominant onshore wind stresses had a significant impact on the subtidal circulation and displayed a strong correlation with the mode 3 amplitude, explaining 11% of the total variability. Wind-driven circulation exhibited a vertically sheared profile, characterized by downwind near-surface flow and upwind flow underneath in the lagoon mouth and head, and a homogeneous downwind profile in the central part of the lagoon. It is concluded that tidal stresses governed the subtidal circulation, followed by density gradients, and wind stresses modified spatial circulation patterns induced by tides and density gradients. This research provides new observational evidence on tidal residuals in long channels and contributes to the understanding of the interactions between the main subtidal circulation drivers in hypersaline lagoons. • Tidal advection governed the subtidal circulation of a hypersaline coastal lagoon. • Density gradients became relevant during neap tide and dominated the circulation near the head. • Wind-driven flow appeared during strong wind conditions and displayed a vertically sheared profile near the mouth. [ABSTRACT FROM AUTHOR]

To reveal the mechanism for the transport of water and suspended sediments by wind-induced estuarine residual circulations, in situ moorings of acoustic Doppler current profilers were conducted in microtidal Onsan Bay. Water circulation within the bay was characterized by freshwater discharge from the river and free exchange with the open sea, showing current seaward outflow and landward inflow in the western (M1) and eastern (M2) flanks of the navigation channel, respectively. This circulation was influenced by the intrusion of alongshore currents strengthened (weakened) by the northerly (southerly) winds. In M1, seaward outflow was dominant, except for surface disturbances caused by winds. Meanwhile, the classical estuarine circulation (72% of the entire period) in M2, which prevailed under the southerly winds, was switched into the reversed estuarine circulation when the stress of the northerly wind persistently exceeded 0.005 Pa (> 5 h). The water transport (Tr w) in M1 did not significantly vary depending on the wind direction, but the Tr w in M2 could be reduced by 53% within 10 m above the bed as the southerly winds were transited into northerly winds. Over the entire period, Tr w tended to be mutually compensated for and balanced by the circulation system between M1 and M2. On average, however, the sediment transport (Tr sed) in M2 was 37% higher than M1, indicating that the suspended sediments were preferentially deposited within the bay. Most of the suspended sediments causing the deposition within the bay were mainly transported by estuarine residual circulation rather than tidal pumping. In M2, the frequent resuspension events of the fine-grained sediments (d 50 = 6 μm) due to the high variability of the along-channel current velocity could maintain a sediment concentration of 18 mg l−1 in the bottom layer. These suspended sediments responded mainly to landward current in bottom layer (classical estuarine circulation) rather than seaward current (reversed estuarine circulation), contributing to increase in Tr sed. Considering that the prevailing winds from 2013 to 2020 in Onsan Bay mainly create a favorable condition for the development of the classical estuarine circulation, the sedimentation is expected to persist. [Display omitted] • Wind forcing induced switch between classical and reversed estuarine circulations. • Water transport was balanced by circulation system between water inflow and outflow. • Suspended sediments were controlled by residual currents rather than tidal pumping. • Sediment transported by classical estuarine circulation caused sedimentation in bay. [ABSTRACT FROM AUTHOR]

The typical bidirectional pattern of subtidal flow observed at the mouths of estuaries is produced by inflows during high-tide (flood) and outflows during low-tide (ebb) periods. This flow pattern can be modulated in the presence of a strong river discharge, as observed for the Alvarado Estuary (western Gulf of Mexico) during the rainy season. Herein, six transect repetitions were obtained using a towed Acoustic Doppler Current Profiler (ADCP) at an average speed of approximately 2 m/s during a diurnal tidal cycle from September 20 to 21, 2009. The water column during this period was strongly stratified at the mouth of the estuary due to the amount of discharge from the Papaloapan River. Furthermore, the temperatures within the estuary were directly related to the distance from the Papaloapan River. Based on salinity data obtained at four locations along the estuary, the Alvarado Estuary could be considered for the first time as a transitional system from oligohaline to mesohaline (salinities between 5 and 18). The currents and backscatter intensities of the ADCP revealed concentrated areas of suspended particles in the deepest area of the estuary during flood tide periods. During ebb tide periods, estuarine water primarily exited through its mouth (∼90%), followed by the artificial channel (∼10%) located in the northernmost area of the estuary. The residence time during this season was 3.99 days. Finally, the typical estuarine flow pattern observed during flood and ebb tide periods was only depicted near the bottom of the mouth of the estuary. However, the near-surface flow was always toward the ocean (outflow), regardless of the tidal stage. • The Alvarado estuary is a transitional system from oligohaline to mesohaline. • There is a constant surface seaward flow at the mouth, regardless of the tidal stage. • The artificial channel has a common ocean-estuarine flow pattern. • 90% of the water exit through its mouth and only 10% through the artificial channel. [ABSTRACT FROM AUTHOR]

A previously unreported internal tidal bore, which evolves into solitary internal wave packets, was observed in Port Susan, Puget Sound, and the timing, speed, and amplitude of the waves were measured by CTD and visual observation. Acoustic Doppler current profiler (ADCP) measurements were attempted, but unsuccessful. The waves appear to be generated with the ebb flow along the tidal flats of the Stillaguamish River, and the speed and width of the resulting waves can be predicted from second-order KdV theory. Their eventual dissipation may contribute significantly to surface mixing locally, particularly in comparison with the local dissipation due to the tides. Visually the waves appear in fair weather as a strong foam front, which is less visible the farther they propagate. [ABSTRACT FROM AUTHOR]

We analyzed strong events of coastal poleward along-shore currents above 10 cm s −1 and up to more than 50 cm s −1 on the inner shelf (50–80 m depth) of the Bay of Biscay (BoB) from the Spanish coast to the Brittany coast. We used data from four acoustic Doppler current profilers (ADCPs) deployed from July 2009 to August 2011. The goal of this study was to analyze current variability at meso- and subinertial scales and its generation mechanism. These currents occurred all year long and were classified into three types. Events occurring principally in the southern part of the BoB were classified as southern events . Bay-scale events were defined when strong poleward currents were detected over all the shelf, typically stronger on the Spanish and the southern Brittany shelves. Strong events were characterized by depth averaged current velocities over 40 cm s −1 in the southern part of the BoB. At short time lags, the along-shore currents were clearly related to along-shore wind stress at upstream locations. An explanation is provided for longer time lags in terms of coastal trapped wave (CTW) dynamics. The first CTW mode phase speeds were in agreement with the propagation speeds of the fastest events (> 5 m s −1 ), while inner shelf modes could explain the slowest events (∼ 1–3 m s −1 ). The cross-shelf density gradient and the extension of the IPC were also associated with strong coastal poleward along-shore currents. The duration of the events, the vertical structure of the currents and the associated coastal trapped waves were studied in relation with the stratification. [ABSTRACT FROM AUTHOR]

Larval retention plays a fundamental role in the persistence of coastal fish assemblages. Here, we examine larval fish distribution and abundance patterns in Palma Bay, a large (∼20 km) wind-driven microtidal bay in the southern coast of Mallorca (Spain, NW Mediterranean Sea). Larval fish assemblage structure in the bay were analyzed during July 2010 and interpreted in the context of the observed circulation patterns, adult habitat distribution and spawning traits. Acoustic Doppler Current Profiler (ADCP) observations showed the presence of retentive flow patterns in the middle of the bay enhancing local larval accumulation and self-recruitment. In consequence, larval abundances were higher in this central part of the bay (∼40 m depth, mean abundance 607.6 ± 383 ind. 10 m −2 ) than along the coastal fringe (<35 m 113.7 ± 91 ind. 10 m −2 ). Also, a multivariate predictive approach based on Redundancy Analysis (RDA) revealed differences between the larval fish assemblages in areas inside the bay, constituted by small pelagic and benthopelagic taxa (gobids, Chromis chromis and Serranus hepatus ) and offshore larvae, mostly from meso and large pelagic fish. These larval fish assemblages were structured according to depth variations and zooplankton abundance, and remained relatively unmixed because of the circulation patterns in the mouth of the bay that uncouple its dynamics from alongshelf circulation. Even larvae of typically pelagic species that spawn close to the coast ( Sardinella aurita , Auxis rochei ) were associated with the retentive effect of the bay. Our study highlights the important role of coastal bays in the regulation of coastal fish population dynamics and as hotspots for the maintenance of diversity in the Mediterranean Sea. [ABSTRACT FROM AUTHOR]

Field observations were conducted at an inflection point of two bends of the Oujiang River Estuary, China, to investigate lateral circulation and its driving mechanisms, and the effect of stratification in a macrotidal estuary. Repeated cross-channel transect measurements of velocities were obtained by vessel-mounted acoustic Doppler current profiler at the inflection point during neap and spring tides. Conductivity, temperature, and depth were collected at both ends of the transects to estimate the lateral moment budget. We observed asymmetry of lateral circulation in intra-tidal and inter-tidal regions, with strong single-cell lateral circulation during the neap ebb tide. In contrast, during the flood, lateral circulation was weak but structurally complex. During the spring tide, lateral circulation only appeared at peak ebb and peak flood. This asymmetric variation in lateral circulation is induced by an asymmetric driving mechanism, investigated thoroughly and connected to water stratification. The significant effect of nonlinear advective acceleration in both highly stratified water and well-mixed water indicates the important influence of an inflection point of two bends on lateral circulation. Besides, the strong stratification plays a role to suppress the lateral circulation due to the tilted pycnocline spin opposite to lateral circulation. The Oujiang River Estuary is an important shipping area, and the results have practical significance for the development and utilisation of port and channel resources. • Field data were collected at an inflection bend of the Oujiang River Estuary. • Cross-channel measurements of current velocity obtained by vessel-mounted ADCP. • Strong, asymmetrical lateral circulation occurs during the neap ebb tide. • During the flood, lateral circulation is weak but structurally complex. • Findings have practical significance for port and channel resources. [ABSTRACT FROM AUTHOR]

Sub-inertial currents (SICs) over the continental shelf of the northern South China Sea (NSCS) are investigated using the in-situ current observations of acoustic Doppler current profiler (ADCP) mooring arrays off the Pearl River Estuary in 2006 and 2007. The survey was carried out in four separate time periods: summer, winter, spring (before the onset of the southwesterly monsoon), and fall (after the establishment of the northeasterly monsoon). The observations showed that the current directions were generally along the shelf, consistent with the directions of monsoonal winds. The currents were also affected by a few of mesoscale eddy events. In summer 2006, the volume transport was northeastward with a mean magnitude of 1.4 Sv through a cross-shelf section from the site of the depth of 135 m to the coast; in winter 2006/2007, spring 2007 and fall 2007, the volume transports were all southwestward with magnitudes of 2.0, 2.1, and 0.9 Sv, respectively, through a cross-shelf section from the site of the depth of 290 m to the coast. The standard deviations of the SICs were generally smaller than the velocities of the mean currents, and the variability of SICs showed significant correlation with the local sea surface winds. No persistent counter-wind currents were observed in the study area during the fall and winter observational periods. [ABSTRACT FROM AUTHOR]

In shallow estuarine environments, the time scales of hydrodynamic processes that control particle distribution may outpace the time scales of phytoplankton patch formation through reproduction. Consequently, physical processes can dominate the distribution of the phytoplankton, but these processes and their dynamics are not well understood. Here we used flow measurements with a bottom mounted Acoustic Doppler Current Profiler (ADCP), shipboard hydrographic transects, drifter releases, and Rhodamine dye to characterize the small-scale flow environment and its effect on dispersion processes in a shallow estuarine environment, Apalachicola Bay, Florida. Spatial spectra of salinity and chlorophyll followed a power law behavior of −3 at length scales of 250 m–5 km. The ADCP data revealed the presence of a vertically sheared flow that was strongly modulated by tides and bottom topography. Tidal flows had a characteristic magnitude of 20–40 cm s−1, with durations of flow reversals between the near-surface and bottom flows. Drifter triplets indicated shear and strain rates on the order of 10−3 – 10−4 s−1, and single particle dispersion rates (diffusivity) of 0.1 m2 s−1. The area evolution of the dye patch observed by a drone corresponded to eddy diffusivity comparable to those estimated from drifters, or about 0.1 m2 s−1. The dye patch experiments demonstrate how physical processes at scales of 1–100 m can affect the shape and development of phytoplankton patches in the bay. Vertical shear, produced by wind directions deviating from flow direction, can broaden and divide a plankton patch by transporting different depths of a patch in different directions. When winds and currents are aligned, shear leads to elongation and narrowing of the patch. The results indicate that the small-scale flow environment in estuaries can be pivotal in controlling the distribution and dispersal of planktonic organisms and thereby becomes a decisive factor for the development and breakdown of phytoplankton communities. [Display omitted] • Study reveals controls of phytoplankton distribution in Apalachicola Bay, Florida. • Small-scale flow environment is key for shaping estuarine phytoplankton patchiness. • Shear and strain rates were estimated 10−3 – 10−4 s−1, eddy diffusivities ∼0.1 m2 s−1. • Dye patch experiments explain movement and shape of estuarine phytoplankton patches. [ABSTRACT FROM AUTHOR]

The overall goal of this study was to strengthen understanding of the hydrographic structure in shallow estuaries as influenced by seasonal and depth-dependent variability, and by variability from extreme meteorological events. The mesohaline Neuse Estuary, North Carolina, U.S.A., which was the focus, receives surface inputs from upriver and tributary freshwater sources and bottom inputs from downriver high-salinity sound water sources, resulting in varying degrees of stratification. To assess depth-dependent, estuary-wide changes in salinity, a multiple time series was created using data from four discrete depths (surface and 1, 2, and 3 m±0.25 m). The database was developed from weekly to biweekly sampling of the entire water column, and included side-channel as well as mid-channel data. We characterized seasonal differences in halocline depth affecting the hydrographic structure of the mesohaline estuary and site-specific variation in nutrient concentrations, based on a comprehensive eight-year physical/chemical database. The first two years of the record showed an expected seasonal signal and included events that impacted the surface layer from freshwater inputs. Remaining years had greater variability over seasons and depths, with freshening events that affected all depths. Halocline depth was compared at specific locations, and a “snapshot” view was provided of the relative depth of these water masses within the estuary by season. We also examined flow patterns at the same cross-estuary sites over a three-year period, using a boat-mounted acoustic Doppler current profiler (ADCP) with bottom-tracking capability. Composite visualizations constructed with single-transect ADCP data revealed a classical estuarine circulation pattern of outflow at the surface/southern shore and inflow at the bottom/northern shore. Although this pattern deviated under extreme climatological events and was sometimes variable, the estuary generally exhibited a high probability of direction of flow. Wind fields, hurricanes, and small-scale, high-precipitation events represented significant forcing variables. [Copyright &y& Elsevier]

Areas around headlands often have higher tidal flow, making them important foraging locations for seabirds and marine mammals, as well as of interest for tidal energy extraction. Using in situ field measurements and hydrodynamic modelling, this study investigated tidal features around the most northerly promontory headland on the United Kingdom mainland. An acoustic wave and current (AWAC) instrument and an acoustic Doppler current profiler (ADCP) were deployed for 109 days in upward-facing configurations. These data were used to calibrate a hydrodynamic model developed using the TELEMAC-MASCARET modelling suite. This study used a non-hydrostatic model with a vertical layer configuration to reproduce upwelling present in field data. Data showed complex hydrodynamics with occurrences of vertical water velocities of up to 0.219 m/s. Modelling suggests upwelling begins in association with headland eddy formation as a result of secondary circulation due to flow curvature, which occurs off the tip of the headland. Upwelling continues to develop as the eddy dissipates and forms an upwelling front which moves away from the headland as the tide starts to turn. The modelling work successfully reproduces upwelling recorded by field instruments. Models of sufficient complexity are required to resolve fine-scale hydrodynamics associated with such features, which is important for tidal stream renewable energy developments in these environments, as well as understanding foraging activity of seabirds and marine mammals. • Headlands have higher tidal flow and are important seabird foraging locations. • Combining in-situ measurements with 3D modelling can investigate tidal features. • Observations show a large area of upwelling associated with the headland. • However, sufficient model complexity is required to resolve fine-scale hydrodynamics. [ABSTRACT FROM AUTHOR]

In this article, based on the example of the Toliara lagoon in Madagascar, we identified the major impacts of the two main drivers of the water renewal in a typical channel lagoon: the tide and the swell breaking on the reef. For this purpose, we applied the numerical model Croco which is based on ROMS-AGRIF and uses a realistic and high-resolution configuration. The validation was performed with in situ ADCP data, then, realistic and sensitivity simulations were run to infer tides and waves role in ocean-lagoon exchanges and to estimate water age and water origin. The modelling approach adopted enabled an understanding of the effects of swell and tide on the water renewal. During low wave conditions, tide and waves counteract each other on cross-reef flow: the cross-reef inflow tends to increase with waves breaking on the reef, whereas cross-reef flow tends to decrease or to be outflowing with tidal action. Hence, the lagoon-ocean exchange is reduced and is at a minimum when wave height reaches 0.85 m and the tide has minimum amplitude. During high wave conditions, both tide and waves increase ocean-lagoon exchanges by increasing the cross-reef inflow, whereas during low wave conditions, the cross-reef flow is mainly outward with increase with tidal amplitude and decrease with wave height until the threshold of 0.85 m. • From numerical experiments carried up on Toliara lagoon, we examine: • Tide and waves breaking on the coral reef barrier drive the hydrodynamic circulation on typical channel lagoons. • During wavy events, the main part of the water entering the lagoon is cross-reef water. • During low-wave events, the main part of the water entering the lagoon is through passage water. • During wavy events, the water age decreases with both tidal range and wave height. •During low swell episodes, the water age decreases with tidal range but increases with wave height. [ABSTRACT FROM AUTHOR]

Subinertial hydrodynamics were investigated at an inner-shelf around a cape on the Florida Atlantic coast during spring 2014. The area to the northeast and southeast of Cape Canaveral was instrumented with four moored Acoustic Doppler Current Profilers (ADCPs) within the swales on either side of shoals. The purpose was to examine the influence of a western boundary current (WBC) on inner-shelf subinertial flow, which was mainly along the shelf. A Concatenated Hilbert Empirical Orthogonal Function (CHEOF) analysis showed that Mode 1, which accounted for 95.7% of flow variance, described variations in the along-shelf current. Wavelet coherence analysis was used to examine relationships between CHEOF Mode 1 and potential environmental forcings. Analysis of the cross-shelf momentum terms showed geostrophic balance throughout the deployment with occasional contribution of the gradient of the wave radiation stress S xx. Along-shelf momentum was beyond frictional, as other terms in the balance were also influential. When along-shelf winds and the Florida current are oriented in the same direction, an increase in Florida current transport is observed and vice versa. Enhancement of the Florida current is associated with an increased positive across-shelf sea surface slope in accordance with geostrophy, driving the inner-shelf subinertial flow in the same direction as the western boundary current (WBC). Conversely, southward along-shelf winds coincided with weakening of the Florida current and a negative inner-shelf sea surface slope that maintained geostrophy, driving a southward inner-shelf flow. • This study investigates the influence of the Florida current on subinertial hydrodynamics off a cape in an inner shelf. • Statistical analyses were used to examine relationships between the subinertial flow and different environmental forcings. • Cross-shelf momentum showed that subinertial flow was in geostrophy with occasional contribution of the gradient of S xx. • The along-shelf momentum was beyond frictional. • Results indicated that the Florida current had a clear influence on subinertial hydrodynamics through water-level slopes. [ABSTRACT FROM AUTHOR]

Our study aims to determine the development of sediment-rich freshwater plumes in a non-tidal brackish water-dominated (salinity < 6) estuary in the Halikonlahti Bay, Northern Baltic Sea. We studied three seasons with different wind conditions and discharges: two open water periods, one with low (~0.2 m³/s) and one with high (31–40 m³/s) river discharges, and one ice-covered period with high (28–40 m³/s) river discharge. To conduct our analyses, we measured suspended sediment concentration (SSC), turbidity, salinity and temperature of bottom and surface waters together with current measurements along the estuary. Water samples were collected with LIMNOS water sampler and current measurements were done with acoustic Doppler current profiler. The results indicate that river plume develops under high river discharge, while during low river discharge the plume is very limited in extent. In open water conditions, SSC increased approximately ten-fold in the estuary head, with increased discharge from 0.2 m³/s to 31 m³/s. Buoyant plumes developed in both open channel and ice-cover conditions during high river discharge periods even in a weakly stratified environment, where the salinity difference was less than five over the entire water column. Unlike salinity, small temperature differences between river and seawater did not contribute the development of buoyant sediment plume. Weak stratification together with reduced wind-induced mixing was found to limit the sediment mixing between fresh surface and saline (~5) bottom layers in both ice-covered and open water conditions. For example, even 2–5 times higher SSCs were found at surface waters compared to bottom waters over a shallow (~4 m) water column. Wind and river discharge induced estuarine currents were found. Inverse estuary circulation developed under the conditions of low river discharge and inshore directed wind. High river discharge together with salinity stratification formed a positive estuarine circulation pattern, with surface outflow and bottom inflow. • Buoyant river plumes form in both ice-covered and open water conditions. • Buoyant river plumes form even in conditions of low salinity stratification. • Low salinity stratification reduces sediment mixing within water column. • Small water temperature differences do not contribute the behaviour of river plumes. [ABSTRACT FROM AUTHOR]

While estuarine dams have been noted for often resulting in siltation, the flux mechanism and the effect of the dam on the fluxes is not well understood. To gain process-based insight into the spatiotemporal variation and factors that control sediment flux, high resolution in-situ flux measurements were collected at stations 1.5 km and 6.5 km seaward of the Geum estuarine dam, Korea. Fluxes were calculated based on flow collected by Acoustic Doppler Current Profilers (ADCPs) and suspended sediment concentration derived from ADCP acoustic backscatter which was calibrated with water samples. The fluxes were decomposed into components occurring on timescales longer than the tidal cycle (mean-flow flux) and those occurring on the tidal timescale (correlation flux), and ship-borne surveys were conducted to investigate the role of the dam-induced horizontal salinity gradient to stratification and sediment dynamics. Results suggest that the landward correlation fluxes increased because the dam construction amplified the tide and because the correlation flux was proportional to the tidal amplitude, through tidal asymmetry. The dam discharge was found to control the seaward mean-flow fluxes, which were characteristically unsteady and proportional to the discharged volume. Discharge also was found to result in periodic stratification and flocculation asymmetry. The bed level change between the two stations was a balance between the effects of tidal asymmetry and freshwater discharge. For shallow, macrotidal estuaries with small discharge, estuary siltation is expected primarily due to correlation fluxes, and the effect is enhanced when the dam installation amplifies the tides. • The mean-flow flux was seaward, whereas the correlation flux was landward. • The fluxes were proportional to discharged volume and tidal asymmetry, respectively. • The correlation flux was dominant but was modified by mean-flow fluxes from the dam. • The net total flux was convergent and resulted in deposition. • Dam discharge led to tidal straining and resuspension and flocculation asymmetries. [ABSTRACT FROM AUTHOR]

Analyses of seasonal data obtained using acoustic Doppler current profilers provide an understanding of the behavior of suspended particulate matter concentration (SPMC) toward different forcings in tide-controlled floodways. In this work, the relative contributions of external forcings on SPMC variability were quantified in a tidal river system using singular spectrum analysis (SSA). The main environmental features affecting SPMC were identified as i) spring-neap tidal oscillation, ii) the ebb/flood velocities, and iii) tidal straining. Large SPMC fluctuations occurred under strong mixing states and were directly related to the sediment resuspension stirred up by spring-neap tidal cycles (73.6%–81.9%) and ebb/flood velocities (9.6%–19.5%). On the seasonal scale, river discharge is the key variable explaining the downstream flushing and promoting the occurrence of a convergence zone at the floodway. Upstream from the floodway (greater than 4.8 km from the river mouth), the spring-neap tidal oscillation dominated the suspended particulate matter (SPM) mobility under low river discharge. Two interesting findings were revealed in this work: (i) the SPMC/SPM transport variation responses to tidal forcing (tidal asymmetry) were dominated and modified by river discharge and (ii) the effect of river discharge on the SPMC/SPM transport did not result in a uniform state along the floodway. It is believed that these findings provide further understanding of the dynamics of suspended sediments in shallow tidal systems. Image 1 • Contributions of environmental forcing on SPMC variability are quantified. • SPMC variability is examined using Singular spectrum analysis. • Seasonal variations of SPMC are exhibited. • SPM mobility distance under the tidal asymmetry and tidal pumping is presented. • Role of control forcing on SPMC is discussed. [ABSTRACT FROM AUTHOR]

The characteristics of a shear front system in a tidal channel have been investigated through a field study. High-resolution sampling, including three-dimensional measurements of current, temperature, salinity, and turbidity, was performed on repeated transects across the frontal zone both with fronts apparent and with fronts absent. The density measured using conductivity/temperature/depth sensors provides a field proof that the shear front is present whether a density gradient exists. It is distinguishable from conventional buoyant shear fronts, which require a strong along-channel density gradient. The velocities measured using an Acoustic Doppler Current Profiler (ADCP) and trapping Global Position System (GPS) drifters indicate that shear front evolution has a four-stage dynamic process in a tidal cycle: (1) a directional shear front of shoal-flood and channel-ebb during the slack before flood and early flood, (2) a speed shear front of shoal-quick and channel-slow after the early flood, (3) a directional shear front of shoal-ebb and channel-flood during the slack before ebb and early ebb, and (4) a speed shear front of shoal-slow and channel-quick ebb during the late ebb. Thus, the velocity shear, a variation in the current speed and direction on either side of the front, is the generation mechanism of the observed shear front system. It is caused by the unequal tidal durations between deep channels and shallow shoals in the channel discussed herein. The Coriolis effect is found to be negligible through the momentum balance analysis. The unequal tidal durations can be caused by the following alternative mechanisms, according to the analysis and model simulation: (1) differential friction, owing to the steep bathymetry between the shallow shoal and the deep channel, and (2) circulation, owing to the coastline bend of the shallow shoal. • Conventional buoyant shear fronts require a strong along channel density gradient. • Xiazhimen tidal channel shear front system investigated through field study. • The shear front evolution is a 4-stage dynamic process linked to the tidal cycle. • The observed shear front system is caused by velocity shear only. • The velocity shear is caused by the unequal tidal durations between deep channels and shallow shoals. [ABSTRACT FROM AUTHOR]