Simulating sediment dynamics in a large and energetic estuary system remains challenging, primarily due to the spatial and temporal complexities of the interaction between flow and sediment transport, especially for sand-mud mixtures. This study uses a three-dimensional (3D) numerical model, based on the open TELEMAC system, to investigate the dynamics of suspended sediment concentration (SSC) in the Gironde Estuary, a complex estuarine environment characterized by an estuarine turbidity maximum (ETM) and significant variations in river discharge. The main contributions of this study include addressing the challenges of coupling bed friction with sediment transport of the sand-mud mixture for feedback on bed roughness and bottom depth changes and the ability of the model to capture the migration of ETM from high to low flow. Additionally, the current study analyzes the ability of the model to capture the migration of ETM from high to low flow, and it utilizes a calibration strategy that minimizes parameters by using in situ data and encompassing hydro–morpho-sedimentary interactions. A sensitivity analysis was done using different settling velocity approaches and sediment classes to establish an optimal model configuration and the uncertainty associated with the reduced model parameterization is discussed. The model satisfactorily reproduces the hydrodynamic features, particularly when the hydro-sedimentary feedbacks are taken into account, the seasonal trend of SSC, spring-neap variations, and the development of a well-defined ETM. The selection of a specific formulation for the settling velocity influences the location and magnitude of ETM. The van Leussen formula not only predicts a broad movement of ETM from high to low river flow, but also predicts high turbidity for extended periods during low river flow. Conversely, two empirical formulas from Le Hir and Defontaine predicted the highest turbidity during neap tides but sediment losses during prolonged simulations. The results of this study contribute to a deeper understanding of sediment dynamics in the Gironde Estuary, providing valuable information for future estuarine modeling and management.

This study presents a novel analytical framework for modeling one-dimensional solute transport in rivers, integrating advection, rate-limited adsorption on suspended sediments, and first-order degradation. Analytical solutions are used to validate the numerical scheme's accuracy under idealized conditions, tested for instantaneous and continuous pollutant discharges. The research importantly investigates short-term solute accumulation in riverbeds, a critical yet understudied process that affects sediment transport and pollutant fate. Applicable to a wide range of contaminants (e.g., nutrients, pesticides), the framework aids water quality assessment, pollution control, and risk mitigation. Implemented in the open-source SWASHES library, these solutions provide practical tools for decision-support systems and serve as reliable benchmarks to validate numerical models. By addressing transient and persistent pollutant scenarios, this work enhances predictive capabilities for environmental management. The approach bridges analytical and numerical methods, offering robust foundation for simulating solute transport across industrial and ecological contexts, advancing sustainable water resource management.

Active geomorphological interventions, such as reprofiling of river bars, are often used to increase bar dynamics and prevent vegetation encroachment. River restoration management should be planned based on the knowledge of what processes will follow the intervention and on the anticipation of the consequences. However, in many cases, the associated physical processes are not clearly identified whereas their consequences on bar morphodynamics are still not fully understood. This study aims to bring new insights into the biomorphodynamics evolution of the riverbed after restoration works by using a 2D biomorphodynamic model developed in the TELEMAC-MASCARET system. It seeks to compare and evaluate the performance of five bar reprofiling scenarios in which the bar elevation is lowered to just below the water level at specified design discharges. The study area is located in the channelized and regulated alpine gravel-bed Isere River (France). Bar dynamics and early stages of vegetation establishment are analyzed for the first 2 years after each restoration scenario. The results indicate that plant colonization would occur in all cases. Overall, maximizing the reduction of bar height is the most effective way to improve the bar dynamics and limit future vegetation encroachment.

In rivers, coastal seas and transitional waters, sediment transport processes involve a variety of interacting factors that dynamically vary over time and space. The flow dynamics within these highly heterogeneous natural systems influences the spatial patterns of erosion and deposition of the bed sediments, which in turn shapes and conditions the bottom morphology. By taking advantage of the established modelling framework in both two- and three dimensions for unstructured meshes proposed within the Telemac-Mascaret system, the new module Gaia provides a code structure for solving sediment transport and morphological evolution problems. By a clear treatment of sedimentary processes that happen in the water column, in the bed structure, and at the water–bed interface, Gaia efficiently manages the spatial and temporal variability of sediment size classes, properties and transport modes for two- and three dimensions. In addition, this module can easily be expanded and customised to particular requirements by modifying user-friendly, easy-to-read, and well-documented FORTRAN-90 subroutines.

The present study investigates the morphodynamic interactions between migrating bars in a sandy gravel and nearly straight channel of the Loire River (France). From a large dataset collected from field campaigns performed between 2016 and 2020, we analysed the deflection power exerted by bars on flow and sediment transport that influences the dynamics of other bars present in the channel. We also investigated the role of low flows in bar formation. To this aim, the riverbed evolution has been documented using four years of bathymetrical and aDcp surveys. Flow deflection induced by the presence of bars in the channel differs for pre-existing (first-order) bars and bedload sediment accumulations identified as new developing bars (second-order bars). The latter needs to reach an equilibrium size to influence flow direction. During low flows, first-order bars emerge and create contraction/expansion zones in the channel, inducing a temporary local geometrical forcing that leads to sediment deposition at the outlet of these zones. Sediment deposited in these specific areas can constitute a nucleus for second-order bar formation or contribute to first-order bar aggradation. According to the linear free bar theory, the critical discharge that contributes to expressing an alternate bar pattern is exceeded during moderate flood, leading to the observation of a transitional bar pattern between alternate and central bar pattern.

Tides, winds, and waves drive the hydrodynamics in the coastal zone.Under their action, the seabed can be mobilised to form what is collectively known as "bed forms".This study focuses on one type of bed form: marine dunes in shallow shelf seas.Marine dunes are dynamic, rhythmic, large-scale, flow-transverse sedimentary structures.They are characterised by wavelengths in hundreds of metres, heights of a few metres and can migrate up to tens of metres per year.They have been found in shallow shelf seas all over the world, e.g. in the North Sea, the Barents Sea, the South China Sea, and the Bisanseto Sea.As mapping of the world's continental shelves progresses, we may find that marine dunes are recurrent shallow shelf sea features.This study aims at improving our understanding of the hydrodynamics, the sediment transport and morphological processes at play in a marine dune environment.Indeed, the morphology and dynamics of dunes are still poorly understood in open marine environments.A three-dimensional coastal area model is being developed for an application in the southern North Sea.The site is subjected to relatively strong tidal flows, with a predominance of the flood towards the North-East.Waves primarily come from the South-West, travelling through the English Channel, but some significant events have been noted from the North-North-East.Recurrent bathymetric surveys indicate dune migration rates of up to 30 metres per year towards the North-East.Different steps towards the development and validation of this numerical model are presented.

Marine dunes are sedimentary forms typically encountered on continental shelves. They migrate under the combined action of tidal currents and waves. Such an active environment poses a challenge to the design, safety, and maintenance of offshore and coastal works. Due to the continuous seabed evolution created by marine dune dynamics, offshore wind farm (OWF) elements, such as pile foundations and cables, are at risk of becoming exposed, weakening their integrity and stability, or on the contrary overburied, generating additional mechanical and thermal loads. Local scour at the toe of individual structures, and global scour resulting in the general lowering of the seabed around a group of structures, can also be elements of concern. Being able to predict the evolution and migration of marine dunes is, therefore, critical to limit damage to the infrastructures and to design effective protection works where needed. In this context, the present work will investigate marine dune dynamics at different spatial and temporal scales (from metres to kilometres, from days to years) using a complex process-based model: the suite of open-source numerical solvers TELEMAC-MASCARET. The objective is to gain a better understanding of the hydrodynamics, the sediment transport and morphological processes at play in a marine dune environment, as well as of the mutual interactions between the dune field and the OWF elements. The model capabilities to reproduce large-scale sediment transport processes in OWF environments and to obtain accurate diachronic predictions of the dunes’ evolution will be assessed in this work, and further developed if necessary. A large dataset (bathymetric surveys over several periods, metocean data, and sediment data) has been collected in the last few years for a proposed OWF project off Dunkirk, France. These data will prove invaluable to assess the model performance. They indicate dune migration rates of tens of metres per year in places. The site is subjected to relatively strong tidal flows, with a predominance of the flood towards the North-East. Waves are primarily from the South-West, travelling in the Channel, but some significant events from the North-North-East have been noted. This work is part of the 3-year MODULLES project: MOdelling of marine DUnes: Local and Large-scale EvolutionS in an OWF context, funded by France Energies Marines and the French government, under the “Investissements d’Avenir” programme managed by the French National Research Agency ANR. It is hosted by the Saint-Venant Hydraulics Laboratory (LHSV).

This study investigates the effects of vegetation patch patterns on the morphological evolution of alluvial river channels at the reach‐scale. For this, a new two‐dimensional numerical biomorphodynamic model has been developed using the Telemac‐Mascaret system. Considering the newest development in the topic, the effects of vegetation on bedload transport are included by extending Einstein's parameters for the sediment transport formula. The model was subsequently validated by published laboratory experiments reproducing alternate bar dynamics with different vegetation establishment scenarios. The validated model was then used to study the influence of vegetation patch patterns on the channel morphological evolution considering the two most observed ones: (a) the filled pattern with plants well distributed within the patch, and (b) the stripe pattern with plants established only along the patch edges. 14 scenarios were simulated in total, including sensitivity analyses on the coefficients of vegetation characteristics. The results indicate that the morphological responses of an alternate bar system to the stripe pattern consist of channel widening, steeper slope and reduced water depth, with increased sediment transport rates. The effects of the filled pattern are similar but weaker. The results also show that with the stripe pattern, the alternate bars tend to migrate toward the centerline forming center bars. Besides, the scroll bars forming downstream are shorter, corresponding to less visible chute channels, compared to the filled pattern. Despite much less vegetation coverage, the stripe pattern decreases the bar elongation rates in a way similar to the filled pattern.

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.

Flow, suspended sediment transport and associated morphological changes in the Vietnamese Mekong Delta (VMD) are studied using field survey data and a two-dimensional (2D) depth-averaged hydromorphodynamic numerical model. The results show that approximately 61-81 % of the suspended sediment load in the Hau River during the flood seasons is diverted from the Tien River by a water and suspended sediment diversion channel. Tidal effects on flow and suspended sediment load are more pronounced in the Hau River than in the Tien River. The results show the formation of nine scour holes in the Tien River and seven scour holes in the Hau River from 2014 to 2017. Additional six scour holes are likely to form by the end of 2026 if the suspended sediment supply is reduced by 85 % due to damming. Notably, the scour holes are likely to form at locations of severe riverbank erosion. In the entire study area, the simulated total net incision volume in 2014-2017 is approximately 196 Mm(3) (equivalent to 65.3 Mm(3)/yr). The predicted total net incision volumes from 2017 to 2026 are approximately 2472 and 3316 Mm(3) under the 18 % and 85 % suspended sediment reduction scenarios, respectively, thereby likely threatening the delta sustainability. The methodology developed in this study is helpful in providing researchers and decision-makers with one way to predict numerically the scour hole formation and its association with riverbank stability in river deltas. Of equal importance, this research serves as a useful reference on the role of water and suspended sediment diversion channels in balancing landforms in river-delta systems, particularly where artificial diversion channels are planned.

The Rance estuary is a small steep-sided ria, located in the Brittany coast of northern France, with a maximum spring tidal range of 13.5m and an average river discharge of 7m3/s. Taking advantage of this significant tidal range, the Rance tidal power station (RTPS) was built in the 1960s as the world’s first and largest tidal power plant, with peak output capacity of 240 Megawatts. It is currently the second world’s largest tidal power installation after the Sihwa-Lake tidal barrage. The RTPS has two active parts: a barrage of 6 sluice gates and a structure of 24 turbines. Despite a well-known effect of the plant on damping estuarine water levels, little attention has been given to currents vertical distribution and the plant's impact on the dynamics of freshwater-saltwater interface. Therefore, a three-dimensional model of the Rance estuary was developed. Moreover, currents and salinity measurements were carried out to validate the numerical model. Simulated and measured currents showed that (i)the RTPS induces an acceleration of flood currents directly upstream of the sluice gates and (ii)ebb currents are strengthened by the narrowness of the Saint-Hubert-Port. Finally, salinity analyses assessed the dynamics of the freshwater-saltwater interface which is pushed further upstream during summer.

A 3D model of the Gironde estuary has been built to predict the flow pattern, sediment transport and associated bed evolution. The bed friction is estimated using the bed roughness predictor of van Rijn. It requires to provide a distribution of the sediment at the bottom as the percentage of mud content influences the bed friction and associated sediment transport. Due to limited data availability, some assumptions about the bed mixture are necessary to feed the numerical model. The accuracy of the methodology has been assessed through comparisons with in situ data (water level, velocity and salinity). Accuracy is water levels is about 15 cm and the relative errors are lower than 20% for velocity and salinity for most of the stations. The numerical model has then been used to highlight the sensitivity of the bed distribution: initial bed composition, bed load and fluid mud attenuation. Finally, the evolutions of flow and bed patterns have been analyzed for two contrasted hydrological conditions.

An automatic procedure to identify the bed friction coefficient is tested on a 2D hydrodynamic model of the Gironde estuary (France). The proposed procedure involves an optimization algorithm based on evolution strategy, namely Covariance Matrix Adaptation Evolution Strategy. Without optimization, application of the same friction distribution to different hydrological conditions leads to significant relative error in water level prediction up to 20%-30%. For the tested configuration, 300 runs seemed to be sufficient to reach an optimal value whereas an additional 200 runs would help to gain an accuracy of a few millimeters (or 0.3%). In order to reach the same level of accuracy for the different hydrological configurations, it is necessary to adapt for each configuration of the bed friction coefficient. Such behavior tends to confirm a seasonal variation of the friction coefficient and this is particularly the case in the central part of the estuary. Different relationships of the friction coefficient according to the flowrate have been incorporated inside the 2D hydrodynamic model. These relationships effectively maintain an accurate prediction of the water levels close to 10% for a wide range of hydrological configurations. (C) 2021 American Society of Civil Engineers.

Data assimilation (DA) is widely used to combine physical knowledge and observations. It is nowadays commonly used in geosciences to perform parametric calibration. In a context of climate change, old calibrations can not necessarily be used for new scenarios. This raises the question of DA computational cost, as costly physics-based numerical models need to be reanalyzed. Reduction and metamodelling represent therefore interesting perspectives, for example proposed in recent contributions as hybridization between ensemble and variational methods, to combine their advantages (efficiency, non-linear framework). They are however often based on Monte Carlo (MC) type sampling, which often requires considerable increase of the ensemble size for better efficiency, therefore representing a computational burden in ensemble-based methods as well. To address these issues, two methods to replace the complex model by a surrogate are proposed and confronted : (i) PODEn3DVAR directly inspired from PODEn4DVAR, relies on an ensemble-based joint parameter-state Proper Orthogonal Decomposition (POD), which provides a linear metamodel ; (ii) POD-PCE-3DVAR, where the model states are POD reduced then learned using Polynomial Chaos Expansion (PCE), resulting in a non-linear metamodel. Both metamodels allow to write an approximate cost function whose minimum can be analytically computed, or deduced by a gradient descent at negligible cost. Furthermore, adapted metamodelling error covariance matrix is given for POD-PCE-3DVAR, allowing to substantially improve the metamodel-based DA analysis. Proposed methods are confronted on a twin experiment, and compared to classical 3DVAR on a measurement-based problem. Results are promising, in particular superior with POD-PCE-3DVAR, showing good convergence to classical 3DVAR and robustness to noise.

Despite the inherent difficulties in quantifying its value, bedload transport is essential for understanding fluvial systems. In this study, we assessed different indirect bedload measurement techniques with a reference direct bedload measurement in a reach of a large sandy-gravel-bed river. An acoustic Doppler current profiler (aDcp), the dune tracking method (DTM) and hydrophone measurement techniques were used to determine bedload transport rates by using calibration with the reference method or by using empirical formulas. This study is the first work which attempted to use a hydrophone to quantify bedload rates in a large sandy-gravel-bed river. Results show that the hydrophone is the most efficient and accurate method for determining bedload fluxes in the Loire River. Although further work is needed to identify the parameters controlling self-generated sediment noise, the calibration procedure adopted in this study allows a satisfactory estimation of bedload transport rates. Moreover, aDcp and hydrophone measurement techniques are accurate enough to quantify bedload variations associated with dune migration.

Morphodynamics and hydro-sedimentological processes in meandering rivers are one of the most complex phenomena observed in alluvial channels. Because of the complex nature of the interaction between flow structure, morphology, sediment transport, and bank roughness, a better understanding of these morphological units is needed, particularly for low-gradient rivers characterized by large meandering bends with high width-to-depth ratios. The present research provides an accurate description of the interaction between suspended bed-sediment transport, flow structure, and bed morphology on three consecutive bends characterized by width-to-depth ratios higher than 50. The study focuses on a selected reach of the Colastiné River, which is a secondary channel of the Paraná River, Argentina. Acoustic measurement techniques with high spatial-time resolution were employed during two different events - a bankfull and a medium-flow stage event - to capture the three-dimensionality of the flow velocity, suspended bed-sediment transport, and variations in bed morphology. Although the core of maximum velocity shifts from bank to bank at the bend entrance, following the thalweg shifting, the suspended bed-sediment concentration remains in the center of the channel because of the strong influence of the secondary currents and the bed morphology. Two types of secondary flows are well-defined in the cross section: a unidirectional flow toward the outer bank generated by the topographic steering effect along the point bar, and the classical helical motion confined to the thalweg zone. Close to the outer bank, the core of maximum velocity shifts toward the center of the channel because of the presence of macro-roughness induced by banklines and downed trees, which in turn generate high turbulence patterns along the outer bank. The bed morphology shows an extended point bar occupying almost half of the channel width (in the cross section apex) and spreading downstream to the entrance of the following bend. The thalweg shows an abrupt change from bank to bank, producing high curvature at each bend entrance. The findings reported herein show a lack of correlation between the cores of maximum velocity and suspended bed sediment, exhibiting different behaviors than those observed in smaller alluvial channels with lower width-to-depth ratios.

NavTEL is a new decision support tool for the short-term (36 h) planning of ship routes and the management of underkeel clearance in estuarine navigation channels. NavTEL uses a deterministic method and is coupled with the TELEMAC-MASCARET system for numerical modeling of hydrodynamic and sediment transport in the estuary with a two-dimensional approach. In its present version, NavTEL allows (i) daily simulations to be automatically prepared and launched; and (ii) simulation outputs to be postprocessed to find the safest ship route and to predict underkeel clearances at specified locations. As the reliability of the results relies on the accuracy of water-level predictions, numerical simulations were performed with measured river discharges, storm surge forecasts, and time-varying friction coefficients for bed roughness. Even though NavTEL was initially developed for the Atlantic Port of Bordeaux located in the Gironde Estuary, its kernel has a modular structure allowing the tool to be adjusted to different port configurations and types of water bodies. Finally, examples of graphical outputs and reports generated by NavTEL are shown for an application of a container ship coming into the port of Bordeaux.

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 13.5 m. Taking advantage of this hyper-tidal regime, the first and currently the second largest operational tidal power station in the world was built at the estuary's mouth and has been in operation since the 1960s. Despite the well-known effect of damping of estuarine water levels, little attention has been given to quantifying the influence of the plant on the propagation and asymmetry of the tidal wave inside the estuary. In this study, hydrodynamics and tidal wave patterns were analyzed in this anthropogenically influenced estuarine system. A two-dimensional depth-averaged numerical model of the Rance estuary was developed. Two scenarios without the tidal power plant involving the dam's pre- and post-construction bathymetry (1957 and 2018 respectively) and present-day conditions scenarios were designed, to highlight the impact of bed evolution and the tidal power station on hydrodynamics and tidal asymmetry. Numerical results showed that, without the structure, bathymetric evolution did not substantially influence estuarine hydrodynamics. Nevertheless, on the estuary-side of the dam, the presence of the tidal power plant induced (i) a decrease in both tidal range and tidal prism, (ii) an increase of low water levels, and (iii) a decrease in both flood and ebb currents. Contrastingly, the region close to the structure reacted differently to plant operating modes, with an increase in flood currents (ebb currents) upstream of the sluice gates (downstream of the turbines). For both the natural condition and the artificially-induced hydrodynamic forcing due to the presence of the plant, numerical results showed that the Rance estuary mainly exhibits flood-dominant behavior, with a longer duration of falling than rising water and stronger peak flood currents than ebb currents. Spanning a period of approximately 60 years, this study presents a quantitative analysis of the influence of the tidal power station on the hydrodynamics in the Rance estuary, and its possible consequences for sediment dynamics. This approach is novel for this particular enclosed water body, characterized by the presence of a dam at its mouth and a lock at its uppermost limit.