Human interferences have set off a multitude of morphological responses of the lower Rhine in Germany and the Netherlands. We share insights from thirty years of studies on these responses in the Niederrhein below Xanten and the branches in the delta. Elementary analyses of the 1D Saint-Venant–Exner equations explain the downstream flattening and upstream steepening of the longitudinal bed profile due to retrogressive erosion in response to river training, bend cut-offs and sediment mining. Three reasons make a 2D approach necessary for modelling the seemingly 1D problem of large-scale morphological response: (i) transverse variations in bed sediment composition, (ii) sediment division at river bifurcations, and (iii) the possibility that non-erodible layers in bends cause either erosion or sedimentation of the longitudinal bed profile. The Pannerdense Kop and IJsselkop bifurcations are in a state of quasi-equilibrium, essentially unstable but developing slowly. Considerable spatiotemporal variations in the sediment composition of the riverbed surface pose a challenge to stabilizing the longitudinal bed profile by matching gradients in flow velocity to gradients in bed sediment composition. As these variations form a major knowledge gap, we recommend research on the state and dynamics of sediment size and layer structure in the upper metres of the riverbed.

The Pannerdense Kop is a key bifurcation in the engineered Dutch Rhine system where the Bovenrijn divides into the Waal and the Pannerden Canal. The discharge partitioning at the Pannerdense Kop is important for the Room for the River 2.0 programme, as it influences navigation, flood safety, and freshwater availability. Observations show that since the 1990s an increasing share of discharge is routed towards the Waal (Fig. 1), accompanied by a stronger erosional trend in the Waal than in the Pannerden Canal (Becker, 2021; Sloff, 2019; Chowdhury et al., 2023). A mechanism which may have caused this change is related to the peak flows in the 1990s, when the incoming sediment flux may have exceeded the transport capacity in the Pannerden Canal (Chowdhury et al., 2023; Blom et al., 2024). This stresses the importance of the morphological behaviour during peak flows. During peak flows, morphological adjustments around the bifurcation occur on multiple spatial scales, from dune dynamics affecting roughness (Julien et al., 2002; Frings & Kleinhans, 2008) to patterns related to floodplains, groynes, and bends (Ahrendt et al., 2022; Parker et al., 2011), yet existing knowledge is fragmented across individual processes and time periods. This research therefore provides a comprehensive multiscale analysis of morphological behaviour at the Pannerdense Kop by combining multiple field datasets with output from 1D and 2D morphodynamic models and systematically comparing their responses.

The upper Dutch Rhine branches show continuous bed degradation (1-2 cm/year) threatening multiple river functions. The bed degradation is a response to past sand mining, channel narrowing and straightening, which increased sediment transport capacity and created a sediment transport gradient that drives ongoing erosion (Figure 1). Room for the River 2.0 (RftR 2.0) aims to create a climateresilient river system, supporting river functions. Arresting bed degradation is essential to prevent further deterioration of river functions. For the Waal River, two principal intervention strategies were investigated: (1) continuous sediment management and (2) a large-scale implementation of the multi-channel approach. While sediment nourishment counteracts ongoing erosion by supplying the deficit in the sediment balance, the multi-channel approach reduces sediment transport by diverting discharge from the main channel to a parallel side channel. The objective of this abstract is to determine the effectiveness of both interventions in arresting bed degradation. We present the results of the intervention strategies evaluated with a quasi-3D morphodynamic model (Delft3D) in which the multi-channel approach is schematised as longitudinal training walls separating a side channel from the main channel.

The Room for the River 2.0 (RftR 2.0) program aims to stop large-scale bed erosion of the Rhine branches in the Netherlands. The authors have contributed to selecting and developing feasible solutions now under consideration for RftR 2.0. These solutions are grounded in a solid understanding of the flow and sediment transport processes driving the erosion. In this paper we show that not the total sediment transport, but the longitudinal gradient in transport is key to the large-scale erosion of the Waal River. We also discuss how this affects potential solutions and how uncertainties play a role.

Groynes are commonly found in lowland rivers, where they help maintain a navigable main channel depth and prevent bank erosion. The areas between them, the groyne fields, mainly consist of sediments. The morphodynamics of groyne fields have been studied through laboratory experiments (Yossef & De Vriend, 2010) and numerical models (McCoy et al., 2008; Constantinescu et al., 2009). However, these controlled experiments do not capture the spatial variability observed in natural settings. Based on field measurements Ten Brinke et al. (2004) hypothesized that groyne fields gradually erode under the influence of shipping, while substantial sedimentation occurs during floods. Our objective is to provide a more thorough understanding of the natural variability in groyne field bed level changes with the ultimate purpose to assess the potential and efficacy of groyne field nourishments. To this end, we first establish a baseline representing the natural variability in groyne field bed level changes. Additionally, understanding the factors that govern this baseline is essential.

Deltas are complex and are among the most vulnerable landforms under climate change. Studying them collectively highlights common stressors that drive their most significant challenges. A holistic conceptual framing of a delta and its feeding river basin is fundamental to effective adaptation planning.

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.

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.

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.

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.

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.

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.

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.

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.

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.