Search

• The coupled 2D mophodynamic model considers effects of sediment concentration and bed deformation in the governing equations. • Unstructured meshes better conform with the irregular geometry of the natural rivers. • Channel adjustments play a significant role in sediment transport of alluvial rivers during a flood event. It is difficult to predict the detailed morphodynamic processes induced by heavily sediment-laden floods in alluvial rives, due to the complexity in topography and the significant channel evolution rates, especially in a braided reach of the Lower Yellow River (LYR). A two-dimensional morphodynamic model using a coupled solution approach was firstly developed, with the effects of sediment concentration and bed deformation being directly considered in the hydrodynamic governing equations. The finite volume method with the HLL-MUSCL scheme was adopted in the numerical solution, which can obtain second-order accuracy in space and time. In addition, unstructured meshes were used to accommodate the irregular boundaries. The proposed model was validated using measurements in the braided reach of the LYR during the flood events of 2020 and 2004, with general agreement obtained between the calculated and measured hydrographs of flow and sediment concentration. The highest accuracy was achieved in the simulation of discharge, with a Nash-Sutcliffe efficiency coefficient (NSE) of 0.92. The simulation accuracy in sediment concentration was improved by the coupled approach, with the relative error of the calculated peak value decreasing from 17 % to 9 %. In addition, transport characteristics of graded suspended sediments were well simulated, with the fine fraction being transported downstream and the medium fraction depositing in the study reach. Influences of channel adjustments on the sediment transport processes were investigated during a flood event. The results demonstrated that the incised channel in 2020 was characterized by a larger water depth and smaller velocity, as compared with the shallow channel in 2004. Consequently, the peak discharge decreased and the flood propagation time was extended under the 2020 topography, associated with the lower sediment discharge at the outlet due to the reduced sediment transport capacity. [ABSTRACT FROM AUTHOR]

A three-dimensional coupled hydrodynamic and morphodynamic numerical model was developed to analyze sediment dynamics in the Rance estuary, in response to the tidal power station (TPS) built near the mouth in the 1960s. The Rance estuary is a relatively small low-discharge steep-sided ria , located along the Brittany coast in northern France, with a maximum spring tidal range of 13.5m. Taking advantage of this significant tidal regime, the first and currently the second largest operational tidal power station in the world was built at the estuary's mouth, with peak output capacity of 240MW. After calibration and validation of the model for present-day conditions, suspended sediment concentration (SSC) and bed level evolution were evaluated at tidal and fortnightly scales for different scenarios, with and without TPS. Peak SSC are reached during spring tides and specifically during the estuary's infilling (flood) stage where both turbines and sluice gates are open. Unbalanced with sediment transport during ebb, sediments are accumulated in the main channel of the upper estuary. Under natural tidal forcing (i.e., without TPS), simulations show that an estuarine turbidity maximum forms in the upper estuary, and sediment deposition is significant. However, sedimentation rates are two times lower than those observed in the presence of the TPS. A possible alternative for reducing sediment accumulation in the upper estuary would be the opening of sluice gates simultaneously with the turbines during falling tide, to enhance ebb currents that would allow particles transport towards the estuary's downstream. [ABSTRACT FROM AUTHOR]

• Scale-specific controls of sediment load were identified in a seasonal inland river. • The combined method of MEMD and TDIC effectively address sediment transport process. • Predictions of sediment load using MEMD outperformed those made with the original dataset. Sediment transport is a multi-scale and non-linear process controlled by multiple variables. Given the intricate nature of sediment transport mechanisms and the overlap of different factors' effects on it at various time scales, unraveling the relationship between sediment load and its influencing factors is challenging. This study aimed to elucidate the multi-timescale effects of factors on monthly suspended sediment load in a seasonal inland river. In this study, monthly sediment loads measured by the depth-integrating method were obtained from the Tabu River Basin in northern China from 2007 to 2021, alongside data on four controlling factors: runoff, precipitation, water surface evaporation, and the normalized difference vegetation index (NDVI). The results showed that sediment load, runoff, and precipitation showed strong variability during 2007–2021, while water surface evaporation and NDVI exhibited moderate variability. Multivariate empirical mode decomposition (MEMD) decomposed the temporal patterns of sediment load and associated variables into five intrinsic mode functions (IMF) and a residue. IMF1 (4.7 months), IMF2 (9.0 months) and IMF3 (14.2 months) collectively explained 115.88 %, 91.73 %, 91.28 %, 107.69 %, and 106.09 % of the total variability in sediment load, runoff, precipitation, water surface evaporation, and NDVI, respectively. Subsequently, the time-dependent intrinsic correlation (TDIC) was utilized to illustrate the inherent relationships between sediment load and its controlling factors across various time scales. The associations between sediment load and the controlling factors changed significantly with the scale. Runoff was the dominant factor controlling sediment load at IMF1 (4.7 months), IMF2 (9.0 months), IMF3 (14.2 months), and IMF4 (25.1 months). Water surface evaporation controlled the sediment transport process at IMF5 (63.6 months). The prediction of sediment load on the measurement scale, using scale-specific controls analyzed with MEMD (R2 = 0.993), demonstrated superior performance compared to traditional multiple linear regression prediction (R2 = 0.748). This study will provide a theoretical basis for effective prevention and control of soil erosion and watershed management in inland river basins. [ABSTRACT FROM AUTHOR]

• A novel self-contained hydrodynamic observation platform system was deployed to monitor flow and sediment with ultra-high-resolution in the Yarlung Tsangpo River. • Temporal-spectral analysis with ultra-high-resolution revealed the asynchronous variations of flow and sediment over nearly two months. • The varying behaviors of flow and sediment in different flood events was distinguished using ultra-high-resolution data. Traditional monitoring techniques of flow and sediment are inadequate for the sparsely populated, the rivers in the Qinghai-Tibetan Plateau region which are experiencing amplified climate change. Inspired by the Euchiloglanis fish, this study developed a unique self-contained hydrodynamic observation platform to withstand the high velocities of mountain rivers typically encountered in mountain rivers. The platform, equipped with sensors, was successfully deployed in the Yarlung Tsangpo River, collecting ultra-high-resolution data on flow and sediment over a period of nearly two months. Performance assessments showed that the median value of discharge per unit width measured and suspended sediment concentration by this system were basically consistent with those daily measurements from the hydrological station. Meanwhile, the superiority of the system was reflected in the more significant daily average change, especially at the flooding period. The data unveiled significant minute-level hydrological fluctuations and a complex interplay of factors affecting the river's flow and sediment transport. A wavelet coherence analysis revealed asynchrony changes in short-period flow and sediment variables. Persistent short-period and coherent long-period signals are significantly displayed during flood events. The platform promises future applications, such as automated benthic observations and replacing traditional 'lead-line' methods. Despite acknowledging system limitations of underwater attitude, burying tendency, and real-time data transmission capabilities, the study highlighted the crucial role of this system in understanding and managing river dynamics, particularly amid escalating environmental threats. [ABSTRACT FROM AUTHOR]

Annually 5–6 typhoons strike the Yangtze Estuary (YE) as extreme events. However, their high energy and importance for sediment transportation and geomorphic changes are still not fully understood. In this study, high-resolution observations of wind, wave, flow velocity, and suspended sediment concentration (SSC) at two in-situ stations were carried out during the 2022 Hinnamnor typhoon. Additionally, we simulated the change in SSC, estuarine bed erosion/deposition, and flow and sediment transport with and without a typhoon in the YE using MIKE3 numerical model. The findings revealed that the Hinnamnor typhoon-induced waves increased the SSC of the turbidity maximum zone (TMZ) by a factor of 5.6 times (maximum is 2.8 kg/m3). The TMZ area also extended by 2.68 times (maximum is 7880km2, 70.4% of YE) in the YE. Moreover, the typhoon caused a dramatic change in sediment transport and bed erosion/deposition in the YE. First, in the delta front area where the mean water depth is >5 m, the typhoon significantly increased the southward flux of residual flow and sediment, causing sediment transport into Hangzhou Bay to abruptly increase 26.3 times (increase of 52 million tons, accounting for 1/3 of the present annual flux of the Yangtze River (150 million tons)) during a single spring-neap period. The net erosional area and volume extended to 6770km2 (60.4% of YE) and 91.18 × 106 m3. Second, in the delta shoals (where the mean water depth is <5 m, including east Chongming Shoal, Hengsha Shoal, Jiuduansha Shoal, and east-south Nanhui Shoal), residual flow and sediment flux decreased northward from the typhoon and resulted in the erosion of the shoal. Third, in channels with trumpet-shaped mouths (North Branch (NB), North Channel (NC) and South Passage (SP), except for North Passage (NP)), the upward flux of residual flow and sediment increased due to the typhoon, resulting in bed deposition in these channels (NB, NC and SP). This study highlights the important influence of typhoons on flow and sediment transport and bed erosion in estuarine areas. • Exploring changes of turbidity maximum zone, sediment transport and bed erosion induced by a typhoon in the Yangtze Estuary. • Revealing typhoon event is the critical driving force for sediment transport from estuaries to offshore areas. • Establishing relationships between suspended sediment concentration and wave-current bed shear stress. [ABSTRACT FROM AUTHOR]

An extended three-dimensional unstructured ocean model for simulating impacts of tidal stream turbines on tidal current, turbulence and surface waves has been applied to study the interactions between a tidal turbine farm and its surrounding environment. The present study aims to reveal three-dimensional local and regional changes due to the operation of a proposed turbine farm in natural coastal environment. Fine mesh size is assigned at the turbine farm location to capture the details of local wake dynamics, hydrodynamics and suspended sediment transport. Large geographic coverage of the model provides details of changes in regional features. Results showed that the proposed turbine farm comprised of 18 turbines (15–20 m in diameter) with approximately 20% power extraction from the average available power (averaged over five and half tidal cycles) led to local variation of surface elevation within the range of −10 to 3 mm, flow acceleration on both sides of the turbine farm, flow acceleration of ∼ 0.4 m/s near the bed in the vicinity of the turbine farm which caused bed shear stress to rise up to 2.5 N/m2 (corresponding to the critical stress of a range of fine gravel and finer sediment particles), locally increased TKE of 0.09 m2/s2, reduced wave height of 0.01–0.05 m, and upward sediment transport in the water column. On a regional scale, most of the changes on amplitude and phase of M 2 constituent were observed within 10 km (∼ 15 times the array width) from the centre of the turbine farm, and the wake in terms of 95% flow rate recovery was found to be 9 km long (∼ 14 times the array width). Noticeable changes were also found in surface waves, bed shear stress and suspended sediment transport on regional scale as result of moderation in tidal and flow dynamics, although much less prominent than the local effects. It is recommended that consideration during the Environmental Impact Assessment stage of tidal stream energy projects should be given to an area that extends beyond the immediate vicinity of the planned turbine farm. • Detailed study of hydrodynamic and sedimentary impacts of tidal stream energy extraction. • A three-dimensional ocean model with additional terms for tidal turbine simulation is used. • Both local and regional scale impacts are revealed. • Regional scale impacts are less prominent than local scale impacts. • Environmental Impact Assessment of tidal stream energy projects should consider an area beyond the immediate vicinity of the turbine farm. [ABSTRACT FROM AUTHOR]

The swash zone is an important region for the coastal morphodynamics. Often, model studies of the swash zone use depth-averaged models. These models typically assume a vertically uniform velocity and sand concentration for calculating the sand transport flux. However, this assumption is not always accurate in the swash zone. In order to investigate the vertical distribution of velocity and sand, we use a depth-resolving model that is able to capture these vertical variations. We simulate the flow and suspended sediment transport induced by bichromatic waves using a 2DV depth-resolving RANS model. Our verification of the model shows that special care needs to be taken to deal with bubbles in 2DV simulations. Furthermore, we show that turning off the (Wilcox, 2006, 2008) limiter for turbulence, increases the modelled turbulent kinetic energy that is induced by wave-breaking, resulting in improved predictions of sediment concentrations. Using the depth-resolving model, we show that the vertical distribution of velocity and sand is far from uniform in the swash zone. The results show that if one assumes vertically uniform depth-averaged velocities and concentrations, one can overpredict the sediment flux by 50%. • Air bubbles can stay unrealistically long near the bed in 2DV swash simulations. • Bubbles near the bed lead to lower shear stresses and lower sediment pick up. • Increased wave-breaking turbulence leads to increased onshore sand transport. • A model assuming a vertically uniform flux of sand can underpredict the flux by 50%. [ABSTRACT FROM AUTHOR]

Understanding sediment transport processes on natural sandy beaches is essential for gaining insights into beach recovery and making effective coastal management decisions. This study examines surfzone sediment transport rates related to beachface morphological variations on an embayed mesotidal sandy beach located on the northwestern coast of the Baja California Peninsula in Mexico. Data were collected during a week-long field experiment conducted in June 2016 under low-to-moderate wave energy conditions (H s = 0.4 − 1.3 m). Daily topographical surveys and continuous measurements of near-bottom suspended sediment fluxes were conducted alongside the application of an extended energetics-based model that accounted for velocity and acceleration skewness. Results reveal contrasting accretionary and erosive patterns in the inner surfzone, along with consistent sediment deposition in the swash zone throughout the study period. Onshore sediment transport is found to be related to short-period calm wave conditions (H s < 0.7 m; T p < 10 s) and a weak undertow (<0.2 ms−1). Alongshore nonuniform wave breaking, influenced by irregular bathymetry and moderate-energetic oblique waves from the northwest, contributes to an alongshore gradient in sediment transport rate, leading to erosion in the northern part of the intertidal beach and accretion in the southern part. Suspended sediment flux measurements at 0.2 m above the bed suggest offshore mean transport predominates over oscillatory transport throughout the field experiment. Nevertheless, this observation should be interpreted with caution, as the flux is not vertically integrated across the water column and does not consider fluid acceleration. The model predictions effectively replicate sediment transport rates and consequent volumetric changes (Accuracy = 55– 63 % ; RMSE = 44–69 m3; Bias = − 2 to −61 m3), although they underestimate observed accretion by a factor of three and overestimate erosion by a factor of two. Overall, this research highlights the complexities of natural sandy beach recovery processes in mesotidal environments and emphasizes the importance of considering both cross-shore and longshore components in sediment transport assessments. [Display omitted] • Beachface accretion remains consistent under low-to-moderate wave conditions. • Combined velocity- and acceleration-skewness model reproduces onshore sediment transport, but underestimates beachface accretion. • Erosive and accretive surfzone patterns are linked to nonuniform wave breaking influenced by bathymetry. • Undertow significantly governs offshore sediment flux near the seabed within the surfzone. [ABSTRACT FROM AUTHOR]

Marsh creeks are perceived as important conduits for transporting water and sediment between mudflats and marshes. In order to advance the understanding of the transport mechanisms in creeks, the source and ultimate sink of sediment which moves between mudflats and marshes through creek channels need further investigation. Therefore, two field campaigns were conducted in two intertidal systems with varying sediment availability. The water depth, flow velocity, suspended sediment concentration, and bed level change were measured simultaneously in a marsh creek and on the adjacent mudflat in Chongming Island (China) and in Paulina Saltmarsh (the Netherlands). Paulina Saltmarsh is much smaller, more frequently flooded, and has lower sediment concentration than Chongming. These contrasting conditions allow for a comparison of transport mechanisms and functioning of the creek. Both systems first show that the high suspended sediment concentration (SSC) measured in marsh creeks is mainly the consequence of sediment advection rather than local erosion. In addition, erosion in marsh creeks is usually limited during ebb tides, reducing the export of sediment through these creeks. However, differences have been observed between two systems. The measured SSC was highly asymmetric between flood and ebb tides in Chongming. Large peaks in SSC during the flood period can be observed for most tidal cycles. The marsh creeks in Chongming therefore function as conduits for sediment import. Additionally, there are distinct overbank and underbank tides in Chongming. Sediment was trapped and retained in creeks during underbank tides, which can then be eroded and transported to the marsh during subsequent overbank tides. We also observed that mudflats in Chongming quickly recovered after erosion. These mechanisms have not been observed in Paulina Saltmarsh, where net sediment export via the marsh creek was observed due to a lack of abundant sediment in suspension during flood tides. Furthermore, the remaining bed surface of mudflats after an erosion event was stronger than before, limiting further erosion in Paulina Saltmarsh. These findings from the two systems indicate that the role of creeks in sediment import/export depends on the availability of sediment from mudflats, shedding light on nourishment strategies for salt marshes. • High SSC during flood tides in the studied marsh creeks is attributed to advection of sediment rather than local erosion. • Ebb tides were limited in suspended sediment by various mechanisms, reducing the export of sediment via marsh creeks. • Roles of marsh creeks in sediment delivery depend on the sediment supply from mudflats and hydrodynamic forcing in creeks. [ABSTRACT FROM AUTHOR]

During extreme conditions, the transport of the wave-averaged suspended sediment concentrations in the inner surf zone affects dune erosion. Although large-scale laboratory experiments have provided insight in what drives these sediment concentrations, corresponding field data are lacking. To fill this gap, novel field observations of suspended sediment concentrations are compared to drivers that govern sediment suspension during storm conditions known from literature. A total of 128 time intervals of 20 min are analysed, spread over 10 different high water events with different hydrodynamic conditions. For each time interval, the wave-averaged (i.e. 20 min mean) suspended sediment concentration is computed and compared to three suspension drivers. The studied drivers are (1) bed shear due to near bed velocities that originate from mean currents in combination with wave-induced orbital flow, (2) the horizontal pressure gradients under steep wave fronts that increase the forces on the bed material, and (3) bore-induced turbulence that is generated at the free surface and reaches the bed. The derived bore-induced turbulence generates the greatest correlation with the mean suspended sediment concentrations (r = 0.74, p = 4.47E-23). Samples that deviate from this correlation correspond to time intervals with lower values of derived bore turbulence, less wave energy saturation in the inner surf zone, and stronger mean currents. The correlation with the mean suspended sediment concentrations increases when the shear stress originating from mean currents is used for these time intervals (r = 0.83, p = 1.63E-33). For time intervals during which more energetic conditions persist and the wave energy is saturated in the nearshore, bore turbulence was the dominant mechanism in stirring up sediment. The outcome of this study suggests that, based on the events analysed, dune erosion models may achieve more accurate results if computations of suspended sediment concentrations include a bore-induced turbulence term, or if already included, properly address the relative importance of bore-induced turbulence when compared to bed shearing. • A dune erosion field experiment was conducted to study drivers of wave-averaged suspended sediment concentrations. • The studied drivers are (1) flow-induced bed shearing, (2) horizontal pressure gradients under wave fronts, and (3) bore-induced turbulence. • The variability in bore-induced turbulence generated the greatest correlation with the variability in mean suspended sediment concentrations. • When more energetic conditions persisted and wave energy was saturated in the inner surf zone, bore turbulence appeared the dominant driver. [ABSTRACT FROM AUTHOR]

• 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 process of flocculation in dredge plumes was investigated by field measurements using Laser In-Situ Scattering and Transmissometry (LISST), Optical Back Scatter (OBS) sensors and an Acoustic Current Doppler Profiler (ADCP) during dredging campaigns in Port Curtis, Queensland (Australia). Suspended sediment characteristics differed significantly between natural ambient suspended sediment and sediment suspended by dredging as well as between sediment suspended by mechanical and hydraulic dredgers. LISST measurements enabled distinction of the suspended sediment by origin and showed that the majority of sediment particles naturally in suspension were aggregated flocs made up of multiple smaller particles. Primary particles in natural suspended sediment had a median particle size (d 50) of 8 to 14 μm, while in-situ flocs had a d 50 of 52 to 99 μm. There were both microflocs and macroflocs present in the natural suspended sediment, with the microflocs being dominant (representing 67%–80% of the flocs). Mechanical dredging resulted in the release of individual particles, microflocs and macroflocs, with the potential for a comparable percentage of macroflocs as in natural suspended sediment (11%–27% compared to 20%–33% in natural suspended sediment). Macroflocs released by mechanical dredging were likely to have originated from the seabed. During overflow from the hopper, the suspended sediment released from the hydraulic dredger had smaller flocs compared to the natural suspended sediment, with a reduction in the d 50 of in-situ flocs from 91 to 67 μm and a reduction in the proportion of macroflocs from 32% to 24%. These results indicate that larger macroflocs present on the seabed had been broken up by the hydraulic dredging, with most of the suspended sediment remaining in the form of smaller microflocs, suggesting that the flocs were not completely disaggregated by the shear-stress generated during hydraulic dredging. Ongoing aggregation of suspended sediment was shown to occur in the sediment suspended by both dredging methods within 20–25 min of the plumes being generated. This study demonstrates the importance of considering flocculation in predictive dredge plume modelling studies to prevent significantly underestimating the settling velocity and substantially over-estimating the extent and intensity of dredge plumes if flocculation is not accounted for. • Flocculation occurs in plumes generated by both mechanical and hydraulic dredging • Flocs released by mechanical dredging were comparable to natural flocs • Flocs from hydraulic dredging were smaller than natural flocs • Ongoing aggregation of flocs occurred in the plume for both dredge types [ABSTRACT FROM AUTHOR]

Accurate flood mapping is crucial for enhancing community resilience in the face of flooding. Among the several factors that affect the flood extent, the effect of fluvial dynamics is not clearly recognized so far. Fluvial dynamics play an important role in shaping the patterns and magnitudes of water flow and sediment transport in riverine environments, that consequently affect flood extent. By conducting a scoping review, this paper aims to show how the river's hydrological processes and channel morphology impact floods and flood mapping. To do that, previous studies dealing with flood modeling have been explored to provide a comprehensive understanding of the interactions between fluvial dynamics and flood mapping outcomes. The results showed that morphodynamic variations and sediment transport impact flooding differently depending on local morphological conditions. Some studies indicate that these processes can increase the likelihood of flooding, potentially posing greater risks to nearby areas. Conversely, other research suggests that these morphodynamic variations may help reduce flood occurrences, signaling a nuanced relationship between river morphology and flood dynamics. Additionally, it was found that while bed-load movements significantly influence the synchronization of peak water surface elevation and peak flow times, suspended sediment concentration has minimal impact on the timing and shape of the hydrograph. The results emphasize the crucial role of integrating fluvial hydro-morphodynamics into flood mapping to enhance flood management in riverine environments, contributing to both increased resilience and broader sustainable development goals. [Display omitted] • The influence of sediment dynamics and channel morphology on floods is reviewed. • Sediment impacts flooding differently depending on local morphological conditions. • The findings stress the need to include sediment dynamics in flood studies. [ABSTRACT FROM AUTHOR]

• Decrease of suspended sediment transport in the Mediterranean over the last century. • Time series reconstruction using artificial intelligence. • Crucial role of climate change in the regulation of suspended sediment transport. • Secondary influences of the anthropic impact. The analysis of suspended sediment transport and of its variations over time is crucial for understanding environmental evolution and it is the key to future challenges caused by current global warming. The Mediterranean area is a hot spot for global changes, and the variation of precipitation amount and intensity will modify the environment of this region. In this work, we analyse the time series of suspended sediment transport of two rivers located in central Italy by using statistical and artificial intelligence techniques. Our study aims to re-analyze time series of suspended sediment transport, in order to demonstrate that climate change is responsible for the substantial decrease in the amount of sediment over the past century, in relation to the atmospheric teleconnections of the North Atlantic. Anthropic pressures like reforestation, land use change, and dam building have influenced the sediment transport capacity of rivers, causing a reduction of sediment concentration in water. These results are key factors to determine the future management of the Mediterranean areas, where the future scenarios predict a greater drop in yearly precipitation, and therefore a possible further decrease in the sediment transport capacity of the rivers, with major consequences for coastal and fluvial environments. [ABSTRACT FROM AUTHOR]

• We used a tracer-based approach to quantify melt water proportions related to sediment transport at two contrasting Alpine proglacial streams. • Glacier melt was associated with higher bedload concentrations at the debris-covered glacier and much lower concentrations at the clean ice glacier. • Glacier size, presence of debris cover, and substrate were identified as the main drivers of melt dynamics and sediment transport at both glaciers. Subglacial sediments are a large component of the sediment budget of glacierized catchments but insights into the subglacial origin of sediments (bedload, in particular) linked to proglacial runoff dynamics remain scarce. In this study, we use a tracer-based approach to quantify meltwater proportions related to sediment transport at two proglacial streams, draining glaciers (named debris-covered and clean glacier) of different size, aspect and elevation range with contrasting distribution and thickness of debris cover and lithology of the subglacial sediments (i.e., metamorphic vs. sedimentary), in the Sulden/Solda catchment (Italian Alps). Results indicate that the glacier melt component (75 to 80 %) was associated with bedload concentrations of 1 to 10 kg m−3 at the debris-covered glacier and much lower concentrations of 0.01 to 1 kg m−3 at the clean ice glacier. At the seasonal scale, bedload and suspended sediment concentrations at both sites strongly varied with discharge. While daily bedload concentrations varied by up to two orders of magnitude obscured the seasonal development of bedload concentrations at both sites, a clear seasonality for suspended sediment concentrations was found. At the daily scale, the relationship of discharge, bedload, and suspended sediment was more complex because discharge and sediment transport did not always follow the daily variation of air temperature, or similar daily air temperatures resulted in different discharge and sediment transport responses and vice versa. Glacier size, presence of debris cover, and substrate were identified as the main drivers of meltwater dynamics and sediment transport at both glaciers. This study adds further insights into the interplay of meltwater contributions and sediment transport, which are essential to better assess the impact of climate warming on sediment supply in glacierized catchments. [ABSTRACT FROM AUTHOR]

• Human interventions affect the dynamics of meandering channels. • Unusual inner bank erosion and outer bank deposition occurred due to upstream damming. • Altered flow-sediment regimes and channel boundary drive the unusual meander dynamics. Human activities, such as reservoir construction and bank revetment, significantly alter downstream flow and sediment regimes and river channel changes, impacting river ecosystem services. However, our understanding of how intense human activities affect morphodynamics of meandering channels in big river systems remains incomplete due to the lack of high-resolution in-situ data. Here, we use comprehensive data encompassing hydrology and sediment, historical channel maps, and high-resolution river bathymetry to investigate the evolution of meandering bends in the Middle Yangtze River (MYR) across three distinct phases: the natural period (1490–1975), the bank revetment period (1975–2002), and the post-dam period (2003–2018). The study revealed that, in the absence of human intervention, the meandering channels of the MYR naturally experienced inner bank deposition and outer bank erosion, largely governed by curvature-induced secondary flow patterns in flow and sediment transport mode. In 1975–2002, outer-bank revetments reduced erodibility, stabilizing outer banks, while inner banks continued to experience lateral and vertical deposition, causing a narrower and deeper channel hydraulic geometry. Conversely, meandering bends had exhibited an unusual pattern of 'outer bank deposition and inner bank erosion' in 2003–2018. Channel morphological changes characterized by channel incision and widening were driven by changes from erodible bank boundaries to solidified bank boundaries, substantial reduction in suspended sediment concentration, and an altered and extended effective flow range (10,000–25,000 m3/s). Our findings highlight that human interventions such as damming and bank revetments can alter the conventional erosion–deposition morphodynamics of meandering bends in large river systems, carrying vital implications for sustainable river management in the Anthropocene. [ABSTRACT FROM AUTHOR]

To reconstruct the interaction between Antarctic Bottom Water (AABW) and global climate change, extracting information about past AABW formation from the sedimentary record is crucial. The Prydz Bay region, East Antarctica, has high glacier outflow and is a region of vigorous formation of AABW associated with sea ice formation. The Cape Darnley Bottom Water (CDBW), a primary AABW precursor, flows into Wild Canyon west of Prydz Bay. Thus, the sedimentary record of Wild Canyon can serve as an archive for CDBW flow. In this study, to identify the sedimentary processes and mineral compositions characteristic of CDBW flow, we evaluate deep-sea camera imagery, X-ray computed tomography images, grain-size data, and optical and chemical mineralogical analyses. Heavy minerals in the surface sediments are characterized by pyrope-rich almandine garnets (up to 80% of the heavy-mineral assemblage, 0.2%–3.7% of the dry bulk sediment by weight) typical of nearshore sediment derived from Mac. Robertson Land. In addition, ripples on the channel floor indicate sediment transport occurred as tractional bedload, whereas settling of suspended sediment prevailed on the canyon levee. We propose that modern CDBW flow is the dominant process of sediment reworking through sustained high flow velocities, distinctly different from turbidity currents triggered by ice sheet expansion and rapid retreat. Our results further suggest that fine-grained sediments are transported to the canyon levee as CDBW flows through the submarine canyon. These findings will be a key for reconstructing past AABW formation, which is crucial for understanding the response of AABW to future climate change. • Ripples formed by Cape Darnley Bottom Water are identified in Wild Canyon. • Antarctic Bottom Water flow triggers the reworking of pre-existing sediments. • Sedimentary processes are different during AABW active/inactive periods. [ABSTRACT FROM AUTHOR]

• Either climate data or streamflow data alone is not sufficient to accurately estimate SSL. • Gradient boosting models accurately predict SSL using hydro-climate data. • Hydrological metrics improve the accuracy SSL modeling during model calibration. • The prediction uncertainty of SSL increases with the basin's area. • Average ensemble modeling enhanced the accuracy of models in rivers. Machine learning (ML) models have the potential to improve the accuracy of Suspended Sediment Load (SSL) predictions. However, their ability to incorporate climate data to enhance SSL predictions remains underexplored. This study investigates the efficacy of ML models in modeling sediment transport using hydro-climate variables as input data. Boosted decision tree models, including Xgboost, LightGBM, and Catboost, were employed to estimate SSL using hydro-climate variables such as streamflow and precipitation, as well as maximum and minimum temperatures. The models were trained, optimized, and evaluated using either climate/streamflow data or a combination of both, encompassing 47 gauges across the contiguous United States (CONUS) between 1950 and 2022. The results demonstrate that streamflow data alone is insufficient for accurate SSL estimation in all rivers. Consequently, integrating climate data and streamflow data proved effective in predicting SSL at the daily scale across diverse climate zones with commendable accuracy. The models achieved an average Nash-Sutcliffe Efficiency (NSE) of > 0.66, Kling-Gupta Efficiency (KGE) of > 0.67, and Willmott Index (WI) of > 0.83 during the unseen (test) phase. These findings highlight the potential of ML models, particularly when combining climate data with streamflow, to enhance SSL predictions and improve our understanding of sediment transport dynamics. [ABSTRACT FROM AUTHOR]

In this numerical study, sediment transport and bed evolution in the surf zone under plunging breakers are investigated; this is a challenging topic that involves complex hydrodynamics coupled with sediment dynamics. Here, a Eulerian–Eulerian multi-phase model for sediment transport, with the capability to capture the free surface, is used. Waves are generated using an open-source toolbox 'waves2foam', based on a relaxation method. The model is validated with published experimental results in three cases: (i) surf-zone hydrodynamics over a sloped-rigid bed, (ii) suspended sediment concentration over a sloped-sediment pit, and (iii) bed profile change of a sloped-mobile bed. The profiles of time-averaged velocity, concentration, sediment fluxes, and the time series of concentration are presented. The numerical results suggest that the undertow plays a key role in offshore sediment transport. The instantaneous flow fields show that the vortices under plunging breakers roll up the sediment layers originally near the bed; in such an event, the buoyancy provides a centripetal force to sediment particles. The vortices further transport the suspended sediment onshore and cause strong mixing in water column. This is marked as the underlying mechanism for significant wave-driven flux in the surf zone. • Surf-zone sediment transport under plunging breakers is simulated by using a multi-phase model. • Undertow drives sediment transport. • The vortices roll up sediment layers through a centripetal buoyancy. • Onshore transport and strong mixing induced by the vortices lead to significant wave-driven flux in the surf zone. [ABSTRACT FROM AUTHOR]

An in-situ observational study was conducted to understand the sediment transport processes disturbed by the flood generation type of Sihwa tidal power plant (TPP). A surface mooring was deployed in Lake Sihwa (inside of TPP) to collect time-series data of the current velocity and suspended sediment concentration (SSC). A TPP with a capacity of 254 MW has four sequential phases (power generation–standby–drain–standby) of operation. The SSC during power generation was one order of magnitude higher than that during drain, which led to heavily unbalanced sediment fluxes. The total residual sediment flux was always negative (into the lake), which was attributed to the mean advection processes associated with the discharge. Over 18 day mooring period, 78.28 tons m−1 of sediment was delivered to the lake. The source of delivered sediment might be attributed to more local resuspension than the sediment inflow through the gates of TPP. Since the maximum sediment influx during power generation occurred within the water head difference of 4–6 m, the reduction in discharge rate can be a possible measure for solving the sediment accumulation within Lake Sihwa. • The operation of tidal power plant induced the unbalanced sediment flux. • The maximum sediment influx occurred within the water head difference of 4–6 m. • Sediment budget into the lake tends to increase due to the advective effect. [ABSTRACT FROM AUTHOR]

Estuaries that feature large tidal ranges (macrotidal) have recently received attention for their potential to generate energy by tidal stream turbines. As estuaries are delicate ecosystems and often have large human populations residing on their shores, determining where to place the turbines and how they will impact the hydrodynamics and sediment transport of these systems is vital to their successful implementation. The aim of this study is to provide a framework for assessing an estuary for turbine placement, including determining how the turbines will impact the environment (focusing on the physics), and examining other items that could peripherally affect the tidal turbine farm location or impact. This work was carried out using the Gironde Estuary as a case study and included a combination of in-situ data collection and numerical modeling. In terms of the turbine farm impact on the hydrodynamics and Suspended Sediment Concentration (SSC), results showed that the inclusion of a farm reduced currents up to 10% and decreased SSC up to 15 mg/l. The decrease in SSC was due to the attenuated current velocities decreasing friction velocity and thus near bottom generated turbulence that suspends bottom sediment. Over time, the decreased SSC will lead to sediment accumulation and new bottom features. • Framework established for assessing an estuary for turbine placement and environmental impacts. • Turbines decreased current velocities and suspended sediment concentration in the near and far field from turbine locations. • Decreased suspended sediment concentration due to decreased friction velocity and near bottom turbulence production. [ABSTRACT FROM AUTHOR]

Global climate change is altering the frequency and intensity of extreme weather events such as typhoons and cold fronts, and this is inducing physical changes and adaptions in estuaries and coasts. We conducted a field campaign on the subaqueous Changjiang Delta front in September 2019 to improve understanding of storm impact on short-term hydro-morphodynamics. Over an 11-day period, during which both typhoon and cold front events occurred, in-situ data including flow velocities, suspended sediment concentrations (SSC), and bed-level changes were acquired using bottom tripod and buoyage systems, Significant wave height reached 5.0 m during the typhoon, and the depth-averaged current velocity increased to 1.7 m/s. The net near-bottom sediment flux was in the same direction as the wind and was 5.5 times of that under calm weather. During the cold-front, significant wave height reached 2.5 m, and the near-bottom SSC increased to 7.0 kg/m3. Bed-level changes were small (<2 cm) under pre-storm weather, while net deposition and erosion reached 15.8 cm and 16.8 cm, respectively, during the typhoon. Short-term changes in the sediment source-to-sink regime were detected in the subaqueous Changjiang Delta. The cold front enhances seaward sediment flushing from the delta towards the sea, while the typhoon drives sediment transport from the subaqueous delta towards Hangzhou Bay. We also observed rapid bed-level recovery following these extreme events. These findings improve our understanding of sediment transport under stormy conditions. • Both cold front and typhoon events alter the sediment exchange pattern in the Changjiang Delta. • Typhoon enhances near-bottom sediment resuspension transport from the Changjiang Delta towards the Hangzhou Bay. • Local bed level recovers in a few days from the disturbance caused by extreme events. [ABSTRACT FROM AUTHOR]

A tidal shear front (TSF) can be defined as an obvious horizontal gradient of flow velocity formed by opposing tidal currents (reversed TSF) or same-direction tidal currents (homogenous TSF) on both sides of the front with limited impact by river runoff. Observational and modeling studies have been conducted to investigate the impact of a reversed TSF on the transport of freshwater and sediment in the Yellow River Mouth (YRM). The reversed TSF existence period is only about 40% of a tidal cycle (around slack of flood and ebb); however, when this front disappears (around maximum flood and ebb), sediment transport across the frontal zone is still rare. This suggests that the dynamic mechanism of cross-frontal sediment transport in a tidal cycle remains unclear. In this study, synchronous observations of sea current and suspended sediment concentrations along the river mouth profiles at the YRM in 1995, 2003, and 2019 were used to investigate the long-term variation characteristics, mechanisms, and impacts of the TSF on cross-frontal sediment transport. The results indicate that in addition to the reversed TSF, there are also homogenous TSF during flood and ebb tides, with the ebb homogenous TSF being more significant due to tidal asymmetry in the YRM. The ebb homogenous TSF is located between shallow shoals and the slopes of the subaqueous delta front and lasts for about 3 h around the maximum ebb. Its maximum shear strength is similar to that of the reversed TSF, and the formation mechanism is also similar. The presence of the ebb homogenous TSF reduces the efficiency of cross-frontal sediment transport during the ebb. The reversed and homogenous TSFs alternately block sediment moving towards the sea. Thus, terrigenous sediment is difficult to cross the frontal zone to the central Bohai Sea by tidal currents solely. Over the past two decades of low discharge, with the Yellow River implementing artificial water and sediment regulation, the homogenous TSF during the ebb tide has become more and more important and cannot be neglected for cross-frontal sediment transport under normal conditions. Furthermore, from a long-term perspective, the shear strength of the TSF is positively correlated with the distance of river mouth siltation into the sea. Additionally, salinity and turbidity gradients are also positively correlated with the shear strength in the estuary. This indicates that the blocking of cross-frontal sediment transport by the TSF is a key process in river-sea interaction in the YRM. • The tidal shear fronts (TSFs) evolution and their impact on cross-frontal sediment transport are revisited. • The homogenous TSFs are found as one of the key sedimentary dynamic factors too. • Fluvial sediment seaward transport is hindered not only by reverse TSFs, but also ebb homogeneous TSFs. • Interaction between topography, TSFs, and sediment transport is a key process in long-term river-estuary-sea interaction. [ABSTRACT FROM AUTHOR]

• Contour currents with different physical natures observed over a drift-moat system • High SSC on the drift due to long-range sediment transport by contour currents • High SSC in the moat due to moat wall erosion by tidal-induced secondary flows • Formation of drifts forced by upslope sediment transport of tidal currents Contourite drifts are ubiquitous sedimentary features in the world′s oceans, and their formation are usually ascribed to sedimentation from contour currents. However, in-situ observations of contour currents and their associated sedimentary processes were inadequate. Here, we present mooring observation results from a drift-moat contourite system in the South China Sea to gain insight into its sediment dynamics and formation mechanism. We found that quasi-persistent contour currents develop in the moat, on and below the drift, yet subject to diverse physical oceanographic processes. High suspended sediment concentrations observed on the drift mainly occur in winter, induced by high levels of sediments transported by contour currents from Taiwan. In the moat, however, high suspended sediment concentrations are mostly caused by erosion of the moat wall during periods of strong tidal currents and low shear variance. Such results underscore the complexity of current patterns and sedimentary processes across various topographic units in the drift-moat contourite system. A novel formation mechanism of the contourite drift is thereby proposed, wherein sediment transport by contour currents brings materials for constructing the drift, while its formation is driven by near-critical reflection and upslope transport of sediments by tidal currents. [ABSTRACT FROM AUTHOR]

[Display omitted] • Developing an integrated sediment source assessment index based on long series hydrologic signals. • Developing Karst Surface Characteristic Index based on karst desertification factors. • Karst Surface Characteristic Index is closely related to hydrological processes. • Clockwise hysteresis is the most frequent and efficient type of sediment transport. The source and transport processes of suspended sediment are issues that require further study. The complex relationship between the integrated surface characteristics and the mechanisms of sediment sources and transport in heterogeneous karst watersheds remains unclear. This study proposes a comprehensive assessment framework for sediment sources that combines the surface characteristics of karst watersheds, such as exposed rock and shallow topsoil in karst areas. Additionally, the Karst Surface Characteristic Index (KSCI) is developed to evaluate the potential supply capacity of sediment in karst watersheds. The hysteresis index (HI mid) is calculated using a long series of hydrological data to study runoff and sediment relationships in eight watersheds in the karst region of southern China. Results indicated that plan curvature, terrain relief, NDVI, and rock exposure contributed most to KSCI, with these four factors contributing 65%. The Q-SSC hysteresis analysis results based on 445 hydrological events showed that a clockwise hysteresis pattern was the most frequent and efficient pattern of sediment transport in the studied watershed (clockwise hysteresis pattern accounted for 72.3% of 445 hydrological events) owing to the limited sediment supply in the karst watersheds. The quantitative analysis results of HI mid and KSCI showed that HI mid was closely related to KSCI. With an increase in KSCI, a clockwise hysteresis pattern was more likely to appear, indicating that watershed characteristics exerted a significant influence on sediment transport velocity and efficiency. In terms of spatial distribution, near the outlet of the watershed or the river channel, high KSCI values were distributed, and clockwise hysteresis was more likely to occur. In contrast, at the far end of the watershed outlet or the river channel, high values of KSCI were distributed, and anticlockwise hysteresis was more likely to occur. Our findings further enrich the theory of soil erosion and the runoff-sediment relationship at the watershed scale in karst areas, and serve as valuable decision-making references for integrated water and soil sources management. [ABSTRACT FROM AUTHOR]

The flow field and the sediment motion generated over a rippled bed by a uniform and oscillating pressure gradient are evaluated by means of a direct numerical simulation of Navier–Stokes and continuity equations and assuming that small spherical particles, free to move, mimic the sediment grains. The numerical approach determines the flow around each sediment particle, along with the force and torque that the fluid exerts on it. Then, the particle trajectories are computed by means of Newton-Euler's laws. The parameters of the problem are the same as those of one of the laboratory experiments described by Blondeaux and Vittori (1991a) and the initial ripple profile is similar to that observed during the experiment. The numerical results are qualitatively compared with the experimental observations and support the reliability of the numerical simulation. Moreover, the results provide further valuable information on sediment dynamics under surface waves which propagate over a rippled bed. In particular, the obtained results allow a detailed analysis of the dynamics of the vortices generated by flow separation at the ripple crest and of the mechanism through which the sediments are lifted up from the bottom and carried into suspension. [Display omitted] • Large vortices shed from the ripple crests strongly affect the sediment transport mechanics. • Sediment pick-up and deposition rates were found lagged by a constant interval throughout the wave-cycle. • Indications for the modelling of suspended sediment concentration over a rippled bed are provided. [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]

Sediment motion on rippled bed under wave action plays an important role in coastal landforms, but the application of traditional numerical model is limited by large computational work or bottom sediment condition due to unsteady effects. A single-phase model is proposed to maintain low computation and high accuracy for the vortex structure development and suspended sediment motion on vortex rippled bed in wave-induced oscillatory flow. The 2D bottom sediment condition takes into account the acceleration effect and asymmetric boundary layer development under nonlinear waves as well as the sediment phase-lag. The particle inertia and wake flow around sediment particle are considered in sediment turbulence, and the SST k - ω turbulence model which is suitable for inverse pressure gradient flow on rippled bed is selected to close the Reynolds stress. The single-phase model well predicts the velocity on vortex ripples, periodic concentration at fixed points and vertical distribution of averaged sediment concentration. The results show qualitative formation, development and ejection of vortex, and demonstrate quantitative development of vortex center, radius and strength in symmetric and asymmetric flows. In addition, the model well performs the development of sediment cloud referring to vortex structure. • Efficient numerical model is developed for oscillatory sediment transport on vortex ripple and sheet flow. • Bottom sediment condition performs acceleration effect, asymmetric boundary layer and phase-lag. • Particle inertia and wake flow around particle are considered in sediment turbulence. • SST k - ω turbulence model is applied for inverse pressure gradient flow on vortex ripple. • Results show quantitative development of vortex structure and associated sediment cloud. [ABSTRACT FROM AUTHOR]

• Lateral sediment flux from hillslopes is directly connected to the curve number. • The curve number summarized the resistance from the soil-group complex. • The lateral sedimentologic connectivity was well represented by the curve number. • The soil-cover complex in a watershed is closely related to watershed erodibility. The flux of suspended sediment and bedload in a watershed is closely linked to the resistance of soil, known as erodibility. However, the relation between the resistance generated by soil to sediment transport from the hillslope to the river flow is still a challenge. Typically, the bedload flux is related to the shear stress of the main channel. However, the link between lateral sediment flux from hillslopes to suspended sediment flux of the main channel and the relation between bedload and suspended sediment flux to soil erodibility need to be developed. For three Brazilian watersheds, suspended sediment and bedload transport, soils, and hydrological data were obtained to determine watershed soil erodibility. The landscape connectivity expressed by hydrological-sediment connectivity was applied to determine the lateral sediment transport to the primary watershed channel. The flow resistance and lateral suspended sediment discharge were represented by the curve number, which summarized the resistance from the soil-group complex leading to soil erodibility. The curve number addressed the lateral flow and suspended sediment transport to represent the lateral connectivity on the landscape. Sediment transport (sand and suspended sediment) from hillslopes was added to the bedload transport along the longitudinal axis of the watershed, and the watershed soil erodibility (K W) was, thus, obtained for each of the three evaluated watersheds. [ABSTRACT FROM AUTHOR]

• Development of a new mathematical model for shallow water flows over multi-layered sedimentary beds in two space dimensions. • Modeling of a flux function for the bed changes in the vertical direction to allow mass exchange between the sedimentary layers. • Implementation of an accurate finite volume method of characteristics for solving the governing equations. • Numerical simulation of both evolving and steady-state flows over mutlilayer erodible beds. The two-dimensional modelling of shallow water flows over multi-sediment erodible beds is presented. A novel approach is developed for the treatment of multiple sediment types in morphodynamics. The governing equations include the two-dimensional shallow water equations for hydrodynamics, an Exner-type equation for morphodynamics, a two-dimensional transport equation for the suspended sediments, and a set of empirical equations for entrainment and deposition. Multilayer sedimentary beds are formed of different erodible soils with sediment properties and new exchange conditions between the bed layers are developed for the model. The coupled equations yield a hyperbolic system of balance laws with source terms. As a numerical solver for the system, we implement a fast finite volume characteristics method. The numerical fluxes are reconstructed using the method of characteristics which employs projection techniques. The proposed finite volume solver is simple to implement, satisfies the conservation property and can be used for two-dimensional sediment transport problems in non-homogeneous isotropic beds without need of complicated three-dimensional equations. To assess the performance of the proposed models, we present numerical results for a wide variety of shallow water flows over sedimentary layers. Comparisons to experimental data for dam-break problems over movable beds are also included in this study. [ABSTRACT FROM AUTHOR]

A three-dimensional tidal turbine simulation based on an oceanographic numerical model has been tested for suspended sediment calculation, particularly in the wake of a standalone tidal turbine. The results suggest a need for further improvement of the model in order to obtain correct predictions of suspension strength of the wake and suspended sediment concentration under the influence of a turbine (compared to measured data). Due to the wide use of FVCOM in coastal applications where turbines are commonly installed, it proves necessary to address this issue. Two approaches with respect to modifying bed shear stress and turbulent mixing calculations in the presence of a turbine are proposed and tested in this research. Using data collected in the laboratory as reference, the turbulent mixing enhancement approach is shown to be effective. A series of tests are carried out to identify the impact of the turbine on suspended sediment transport in its vicinity. The results suggest that the impact is highly dependent upon the sediment grain size. • A 3D tidal turbine simulation system is tested for suspended sediment calculation. • The system includes 3D representation of tidal turbine effects on flow structure and turbulence. • The system requires modification to produce accurate sediment suspension strength of the wake. • A turbulent mixing enhancement approach is shown to be effective. • The impact of turbine on suspended sediment transport is shown to depend on sediment grain size. [ABSTRACT FROM AUTHOR]

• The contribution of advection and resuspension was calculated by an instantaneous formula. • The proposed formula can reflect the hysteresis between suspended sediment and water flow movement. • The accuracy was improved by hybrid mechanism and ridge regression model. Estuarine and coastal areas are land-sea interaction zones where sediment particles can easily move, leading to water quality and aquatic ecological issues. The suspended sediment concentration (SSC) is an important indicator of the movement of suspended particles and coastal water environments monitoring. However, traditional in situ SSC observations cannot directly separated contributions of advection and sediment resuspension. In this study we aim to propose a hybrid mechanism and ridge regression model to improve the accuracy of instantaneous SSC estuarine and coastal waters. The new model is based on the application of hydrodynamic factors, functional principles and dimensional evaluation methods, which can analysis the effects of horizontal convective transport, vertical suspended upward movement on the SSC. It is compared with existing formulas (models) and mathematical model (Mike21) calculation results, using field data from single stations at the Yangtze River Estuarine. The comparison shows that the accuracy of the new SSC calculation model based on the equations in this paper is high. The Pearson correlation coefficient of the new model has increased by approximately 0.20, and the relative mean square error and the mean absolute error have decreased by approximately 0.26. In addition, the new model can explain the hysteresis of the sediment transport process and water flow change reasonably. [ABSTRACT FROM AUTHOR]

• 24,918 Landsat images focused on the lower Yellow River were used to monitor SSC during the period 1997–2020. • The proposed HGAR method significantly outperforms other retrieval methods in terms of retrieval accuracy. • HGAR is a reliable method for SSC retrieval and can be used for long-term monitoring of SSCs. Monitoring suspended sediment concentrations (SSCs) of channel waters is essential for evaluating important issues such as erosion, sediment transport and deposition, reservoir siltation, and water pollution throughout watersheds. Remote sensing technology provides a new approach to large-scale SSC monitoring, but the accuracy of existing retrieval methods based on remote sensing technology still needs to be improved for complex environments. This study proposes a new hyperparameter and globally adaptive retrieval (HGAR) method for river SSC retrieval that combines a remote sensing technique and machine learning methods. The HGAR method uses the light gradient-boosting machine method as the basic model for SSC retrieval, and the tree-structured Parzen estimator method, coefficient of determination, and residual prediction deviation are combined to optimize the basic model's hyperparameters. The experiment was conducted in the lower Yellow River, where 24,918 Landsat images from 1997 to 2020 were acquired using Google Earth Engine, and 1,772 monthly cloud-free images were obtained in the end. The sequential backward floating selection method was adopted to determine the optimal modeling features. Two regression and three machine learning methods were compared with the proposed method. The results demonstrate that the proposed HGAR method significantly outperforms other retrieval methods in terms of retrieval accuracy (as evidenced by a correlation coefficient of 0.626 and a Nash-Sutcliffe efficiency coefficient of 0.613), generalizability, and robustness. Finally, the model explanation results show that the HGAR method can successfully capture the key information for SSC retrieval and has higher flexibility than traditional statistical models. In addition, we found that the overall fit and the fit of local extreme values are inconsistent in the lower Yellow River, and the proposed improved accuracy indicator balances the two aspects well. In conclusion, the proposed HGAR method can be used effectively to estimate channel SSC and is expected to provide technical support for long-term and large-scale monitoring of the channel surface SSC. [ABSTRACT FROM AUTHOR]

Shore platforms are generally supply-limited environments and few studies have attempted to measure sediment transport within this rock coast setting. This paper summarises a field-based pilot study that successfully collected fine-grain sediment moving across a sub-horizontal platform at Mudstone Bay, Kaikōura, New Zealand. Two large-aperture and two small-aperture Time-Integrated Mass-sediment Samplers (TIMS) were deployed for 9-days, encompassing 2 days of storm conditions (offshore significant wave heights between 2.0 and 3.6 m, with sediment trapped in both landward and seaward facing directions. The net flux of sediment captured in large aperture samplers was greatest in a seaward-direction (15.2 g), whereas the narrow-mouth samplers, that principally collected suspended sediment had a net onshore flux (5.4 g). The sediment traps yielded sufficient sample to undertake geochemical and textural analysis. Interpretation of these data suggests that the origin of transported material was autochthonous to the platform, being >80 % silt-sized and compositionally consistent with material derived from the shore platform and/or cliff colluvium. This pilot experiment demonstrates that mass-sediment samplers can be used in supply-limited intertidal shore platform settings. Further research involving greater sampling frequency and duration and concomitant detailed hydrodynamic measurements are likely to reveal important insights into the erosion environment. • TIMS can be used on an intertidal shore platform to collect saltating and suspended sediment in bi-directional flow. • Sediment caught in sufficient amounts to conduct provenance and geochemical analysis • Storm conditions were most favourable to collect both saltating and suspended sediment at Kaikōura. [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]

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]

Field work combining in-situ mooring and erosion experiments was conducted in the intertidal flat of Asan Bay to reveal the relationship between sediment erodibility and external forcing and the contribution of local erosion to sediment transport. A total of 25 tidal cycles occurred during the mooring period with 6-h submergence and 5.7-h air exposure. The erosion rate (E) for sediment bed during air exposure was 0.007 g m−2 s−1, which was 36% of E (0.019 g m−2 s−1) during submergence. Although the sediment bed was influenced by consolidation during air exposure, the erodibility of the submerged bed was closely associated with the unconsolidated deposits of sediments newly supplied from tidal channel and bed shear stress (τ cw) induced by current-wave interactions. In particular, τ cw could be enhanced by strong winds, which depended on the ratio (H s / h) of significant wave height and water level, directly disturbing the sediment bed. Under weak winds (H s / h < 0.06), the newly deposited sediments with high erodibility increased the bed elevation. A small amount of the sediment bed was eroded (E : 0.006 g m−2 s−1), as a floc erosion, with a slight decrease in suspended sediment concentration (SSC T). This increased the contribution (57.2%) of eroded sediments to SSC T. Under strong winds (H s / h > 0.06) with enhanced τ cw , meanwhile, the erodible sediment in the bed was depleted accompanied by a decrease in bed elevation. A large amount of sediment bed was eroded (E : 0.025 g m−2 s−1), as a surface erosion, resulting in the rapid increase in SSC T and the enhanced seaward sediment flux. The sediments eroded from the bed were supplied to water column accounting for 86.8% of SSC T. This suggests that seaward sediment transport under strong winds could be caused by surface erosion shifting from floc erosion in the intertidal flat. [Display omitted] • In-situ erodibility of submerged bed was unaffected by consolidation in exposure. • Bed erosion type was shifted between floc and surface erosions depending on winds. • Surface erosion by strong wind increased fraction of eroded sediment in suspension. • Seaward exported sediments were mostly originated from bed under strong wind. [ABSTRACT FROM AUTHOR]

The launch of the Geostationary Ocean Color Imager (GOCI) satellite has made it possible to observe suspended sediment concentration (SSC) in the Bohai Sea (BHS) over a long time and a large area. We utilized remote sensing data from GOCI from 2011 to 2021 to study the spatiotemporal variation characteristics of SSC in the BHS and identified the contribution of each oceanic element to the changes in SSC. Our findings indicate that SSC is high in winter and low in summer, and related to the southeastward winds in winter (>7 m/s) and northwestward winds in summer (<4 m/s), with a correlation coefficient of 0.634. The average annual increase in SSC is 0.07 g/m3 (an increase of 1.2% per year). In terms of spatial distribution, SSC is highest in Bohai Bay and Laizhou Bay, followed by the Liaodong Bay, and is greater than that in the Central Bohai Sea, which is related to multiple river estuaries in the three bays. Additionally, the Bohai and Liaodong Bay in the BHS are covered by sea ice during winter, which affects the response of waves and currents to wind and changes the ocean current field, thereby affecting the resuspension and transport of sediment. Our EOF analysis revealed that wind-induced changes are the dominant factors in the variations of SSC, while changes in river runoff, sediment discharge, and sea ice cover are also important factors influencing SSC changes. • The annual increasing trend of the suspended sediment concentration (SSC) in the Bohai Sea has been analyzed using remote sensing data of GOCI satellite. • The periodic changes in wind speed dominate the periodic changes in SSC in the Bohai Sea. • The SSC in Liaodong Bay is lower than that in Bohai Bay and Laizhou Bay due to different ice coverage and sediment grain size. • Wind speed, river runoff, and sediment discharge contribute 71.8% to the SSC in the Bohai Sea through EOF analysis. [ABSTRACT FROM AUTHOR]

• Input and output time series from the conduit system of a karst aquifer are analyzed. • A simple water-balance model of epikarst zone is proposed. • Hourly internal runoff and diffuse drainage from epikarst zone are estimated. • Correlation and partial correlation functions are used. • A more precise insight into the sediment transport process in aquifer is obtained. Information about the transport of suspended sediment through a karst aquifer is obtained by the time series analysis of the input and outputs from the conduit system of vadose and phreatic zone. The input is the internal runoff from epikarst zone at the analyzed location. The outputs are the discharge from karst spring, as well as the turbidity and electrical conductivity of spring water. The diffuse drainage from epikarst zone is considered as a control process that changes the quantity of water in the conduit system, and consequently it affects the outputs. The time series of internal runoff and diffuse drainage are obtained by a simple water-balance model of epikarst zone based on the concept of saturation threshold. The proposed model has two parameters: maximal capacity of epikarst storage, and maximal diffuse drainage from epikarst storage, which are estimated by using the cumulative rainfall and turbidity data, as well as the value of runoff coefficient. The time series are analyzed by means of the correlation and partial correlation functions. The presented results show that the overall capability and accuracy of the time series analysis are significantly improved by considering the conduit system separately. It is possible to register and to characterize processes in karst aquifer that are not recognizable by using classical approaches. The results for Jadro Spring show that the memory of hypothetical discharge generated by the internal runoff is practically same as the memory of time series of turbidity. The memory of hypothetical discharge generated by the diffuse drainage is very similar to the memory of time series of discharge. The duration of turbidity response is 22 days, which entirely corresponds to the duration of fresh rainwater withdrawal from the conduit system. During the first 5 days, turbidity is generated mostly by the internal runoff from the central part of catchment. During the last 12 days, turbidity has been generated by the internal runoff from the eastern part of catchment. Two components in turbidity are recognized, which are manifested as two peaks in the turbidity graph. Both peaks are produced dominantly by autochthonous sediment. [ABSTRACT FROM AUTHOR]

• Based on Taylor series expansion to compute spatial derivatives, the consistent particle method (CPM) is expanded to simulate turbidity current involving sediment convection and turbulence-induced diffusion of water-sediment flow. • The proposed second-order CPM-upwind scheme to compute sediment convection is relatively easy to implement as it does not use kernel function which has the requirements of symmetry and unity. • In computing sediment diffusion, CPM shows very good accuracy for both isotropic diffusivity and anisotropic diffusivity. • In simulating turbidity current due to sudden release of water-sediment mixture into ambient water, the method captures the vortices and Kelvin-Helmholtz instability as well as achieves quantitatively good agreement with experimental results in the frontal motion and deposit density of water-sediment mixture. This paper expands the Consistent Particle Method (CPM) to simulation of turbidity currents with focus on turbulent water-sediment flow. A mixture model is adopted in solving the sediment transport equation to simulate the sediment mass transfer due to settling convection and turbulent diffusion. To compute the gravity-driven convection term, the proposed CPM-upwind scheme is relatively easy to implement as it avoids the use of a kernel function which has the requirements of symmetry and unity. Two sand settling problems demonstrate the good accuracy of the CPM-upwind scheme. For suspended sediment diffusion caused by turbulence, CPM is able to model anisotropic diffusion without the averaging effect caused by the isotropy requirement if a kernel function is used. The validated CPM methodology is then applied to simulate a sudden flow of water-sediment mixture into ambient water. The results of the mixture front motion and sediment deposit density agree well with published experimental results. [ABSTRACT FROM AUTHOR]

• Flow frequency distribution and sediment-transport rate were influenced by floodplain lake. • Frequency distribution of medium-high flow agreed well with broken power law. • Sediment-transport rating curve deviated from power law at high flow. • Effective discharges decreased by 10%–20% indicating flood channel shrinkage. In natural quasi-equilibrium rivers or anthropogenically-influenced nonequilibrium rivers, the occurrence frequency and sediment transport capacity of different flow levels are greatly important to understanding the linkage between forms and processes. However, our knowledge of the controlling factors on flow frequency and sediment-transport magnitude in large river systems remains limited. In this paper, the hydrological and sediment-transport characteristics of the middle and lower Yangtze River (MLYR) were systematically examined before and after impoundment of the Three Georges Reservoir (TGR). More than 30 years of daily discharge and suspended sediment concentration data were collected from six hydrological stations. The results showed that in the pre-TGR period, the flow frequency distributions (FFDs) and sediment-transport rating curves had segmentation characteristics. Generally, the FFDs of medium-high flow agreed well with the broken power law, and the sediment-transport rating curves deviated from the power law at high flow. This was because the floodplains, especially the Dongting Lake, had significant effects on flood attenuation and sediment storage. Compared the inflection points of the FFD and the sediment-transport rating curve, as well as the effective discharge and bankfull discharge, these four values were of the same order of magnitude. This suggested that the local river system had a dominant effect on the hydrological and sediment-transport characteristics. In the post-TGR period, the overall characteristics of the FFDs were preserved, but the segmental characteristics of the sediment-transport rating curves at higher discharges disappeared. Through analysis, the effective discharge was largely determined by the inflection value (a 1) of the FFD. The reduced a 1 induced a 10%–20% decrease in effective discharge, which indicated flood channel shrinkage in the long run. These findings not only further the understanding of water-sediment regimes in a large alluvial river with an extensive floodplain system but also offer guidance for improving dam operation schemes for river restoration. [ABSTRACT FROM AUTHOR]

Suspended sediment concentration (SSC) is one of the fundamental topics in sediment study. The parameterization of the SSC profile of silty sediments is still under-researched. This study focuses on the mean SSC profile for silty sediments under non-breaking wave-dominant conditions. First, inspired by a 1DV model, different types of the distribution of mean eddy viscosity were proposed, i.e., a toe-type distribution over flat bed and a constant-toe type distribution over rippled bed. Then, the time-averaged diffusion equation for suspended sediment transport was analytically solved, and expressions for the mean SSC profiles were derived. The expressions involve several basic physical processes, including the effects of bed forms, stratification, hindered settling and mobile bed. Verification using a number of experimental datasets showed that the proposed expressions can properly calculate the mean SSC for silt and are applicable for sand as well. In conclusion, this research provides an approach to estimate the mean SSC for silty sediments under wave-dominant conditions, which is expected to be applicable for engineering practice and numerical modelling. • Formulations for mean SSC (suspended sediment concentration) profile of silty sediments were derived. • Several essential physical processes were considered in the formulations. • The SSC profiles over rippled bed and hydrodynamically plane bed were simulated. [ABSTRACT FROM AUTHOR]

In situ measurements and ship-based resuspension experiments using annular flumes are used to determine sediment stability and critical erosion thresholds for four sites with significantly different sediment characteristics, located in the Celtic Sea at water depths of 100 m. Seasonal and spatial variability of sediment characteristics and erodability is examined, and found to be the result of changes in percentage of organic carbon in the surface sediments (R2 = 0.82) and bulk density (R2 = 0.73) respectively when individual characteristic bed parameters are considered. Principal component analysis and linear regression analysis are used to determine a predictive model for erosion threshold in the Celtic Sea (R2 = 0.99), based on grain size, sorting, kurtosis, bulk density, porosity, percentage fines, organic carbon content and chlorophyll a concentration. Physical sediment characteristics were found to be more significant controls of bed stability than biological factors. Local hydrodynamic conditions are used to determine the likelihood and frequency of resuspension given these critical erosion thresholds. Resuspension is driven by tidal currents, and is common year-round, leading to a constant re-working of bed sediments in particular at the muddier sites. This is confirmed by in situ measurements of suspended sediment concentration. • Bed erodibility in the Celtic Sea a function of organic carbon and bulk density. • Predicting erosion threshold requires emphasis on physical sediment characteristics. • Bed sediments re-worked year round by tidal resuspension, enhanced by winter waves. [ABSTRACT FROM AUTHOR]

The temporal evolution of water column turbidity was studied on a submarine canyon on the Barcelona continental margin. From April to June 2014, an instrumented mooring array equipped with an autonomous hydrographic profiler with a CTD and a turbidimeter was deployed in the Foix canyon axis at 870 m depth. The instruments were programmed to collect hydrographic profiles once per day to provide a view of the temporal evolution of water column characteristics from 200 to 800 m water depth. The results illustrate a well-defined water turbidity structure of particulate matter distributed in intermediate nepheloid layers (INLs) developed between 300 and 500 m water depth and above the canyon rims, and INLs and near-bottom nepheloid layers (BNLs) confined inside the canyon between 650 and 800 m water depth. Data from fishing vessels activity at the time of the deployment was obtained from Vessel Monitoring System (VMS). The presence and location of the fishing vessels in the study area suggested a relationship between trawling activity and the generation of such layers. Nepheloid layers were absent during the first part of the deployment, when there was no fishing activity within the Foix Canyon axis or at the adjacent continental slope. Later, with the beginning of trawling activity on the fishing grounds close to the canyon, both INLs and BNLs were observed in the profiling casts, suggesting a causative relationship with fishing activities. Additionally, the hydrodynamic conditions within the canyon also seem to favour particle retention and to increase water turbidity in thick BNLs when water circulation is directed up-canyon. Bottom trawling appears to act as a main sediment resuspension mechanism in the Barcelona continental slope regions, increasing suspended sediment concentration at specific water depths where fishing grounds are located. Suspended particles are then advected and propagate along and across-margin by ambient currents via nepheloid layers. • Particulate matter in the Foix Canyon was mainly distributed in INLs and near-BNLs. • INLs and BNLs were observed when fishing activity was detected close to the canyon. • Hydrodynamic conditions seem to favour particle retention and to increase turbidity. • Bottom trawling acts as a main sediment resuspension mechanism in the Barcelona margin. [ABSTRACT FROM AUTHOR]

• Eight of thirteen typhoons occurred in 2016 impacted the Yangtze Delta wetland. • The impacting typhoons were 450–2000 km away from the delta. • The remote typhoons caused 2–5 times maximum increases in sediment dynamics. Intertidal wetlands are widely considered to be vulnerable to storm impacts. While much attention has been given to the effect of hurricane/typhoon landfall on local erosion/deposition in intertidal wetlands, less is known about how distant hurricanes/typhoons can affect an intertidal wetland or about the comprehensive effects of hurricanes/typhoons on hydrodynamics, sediment transport and seabed stability. Here, we investigate the possible impacts of thirteen typhoons that occurred in the western Pacific during summer–autumn 2016 on an intertidal wetland in the Yangtze Delta. We detected the impacts from eight of the typhoons during half of their duration when these typhoons were 450–2000 km away (nearest distance ever for each typhoon) from the delta. These remote typhoons caused 2–5 times maximum increases in hydrodynamics and suspended sediment concentration. Correspondingly, considerable net mudflat erosion and marsh deposition were observed. We therefore conclude that intertidal wetlands can be strongly affected by typhoons, even those passing by hundreds of kilometres away. These findings contribute to understand how far and in what extent hurricanes/typhoons can affect intertidal wetlands and to increase knowledge needed for coastal management. [ABSTRACT FROM AUTHOR]

Sediment transport over Sandy Point dredge pit in the northern Gulf of Mexico during a cold front event in November 2014 was examined using a finely resolved numerical model. The Delft3D model was used to perform numerical experiments that simulate the effect of wind-generated waves, wind-driven currents, river discharge, and tides on sediment dynamics in Sandy Point dredge pit. The hydrodynamics and sediment models were validated and calibrated using field data of current, wave, water level, and suspended sediment concentration. Two potential sources of sediment were examined: fluvial sediment from the Mississippi River and resuspended sediments from the seabed. Results showed that during a cold front, shear stress from wave motions played a significant role in the resuspension of sediments in Sandy Point dredge pit. The maximum cold front-related wave impact on sediment resuspension could increase near-bed sediment concentration in Sandy Point dredge pit by 20–50 times. In addition, the results suggest that the primary source of sediment for Sandy Point dredge pit during a cold front was resuspension from the ambient seabed due to increased bottom shear stress by wind-induced waves and strong southward wind-driven currents. Currents dispersed sediments from the Mississippi River passes and inhibited riverine sediment supply from Sandy Point dredge pit. Results also showed that cold fronts contribute 16%–24% of the annual sedimentation in Sandy Point dredge pit. • A coupled wave-current-sediment model was used to study the sandy dredged pit. • The combined effect of waves and currents on sedimentation was assessed. • The reverse bottom current inside the pit was identified. • Bottom boundary layer was significantly affected by waves. • Sediment accumulation occurs mostly on the north of the pit. [ABSTRACT FROM AUTHOR]

A model is proposed to determine the sediment transport rate generated by the combined action of sea waves and tidal currents, the latter being modelled as a sequence of steady currents because of the large values of the Keulegan-Carpenter number of the tidal flows, which allow to neglect inertial effects in their modelling. The hydrodynamic module solves Reynolds-averaged momentum equations by introducing a two-equation turbulence model, which considers also the flow in the region closest to the sea bottom and enforces the no-slip condition and the vanishing of the turbulent kinetic energy at the bottom without the need to assume the existence of a logarithmic velocity profile and to fix the normal derivative of the turbulent kinetic energy close to the sea bed. Hence, the evaluation of the velocity profile in the near-bed region turns out to be accurate. The sediment transport module determines the sediment concentration and sediment transport for a mixture of non-cohesive and cohesive sediments. To validate the model, its predictions are compared with laboratory measurements and field data collected over rippled beds at two different tidal estuaries. Then, results are obtained for values of the parameters chosen to mimic a site close to Punta Umbría (Huelva, Spain) and considering different hydrodynamic conditions and different sediment mixtures. The influence of the waves on the suspended sediment concentration is lower for the cohesive fraction than for the non-cohesive fraction. For all the sediment mixtures, it turns out that the cohesive fraction is well mixed over the whole water depth, meanwhile the concentration of the non-cohesive fraction has significant values only near the bottom, thus showing that an accurate description of turbulence dynamics in the buffer layer and viscous sublayer (if it exists) is important to quantify the sediment transport rate. • Sediment transport module that reproduces the concentration of cohesive fraction is presented. • Tidal current is modelled as a sequence of steady-currents due to the large values of the Keulegan-Carpenter number. • Menter's turbulence model to predict the turbulence structure both in the wave bottom boundary layer and in the core region. • Turbulence dynamics description in the buffer layer and viscous sublayer is important to quantify the sediment transport. • Applicable to any type of sediment mixture. [ABSTRACT FROM AUTHOR]

Sandy beaches are highly dynamic due to the transport of sediment by waves and currents. While beach erosion is mainly driven by cross-shore currents, beach recovery is driven by the onshore-directed transport of sediment by incoming short waves. This latter process is complex and often inaccurately predicted by morphodynamic models. This is partly because of the omission of the effects of bed form-induced and surface-generated turbulence on sediment pick-up in these models. In this paper, the phase relationship between the oscillatory flow, turbulence and suspended sediment concentration was investigated based on field measurements obtained beneath shoaling waves, spilling breakers and surf bores. Wave-averaged and intra-wave variability in turbulence and suspended sediment concentration were analysed on a wave-by-wave basis, where individual waves were extracted and then grouped according to relative wave height. Beneath shoaling waves, small but steep wave ripples were shown to affect the phasing of suspended sediment concentration and to cause an offshore-directed short-wave suspended sediment flux. Beneath spilling breakers and surf bores, sediment was mainly stirred under the wave crest and the short-wave suspended sediment flux was onshore-directed. Based on the analysed data, several mechanisms were suggested to cause sediment stirring: friction between the wave orbital motion and the seabed, penetration of surface-generated turbulence towards the seabed, increased bed shear stress at the wave front due to acceleration skewness, and during conditions with strong undertow, wave-current interactions. [ABSTRACT FROM AUTHOR]

• A new three-phase model for sediment transport problems with a water-air interface. • A special modification to the interface-compression VOF method. • A verified three-phase model for local scour caused by submerged wall jets. Interactions between fluid and hydraulic structures and the resulting bed scouring are complex phenomena that involve three phases: water, air and sediment. This study presents a new rheology-based three-phase flow model that can (i) track the water-air interface in the presence of suspended sediment and (ii) predict the local scour caused by submerged wall jet. A modified volume-of-fluid (VOF) method was used to track the water-air interface, and a modified k-ϵ turbulence model was employed to capture important features of the turbulent flow, including turbulence modulation due to fluid-sediment interaction. The three-phase model was first used to study the sediment transport in open-channel flows in order to evaluate the performance of two models for particle response time, and then employed to simulate the scouring process downstream a submerged wall jet issued from a sluice gate. The simulated bed profiles showed good agreement with the measured. It was demonstrated that the three-phase model could capture important dynamic features such as sediment avalanche. [ABSTRACT FROM AUTHOR]