Over the past century, the main channel of the Waal has experienced erosion of approx-imately 1-2 metres (Ylla Arb´os et al., 2021; Chowdhury et al., 2023). This erosion leads to various problems such as instability of struc-tures or disruption to shipping. To address this ongoing degradation, a potential solution is the implementation of sediment nourishments. Recent pilot studies have been conducted in 2016 and 2019 to investigate the feasibility of using sediment nourishments in the main channel of the Dutch Rhine (Becker, 2023). Another possibility of nourishing is to add sed-iment to the groyne fields. Under the influence of currents and ship waves, sediment is ex-pected to be transported to the main channel, causing a groyne field to act as a sand mo-tor. To explore this concept, Rijkswaterstaat initiated a pilot project with sediment nourish-ments in three groyne field clusters along the Waal during the fall of 2023. The pilot includes an extensive measurement campaign.@en

The sediment transport direction is affected by the bed slope. This effect is of crucial importance for two- and three-dimensional modelling of the interaction between the flow of water and the alluvial bed. It is not uncommon to find applications of numerical morphodynamic models in the literature that exaggerate the effects of transverse bed slopes on sediment transport compared to results from laboratory experiments. We investigate mathematically the consequences of such an approach, and we analyse through numerical simulations different explanations for the need to apply deviating values. The study reveals that the reason often lies in the setup of the numerical models, such as the choice of mesh resolution or the necessity to comply with specific aspects of the numerical scheme. The missing or inadequate implementation of physical processes in the model is another cause. All of these effects can be compensated by artificial diffusion added through the bed slope effect coefficients. Since increased diffusion strongly alters the physical processes of self-formed bed morphology, we recommend that modellers address the root causes of inflated erosion and deposition. Bed slope effect coefficients should be applied within the range found in the original publications.

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Climate change puts pressure on river systems, as it increasingly alters the river controls. Engineered rivers with a fixed planform respond to climate change and human intervention by adjusting the channel slope and bed surface grain size distribution. This response often consists of channel bed incision, over hundreds of kilometres, and during decades to centuries, resulting in serious disruptions of inland navigation, increased flood risk, and ecological degradation. Here we investigate how the lower Rhine River (Bonn, Germany – Vuren, the Netherlands, including the Pannerden bifurcation) continues to adjust to channelization measures of the 19th century (Ylla Arbós et al., 2021), and responds to different climate scenarios of control change over the 21st century, using a schematized one-dimensional numerical model for mixed size sediment.@en

Human intervention makes river channels adjust their slope and bed surface grain size as they transition to a new equilibrium state in response to engineering measures. Climate change alters the river controls through hydrograph changes and sea level rise. We assess how channel response to climate change compares to channel response to human intervention over this century (2000–2100), focusing on a 300-km reach of the Rhine River. We set up a schematized numerical model representative of the current (1990–2020), non-graded state of the river, and subject it to scenarios for the hydrograph, sediment flux, and sea level rise. We conclude that the lower Rhine River will continue to adjust to past channelization measures in 2100 through channel bed incision. This response slows down as the river approaches its new equilibrium state. Channel response to climate change is dominated by hydrograph changes, which increasingly enhance incision, rather than sea level rise.

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Water Injection Dredging (WID) has been successfully applied for removing sediment deposits in reservoirs, which results in an increase of their storage capacity. This dredging method is based on the fluidization of the top sediment layer by pressurized injection of water by a dredging vessel. The fluidized sediment can be transported towards the dead storage of the reservoir or sluiced out of the reservoir through the bottom outlets of a dam. This flow can either occur by gravity induced flow or especially directed by the dredging strategy of the WID vessel. This dredging technique can increase the water storage capacity of the reservoir and prevent the erosion of the river downstream, hence the sediment blockage. Recent developments in modelling and measuring tools have enabled stakeholders to design, optimize and monitor WID in reservoirs. In this paper, we will demonstrate how modelling and measuring tools can be used to evaluate alternative dredging strategies for reservoir maintenance. In particular, we show how a mid-field and far-field modelling can be applied for designing WID actions and predicting sediment plume dynamics in a given reservoir. Additionally, we will present recently-developed in-situ measuring tools, that are currently used for monitoring turbidity in a water column and sediment properties during and after WID actions. Finally, potential benefit of applying WID in Shihmen Reservoir (Taiwan) is discussed.@en

Predicting the formation and break-up of immobile layers is of crucial importance for river management, as these processes greatly affect the morphodynamic evolution of the river bed. Two models are currently available for studying these processes: Struiksma's and Hirano's model. In this paper, we show that both models present limitations. This is done by numerical modelling of a laboratory experiment and two thought experiments. Struiksma's model does not predict break-up and Hirano's model yields unrealistic results when part of the sediment is immobile. We propose two alternatives that overcome these limitations: the ILSE and HANNEKE models. They differ in the interpretation of the top part of the bed interacting with the flow. Moreover, only the HANNEKE model explicitly predicts the formation of coarse layers, at the expenses of a more limited application range.@en

River deltas commonly have a heterogeneous substratum of alternating peat, clay and sand deposits. This has important consequences for the river bed development and in particular for scour hole formation. When the substratum consists of an erosion resistant top layer, erosion is retarded. Upon breaking through a resistant top layer and reaching an underlying layer with higher erodibilty, deep scour holes may form within a short amount of time. The unpredictability and fast development of these scour holes makes them difficult to manage, particularly where the stability of dikes and infrastructure is at stake. In this paper we determine how subsurface lithology controls the bed elevation in net incising river branches, particularly focusing on scour hole initiation, growth rate, and direction. For this, the Rhine-Meuse Estuary forms an ideal study site, as over 100 scour holes have been identified in this area, and over 40 years of bed level data and thousands of core descriptions are available. It is shown that the subsurface lithology plays a crucial role in the emergence, shape, and evolution of scour holes. Although most scour holes follow the characteristic exponential development of fast initial growth and slower final growth, strong temporal variations are observed, with sudden growth rates of several meters per year in depth and tens of meters in extent. In addition, we relate the characteristic build-up of the subsurface lithology to specific geometric characteristics of scour holes, like large elongated expanding scour holes or confined scour holes with steep slopes. As river deltas commonly have a heterogeneous substratum and often face channel bed erosion, the observations likely apply to many delta rivers. These findings call for thorough knowledge of the subsurface lithology, as without it, scour hole development is hard to predict and can lead to sudden failures of nearby infrastructure and flood defence works.

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Remarkable 3D flow structures occur at river confluences with small density differences due to differences in sediment concentration or temperature. We explain these by comparing numerical simulations for an idealized confluence with aerial photographs of several river confluences where color differences express the pattern of density differences at the surface. We analyzed numerical simulations of the Rio Negro–Solimões confluence near Manaus, Brazil, in more detail. The numerical model of the idealized confluence showed that the dense water flowed under the light water and the light water over the dense water in a spiraling motion, distorting the interface between the two waters. The horizontal part of this interface moves upwards in downstream direction. Constraining of the spiraling motion in a narrow river downstream of the confluence can cause local up- and downwelling near the banks. A mixing layer can develop when the flow velocities of the two tributaries differ, but strong spiraling motion due to the density differences can suppress this development. The aerial photographs and all numerical simulations showed similar density patterns at the water surface. Even small density differences can have a significant impact and hence need to be considered when analyzing and modeling 3D flow at confluences.@en

Longitudinal training dams (LTDs) are a promising alternative for river groynes. Here we summarize findings of a recent study focused on the along river transition from a series of river groynes to an LTD, where the flow divides between the fairway and the side channel between the LTD and the river bank. A scale model is setup using lightweight granulates made of polystyrene to create conditions that are dynamically similar to a prototype situation in the River Waal. The key advantage of using lightweight granulates is that both the Shields number and the Froude number are similar in the model and the prototype. A high flow and a low flow experiment were carried out. The bedforms in the physical model have dimensions that correspond to theoretical dune height predictions, and also the channel incision due to width reduction is in accordance with expectations. The scour holes that develop near the tip of the groynes, however, are too deep, which may relate to improper scaling of the local turbulent vortices, initiated at the groynes. The morphodynamic developments in the flow divergence zone are subtle, and are overwhelmed by the mobile bed response to the presence of groynes. Considering that the physical model over-predicts the erosion caused by groynes, this suggests that the LTD configuration subject to study results in a comparatively stable bed morphology.

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The West African coastal barrier is maintained by significant wave-driven longshore sand transport. This sand originates from rivers and large coastal sand deposits. Today, however, much of the fluvial sand is trapped behind river dams and/or interrupted at several locations by port jetties. As a result, the sandy coastal barrier is eroding almost everywhere along its length.The aim of this study is to derive a large-scale sediment budget analysis, following a consistent approach, for the following countries: Republic of Côte d'Ivoire, Ghana, Togo and Benin, and pointing out the effects of major human interventions and climate change in this large common sediment system. The results are used as a basis to raise awareness among local governments and organizations on the effects and interdependency that major anthropogenic interventions (i.e. port jetties and river dams) and climate change (i.e. sea level rise, changes in wave climate, precipitation and temperature) may have on this shared sediment system. These detrimental effects can even occur in neighboring countries, as shown by some of the results. This estimation was carried out using a quantitative approach, based on one consistent numerical modelling system and validated with regional and local data.Based on the outcomes of the study, and with the support of a number of validation workshops in the different countries, suggestions are also provided for the setting up of a regional sediment management plan for the entire region.

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Hydropowerdams largely impact sand and gravel movement in rivers. The reservoirs andtheir operation trap the sediment and dampen the flow hydrograph. In fullyalluvial rivers, this causes bed degradation, coarsening and change of channelpatterns. However, in many cases these dams are located in bed-rock channels withunder-supplied conditions. Bed-rock channels are generally steep with highvelocities, and sand is only settling in wakes and eddies behind rocks or alongbanks in sand bars (some sand is stored in pores and pockets in the exposedriver bed as well). Without dams, deposition (during floods) and erosion of thesand bars is balanced, but a dam disturbs this balance and causes gradual degradationof the bars. A well referenced and important example of these conditions is theColorado River in the Grand Canyon in the USA (Grams et al, 2015). The existenceof sand bars is crucial for environmental and social functions of the river. Inthis paper the results of a study to develop a practical approach to simulatethe impacts of hydropower dams on sand bars, and to assess the effects ofmitigation measures. The origin of this study comes from the present plans ofconstructing several dams in the main stream of the Mekong in Laos andCambodia. Previous work of the author for sand-bar modelling in the GrandCanyon using Delft3D (Sloff et al, 2009), showed that with 3D high-resolutionDelft3D models, the sand bar development during artificial floods can be quantifiedto some extent. However, for assessment of large stretches of rivers, suchtechniques require too much computational effort. Instead, a coarser modelling approachhas been developed, that mimics the developments of sand bars and associatedprocesses, but only in an approximate way. The main aim of this approach is toquantify the impacts of modified flows and sediment loads downstream of dams. @en

Confluences are fascinating elements in river systems. The merging of tributary rivers produces complex hydrodynamics that can play an important role in sediment transport problems, ecological studies or the routing of a pollutant. If the two merging rivers originate from different regions they can have different characteristics. One such a characteristic can be the water density. The influence of many different properties of a confluence on the hydrodynamics has already been investigated. The effect of bed discordance has been described [Biron et al., 1996], as well as that of the momentum ratio [Best & Reid, 1984]. Also a lot of work has been carried out on the mixing layer that may form (van Prooijen & Uijttewaal, 2002). The effects of density differences, on the contrary, have received little attention. Rare examples are the work by Cook & Richmond (2004) and Lyubimova et al. (2014). The aim of this research is to see how and when density differences are important in confluences with respect to other occurring flow structures. Non-dimensional flow parameters will be linked to certain types of flow. These nondimensional parameters can then be used to determine which flow processes can occur. This research shows the importance that small density differences can have on the local hydrodynamics near a confluence. We explain the processes and present suitable nondimensional parameters to describe the flow. @en

Sediment deposition is one of the key mechanisms to counteract the impact of sea level rise in tidal freshwater wetlands (TFWs). However, information about sediment deposition rates in TFWs is limited, especially for those located in the transition zone between the fluvially dominated and tidally dominated sections of a river delta where sedimentation rates are affected by the combined impact of river discharge, wind, and tides. Using a combined hydrodynamic-morphological model, we examined how hydrometeorological boundary conditions control sedimentation rates and patterns in a TFW located in the Rhine-Meuse estuary in the Netherlands. The modelling results show that net sedimentation rate increases with the magnitude of the river discharge, whereas stronger wind increasingly prevents sedimentation. Sediment trapping efficiency decreases for both increasing river discharge and wind magnitude. The impact of wind storms on the trapping efficiency becomes smaller for higher water discharge. The spatial sedimentation patterns are affected by all controls. Our study illustrates the importance of evaluating both the separate and the joint impact of discharge, wind, and tides when estimating sedimentation rates in a TFW affected by these controls. Such insights are relevant to design measures to reactivate the sedimentation process in these areas.

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