Erosion-control measures in rivers aim to provide sufficient navigation width, reduce local erosion, or to protect neighboring communities from flooding. These measures are typically devised to solve a local problem. However, local channel modifications trigger a large-scale channel response in the form of migrating bed level and sediment sorting waves. Our objective is to investigate the large-scale channel response to such measures. We consider the lower Rhine River from Bonn (Germany) to Gorinchem (the Netherlands), where numerous erosion-control measures have been implemented since the 1980s. We analyze measured bed level data (1999–2020) around four erosion-control measures, comprising scour filling, bendway weirs, and two fixed beds. To get further insight on the physics behind the observed behavior, we set up an idealized one-dimensional numerical model. Finally, we study how the geometry and spacing of the measures affect channel response. We show that erosion-control measures reduce the sediment flux due to (a) lack of erosion over the measure and (b) sediment trapping upstream of the measure, resulting in downstream-migrating incision waves that travel tens of kilometers at decadal timescales. When the measures are in close proximity, their downstream effects may be amplified. We conclude that, despite fulfilling erosion-control goals at the local scale, erosion-control measures may worsen large-scale channel-bed incision.

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Channel adjustment in engineered rivers is often associated with channel bed incision (e.g., Chowdhury et al., 2023, Czapiga et al., 2022a, 2022b, Ylla Arbós et al., 2021). Channel bed incision reduces the stability of in-river structures, exposes river-crossing cables and pipelines, and the spatial variability of channel bed incision due to less erodible reaches creates shipping bottlenecks. Various measures have been implemented to cope with these issues. They range from sediment nourishments to erosion control structures (e.g., Habersack and Piégay, 2007). Our objective is to assess the potential of floodplain lowering and sediment nourishments in mitigating large-scale channel bed incision in engineered rivers affected by climate change, considering a spatial scale of hundreds of kilometres. Our domain of interest is the Rhine River between Bonn, Germany, to Gorinchem, Netherlands. This reach has been extensively channelized during the 18th-20th centuries for improved navigation and flood protection (e.g., Ylla Arbós et al., 2021).@en

River managers today are faced with the challenge of adapting to climate change while also having to sustainably secure all important functions in a healthy river system for society. Nature-based Solutions (NbS) have proven themselves effective across a multitude of contexts; providing integrative approaches for river restoration, conservation and sustainable management, ensuring both climate change adaptation and contribute to climate change mitigation and biodiversity recovery for generations to come. NbS are multi-faceted and more importantly, they are effective when it comes to addressing complex societal challenges (e.g. reducing flood risk, increasing natural values and biodiversity, ecosystem services and human well-being), as they provide a novel, integrative and coherent approach. Despite the significant and rapidly growing base of scientific evidence regarding the effectiveness of NbS in riverine systems management, the actual uptake and application of NbS on a larger (EU) scale is still in its early phase. From where we stand today, a major barrier to the wider uptake and application of NbS in riverine systems remains (a) our limited experience in scaling solutions beyond their local contexts (so called ‘Upscaling’), and (b) make Nbs as a standard work practice within water management organisations throughout North-West Europe (so called ‘Mainstreaming’). Also, our lack of standardised methods for quantitative assessment and monitoring of ecosystem services and benefits related to NbS hinders replication and application at a wider scale.@en

Floods can cause punctuated changes to river channel morphology over short time scales. This work investigates whether spatial variation in river floodplain width drives enhanced morphodynamic change during floods. We examine the relationship between longitudinal variation in floodplain width and bed elevation change within and between flood events using high-resolution, biweekly bathymetry measurements from the Waal River, the Netherlands, over the last 20 years across a 10km study reach. We find that bed erosion during floods tends to occur just downstream of floodplain constrictions while deposition during floods tends to co-occur with spatial floodplain widening. Low flows show inverse bed elevation changes at the same locations resulting in a cyclic, along-channel variation in lowvs. high-flow bed elevation variation. This study suggests that spatial changes in planform channel geometry can help predict relative intra- and inter-flood morphodynamic changes.

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Local river interventions, such as channel narrowing or side channels, are often necessary to maintain safety, ecology, or navigation. Such interventions have different effects on the river's bed morphology during periods of high- and low-discharge events. Mapping the bed-level variations for different discharge levels and understanding these effects can provide new opportunities for the design of interventions in multifunctional rivers. At any moment, the local bed level in a river is composed of bed-level changes that occur at various spatial and temporal scales. These changes consist of bed aggradation/degradation trends on a large scale, on an intermediate scale bed-level variations as a result of discharge fluctuations, and on small-scale moving river bed forms like dunes. Using the river Waal in the Netherlands as a case study, we analyze the intermediate-term bed-level changes resulting from discharge fluctuations (dynamic component) and propose adaptations to the design of floodplain interventions such that possible negative impact on the local bed-level changes is minimized. Time series of bed levels along two 10 km stretches of the case study are considered for a period of 16 years (2005–2020). Using a wavelet transform, we isolate bed-level variations resulting from discharge events. These bed-level variations are presented based on the magnitude of the discharge event and are compiled in an interactive atlas of river morphodynamics, allowing us to mitigate the impact of interventions. This will help river managers in the design of interventions and lead to improved management, operation, and maintenance of multifunctional rivers.

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Recent policy initiatives in Europe emphasize a movement towards nature-based solutions in flood management; however, a quantitative relationship between specific flood management measures and indicators of ecological health and biodiversity is difficult to establish (Penning et al., 2023). In the Netherlands, several studies have been conducted on floodplain vegetation monitoring; however, these studies are primarily focused on monitoring changes to hydraulic roughness for flood risk assessment (Harezlak et al., 2020; Penning & van de Vries, 2020). These works provide an opportunity to expand upon existing research to explicitly connect river management practices with indicators of floodplain biodiversity change in the Netherlands. In this study, we utilize publicly available geospatial data to identify changes in land use, vegetation classification and spectral indicators of vegetation health at restoration sites associated with the Room for the River (RftR) program in the Netherlands. Completed in 2018, RftR involved over 30 river management projects constructed to reduce flood risk by lowering peak water levels (Mosselman, 2022). Our objective is to quantify the impact of ecologically focused RftR projects on habitat heterogeneity and river connectivity in the surrounding floodplains. @en

The International Union for Conservation of Nature (IUCN) published their Global Standard for Nature-based Solutions (NbS) in an effort to further a common understanding and successful application of NbS. Our objective is to analyse the applicability of and considerations and advancements in using the IUCN Standard, as very few studies have examined and reflected on its actual application. As method, we applied the IUCN Standard to three case studies of river restoration projects with a focus on flood risk mitigation: (1) Eddleston Water Project, (2) “Room for the River” Deventer Project, and (3) Missouri River Levee Setback Project. Rather than evaluating the case studies itself, we evaluated the outcome to find the strong and weak points of the IUCN Standard. The gathered data (publicly accessible documents, conducted interviews with experts and stakeholders) was analysed and showed the role of the number of documents and interviews available. This determined the outcome of the IUCN assessment. The consultation of project experts has appeared to be an essential step in the data collection, while stakeholder interviews and field visits were less important, but did increase the degree of substantiation and the ease of data collection, respectively. Although restricted by a limited evaluation of flood risk mitigation studies, using the IUCN Standard for an ex-post assessment can provide credibility to project processes (e.g. stakeholder engagement and adaptive management), reveal project strengths and weaknesses, and provide opportunities for the comparison of projects. Hence, the IUCN Standard aptly evaluates process-based aspects of Nature-based Solutions for riverine flood risk mitigation.@en

Nature-based Solutions (NbS) are actions that harness nature to help address major societal challenges. The assessment frameworks for NbS proposed in the literature differ in scope and intended use. In 2020, the International Union for Conservation of Nature (IUCN) introduced their Global Standard for NbS as a framework that can be used by anyone working on different types of NbS. Since research on the applicability of the IUCN Standard remains limited, the aim of this paper is to analyse whether the IUCN Standard may be used as an overarching assessment framework for NbS in river flood management applications and to identify the main differences in content with other NbS-frameworks. This was achieved through a comparison with 29 assessment frameworks for NbS, that are applicable to physical interventions for riverine flood risk reduction. The comparisons showed that the IUCN Standard has the largest breadth in scope of application and may therefore be used as an overarching framework. In addition, we identified a distinction between frameworks for the assessment of project processes (process-oriented) and project results (results-oriented), where the IUCN Standard can be characterized as process-oriented. This implies that the IUCN Standard may be used to assess the processes (e.g. stakeholder engagement and adaptive management) of planned, ongoing or completed NbS projects for a wide variety of environmental contexts and societal challenges. This will help persuade policy makers to consider NbS as one of the solutions in flood management issues, next to or in combination with e.g. engineering solutions or changing land use. We also identified that, while the IUCN Standard is straightforward to use and incorporates stakeholder input, the environmental context specificity as well as guidance depth on resources for assessment can be improved.@en

Climate change is responsible for global shifts in precipitation patterns and an overall in-crease in global temperatures. The transi-tions are anticipated to modify the river hydro-graph and sea level. The changes to the hy-drograph are also likely to influence sediment flux. These alterations imply shifts in both up-stream and downstream boundaries for river bifurcations. However, the resulting bifurca-tion response remains uncertain and warrants further investigation. Our objective is to un-derstand the extent of large-scale and long-term response of river bifurcations to climate change. We take the Upper Dutch Rhine bifur-cation region as our case study and develop a 1D hydro-morphodynamic model representing the system to achieve this goal.@en

Tipping occurs when a critical point is reached, beyond which a perturbation leads to persistent system change. Here, we present observational indications demonstrating presently ongoing noise-tipping of a real-world system. Noise in a river system is associated with the changing flow rate. In particular, we consider the upper Rhine River delta, where flow and sediment fluxes are partitioned over the two downstream branches (bifurcates) of an important river bifurcation. Field observations show that a sequence of peak flows in the 1990s resulted in sudden sediment deposition in one bifurcate, triggering a persistent and ongoing change in the flow partitioning. This has caused the system to move toward an alternative equilibrium state or attractor. An idealized model confirms that a river bifurcation system under such conditions is prone to tipping, and provides insight on the onset of tipping.

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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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River bifurcations divide the water and sediment over two downstream branches or bifurcates. As the changing climate adjusts the boundary conditions (i.e., base level, hydrograph, and sediment flux) for bifurcations, it will affect their flow and sediment partitioning over the bifurcates. Our objective is to provide insight into the response of a bifurcation to sea level rise (SLR). To this end, we compare the response of an idealized bifurcation in an engineered river (i.e., with a fixed planform and width) to SLR to the one of a single channel, using a one-dimensional numerical model system.@en

A bifurcation in an engineered river system (i.e., fixed planform and width) has fewer degrees of freedom in its response to interventions and natural changes than a natural bifurcation system. Our objective is to provide insight into how a bifurcation in an engineered river responds to peak flows and human interventions. To this end, we analyze the change in hydraulics, bed level, and bed surface grain size in the region of two bifurcations in the upper Rhine delta in the Netherlands over the last century. We show that, over the last two decades, the water discharge in one bifurcate (the Waal branch) has steadily increased at the expense of the other. This gradual increase in the water discharge of the first branch is associated with its erosion rate being larger than the other branch. The quick succession of two or three peak flow events (1993, 1995, and 1998) caused rapid sediment deposition over the upstream part of the bifurcate that has gradually lost discharge, which seems to have triggered the slow change in flow partitioning.@en

Typically the time scale of river response to change of the controls (i.e., flow duration curve, sediment flux, and sea level) is of the order of decades to centuries. Understanding temporal change and, in particular, abrupt change in channel response is increasingly important in engineered river systems, as abrupt change may negatively affect flood risk, navigation, and freshwater supply. The analysis of abrupt change in engineered systems with a bifurcation (i.e., a single channel splitting into two branches or bifurcates) is complicated by the fact that insight and measured data on the partitioning of water and sediment over the bifurcates are typically lacking. Our objective is to provide insight on whether observed abrupt change of the Pannerden bifurcation in the upper Rhine delta (Netherlands) may be associated with tipping.@en

Sediment transport capacity and supply of sediment to a river channel increase significantly during peak flow events. Here we study how a river bifurcation system (partitioning water and sediment over its downstream branches) responds to peak flow events. We focus on the Pannerdense Kop bifurcation in the Dutch Rhine River, an engineered system where planform and channel width are fixed. We analyze water discharge and bed level data measured over the last century. We observe rapid aggradation in one of the branches (Pannerden Channel) following the peak flow events of 1993 and 1995, and little to no bed level change in the other branch (Waal). Prior to the event, both branches eroded, and the upstream part of the Pannerden Channel had a greater erosion rate than the Waal. After the 1993 and 1995 peak flow events, the erosion in the upstream part of the Pannerden Channel slowed significantly, whereas the upstream part of the Waal branch continued to erode (though at a smaller pace than before the peak flow events). This differential erosion has resulted in a gradual increase of water discharge toward the Waal branch. Interestingly, the bifurcation system does not appear to respond equally to all peak flow events. We hypothesize that the bifurcation response to the 1993 and 1995 peak flows differs from previous peak flows because of the sequence of the two events. Between the 1993 and 1995 events, the system may not have had sufficient time to disperse the sediment deposited at the upstream end of the Pannerden channel. Another reason for the response to the 1993 and 1995 peak flows to differ from previous events may be that the channel bed surface within the region of interest has coarsened significantly. This study illustrates the importance of peak flows regarding bifurcation dynamics, and further research is focused on the interaction between bifurcation dynamics and the dynamics of the larger-scale system.@en

We assess whether an observed sudden change in trend of the flow partitioning over the downstream branches of a bifurcation system in the Dutch Rhine River, following two consecutive peak flow events, is evidence of system tipping. For this purpose, we analyze field data of the bifurcation region and relate observed sudden and subsequent slow system changes to the stability properties of equilibrium solutions of a low complexity river bifurcation model. In particular, the two peak flow events led to sediment deposition at the upstream end of one bifurcate. As the resulting larger flow rate toward the other bifurcate enhanced channel bed erosion, that bifurcate has attracted an ever larger portion of the flow rate since. This presumably unstable state of the bifurcation region is of concern, as flood risk management and freshwater supply within the system are based on a certain agreed flow partitioning ratio between the bifurcates. Based on the river bifurcation model, we illustrate that there exist several paths in which a sudden decrease in flow depth in one bifurcate leads to a transition from one stable state with two open branches to another stable state with one closed branch. As the bifurcation model is a heavily schematized one, we cannot prove that the observed bifurcation system change agrees with one of these theoretical trajectories. There are indications that some natural systems are able to prevent tipping behavior, provided that their response is characterized by sufficient degrees of freedom. The fact that the river system is a heavily engineered system with fixed planform and banks (and hence limited degrees of freedom of channel response) may have prevented the system from evading tipping behavior.@en