Engineered rivers are often prone to channel bed incision. This decreases the channel-floodplain connection, hampers navigation where nonerodible reaches increasingly protrude from the bed, and can destabilize structures. Here we inventorize causes and characteristics of channel incision measures. We elaborate on how channel bed incision is a transient channel response toward a new equilibrium channel state. Causes of incision comprise base level fall, channel narrowing (e.g., due to river training), channel shortening (bend cut-offs), an increased channel-forming discharge (e.g. due to climate change), and a decrease (or fining or coarsening) of the sediment flux from the upstream part of the basin. Finally, we discuss two measures that may mitigate channel bed incision: sediment nourishments and longitudinal training walls.

@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

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

Humans have intervened in rivers for centuries. River engineering measures have aimed at protecting populations against flooding, ensuring reliable and safe navigation, providing freshwater for drinking, domestic and industrial use, irrigation, and energy supply, and providing opportunities for recreation. All around the world, measures such as channelization (i.e., channel narrowing and shortening), dam construction, or channel diversion have allowed for the proliferation of human settlements, technological progress, and an improved quality of life. Despite the various socio-economic benefits of human intervention in rivers, engineering measures have side effects, often unaccounted for, or simply unknown before they manifest. This is because, by modifying the channel characteristics (geometry, planform, size of the bed surface sediment), or its controls (water discharge, sediment supply, base level), engineering measures alter the equilibrium state of a river. In response, rivers adjust toward the new equilibrium state through bed incision or aggradation, changes in channel width or sinuosity, or changes in the bed surface grain size distribution. This response may extend over hundreds of kilometers, and develop during decades to centuries....@en

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.

@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

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

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.

@en

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

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

While most of the world's large rivers are heavily engineered, channel response to engineering measures on decadal to century and several 100 km scales is scarcely documented. We investigate the response of the Lower Rhine River (Germany-Netherlands) to engineering measures, in terms of channel slope and bed surface grain size. Field data show domain-wide incision, primarily associated with extensive channel narrowing. Remarkably, the channel slope has increased in the upstream end, which is uncommon under degradational conditions. We attribute the observed response to two competing mechanisms: bedrock at the upstream boundary increases the channel slope over the upstream part of the alluvial reach to compensate for the reduction of net annual sediment mobility, and extensive channel narrowing reduces the equilibrium slope. Another striking feature is the advance and flattening of the gravel-sand transition, suggesting its gradual fading due to an increasingly reduced slope difference between the gravel and sand reaches.

@en

Tsunamis, impulse waves, and dam-break waves are rare but catastrophic events, associated with casualties and damage to infrastructures. An adequate description of these waves is vital to assure human safety and to generate resilient structures. Furthermore, a specific building geometry with openings, such as windows and doors, reduces wave-induced loads and increases the probability that a building withstands. However, waves often carry a large volume of debris, generating supplementary impact forces and creating debris dams around buildings, limiting the beneficial effects of the openings. Herein, a preliminary study on the three-dimensional (3D) effect of debris dams on postpeak wave-induced loads under unsteady flow conditions is presented based on laboratory experiments. Both wooden logs (forest) and shipping containers were tested, showing different behaviors. Shipping containers were associated with severe impact force peaks, whereas the interlocking nature of forest-Type debris provoked a compact debris dam, leading to higher and longer-lasting hydrodynamic forces. The arrangement of the debris also had an influence on the resulting structural loading. All tested scenarios were analyzed in terms of horizontal force, cantilever arm, and impulse acting on the building. This study presents a methodology to support the evaluation of postpeak debris-induced loads for the design of safer resilient buildings.

@en

The Rhine reach comprising the Niederrhein and the Upper Rhine Delta is characterized by a long history of engineering interventions. The area is densely populated and the Rhine, the main inland waterway in Europe, justifying the need for such measures. Interventions have had a strong impact on morphodynamic channel characteristics. We use bed level data collected since 1926 to assess large scale bed level change in the Upper Rhine Delta and Niederrhein. The field data show various trends both at the large scale and locally. A reduction of the main channel slope is observed in the Waal and the IJssel branches of the Rhine Delta, achieved through bed degradation. In the Pannerdensch Kanaal-Nederrijn-Lek branch, the morphoynamics are controlled by three weirs, downstream of which degradation has developed. The downstream half of the Niederrhein has degraded, resulting in a slope increase. Most of the domain has remained relatively stable for at least 20 years. The observed trends provide insight on the river response to interventions, which helps to better understand degradation processes, in model calibration, and in anticipating future change of channel geometry.@en

The Rhine River is the most important inland waterway in Europe, with over 300 million tons of cargo transported annually over its waters (Blom, 2016). The river also serves as a water supply for households, industry and agriculture, evacuates wastewater and is part of several nature conservation areas (Frings et al., 2014). Additionally, its waters create a main ecological corridor and host several hydropower plants (Middelkoop et al., 2001). The proper fulfilment of these functions can be hindered due to human intervention and climate change, which both impact the river morphodynamics.@en