Many run-of-river hydropower plants built without stilling basins now experience progressive scour due to prolonged operation and increasingly frequent floods. The Chancy-Pougny dam on the Rhône River, constructed in the 1920s at the Swiss– French border, exemplifies this issue. Severe flow recirculation was identified as the main cause of erosion, with pressure fluctuations increasing between the original and current stilling basin. While earlier work developed scour protection measures through physical modelling and numerical predictions, the present study focuses on analyzing pressure measurements within the stilling basin to assess how fluctuations can be reduced to limit future scour. Effective mitigation strategies include: (1) raising the basin water level, (2) introducing a guidance wall to restore symmetrical flow, and (3) adding various configu-rations of half-cube concrete prisms to increase roughness and energy dissipation. A life cycle assessment of prism materials and construction methods further supports a sustainable approach to rehabilitating ageing hydraulic infrastructure.

Highly unsteady air-water flows are common in coastal and hydraulic engineering, particularly during dike breaches and wave overtopping events, where sudden releases of water propagate down on slopes. Despite their relevance in erosion processes, structural stability and human safety, the hydrodynamic behaviour and multiphase characteristics of these unsteady flows remain poorly understood. This study presents new experiments on dam-break waves propagating over smooth and grass-covered slopes with a 1:3 inclination, representative of typical dikes in north-western Europe. The experiments were conducted using a dam-break facility equipped with ultrasonic distance sensors and high-speed imaging to capture wave-front celerities and flow depths. Results show that surface roughness substantially influences wave propagation, leading to reduced celerities and enhanced air entrainment. On vegetated slopes, a highly aerated “ white-water ” front developed, extending over a distance equal to 5 times the dam's impoundment depth, followed by a gradual decay of aeration. The measured wave front celerities agreed well with previous analytical solution for dam-break flows on slopes using friction factors consistent with steady flows on artificial grass. Ensemble-averaged flow depth data from up to 320 repetitions revealed statistically robust patterns of free-surface fluctuations. Results also provided a comparison between dam-break waves and unsteady flows generated with the Wave Overtopping Simulator (WOS), showing similarities and highlighting differences. Overall, these findings improve our understanding of multiphase unsteady flows on vegetated slopes and provide valuable data for developing more accurate predictive models for flood impacts and coastal structure design.

Dam-break waves are widely used to study unsteady flows like (flash) floods, tsunamis and storm surges. This technical note advances the classical theory of dry-bed dam-break waves, providing new analytical insights into the momentum and energy fluxes: key quantities governing impact loads and debris motion during extreme events. Building on previous seminal work, explicit expressions are derived for the maxima of momentum and energy fluxes and their occurrence is linked to the propagation characteristics of dam-break waves. Analysis is further extended by incorporating existing models that consider frictional effects in the wave-tip region. This allows the derivation of a transition criterion between the ideal-fluid region and the friction-dominated tip, identifying when friction alters peak fluxes. Comparison with new laboratory data confirms that simplified models capture the dam-break waves’ essential dynamics and results show that friction particularly reduces flux maxima for smaller impoundment depths, with important implications for structural loading, hazard assessment and engineering design.

The flood event of July 2021 in Western Europe was characterized by water levels far above the 100-year design flood and by billions of euros in damages. During the flood, voluminous debris accumulations of up to 4,000 m3 occurred at various bridges, causing backwater rise and increased inundation depths. These accumulations contained large amounts of man-made materials, in particular cars, building rubble and household items in addition to driftwood. Therefore, a multilaboratory test series was conducted in this study, aiming to quantify the effect of debris composition and bridge design on backwater rise. Two model scales and seven different debris compositions were studied with the help of flume experiments, carried out in parallel in three laboratories in Belgium, Germany, and the Netherlands. Based on postflood field observations, man-made debris was represented by plates and cubes, mixed with logs. Results showed similar correlations between debris composition and backwater rise in all three laboratories. Compared to debris mixtures with only logs, an increasing volume fraction of plate-shaped objects increased backwater rise due to their higher interlocking nature, while an increasing share of cuboid objects generated less backwater rise. In all cases, backwater rise increased with increasing Froude numbers above 0.13 for constant debris volumes. In contrast, relative backwater rise decreased with increasing initial water level and for a higher bridge blockage ratio. The design of the bridge deck influenced both clogging behavior as well as backwater rise at the bridge, and closed handrails led to higher backwater rise compared to configurations with porous or no handrails. When comparing the results from all laboratories, only minor differences were observed, showing consistency in the methodology. Finally, this study presents a predictive equation to determine backwater rise at bridges in narrow river sections considering debris composition, bridge design and hydraulic conditions.

Ref: Can. J. Civ. Eng. 00: 1–19 (2025) | dx.doi.org/10.1139/cjce2025–0029. In the originally published article, a label in Fig. 2 indicating the “Position of the transducers” was placed in panel 2e (in error) instead of panel 2c (correct). The original and corrected figures and captions are shown below. The article has been updated, including the correction of a minor typographical error in the figure caption (“closet” corrected to “closest”).

Tsunamis, impulse waves, and extreme floods are catastrophic events that can result in significant loss of life and cause extensive damage. Understanding the effects of these extreme events on infrastructure is crucial for designing resilient buildings in hazard-prone regions. While most previous studies focused on idealized (frontal) impacts, this study experimentally investigated the combined effect of building orientation and openings on the hydrodynamic loading. Visual observations revealed that rotating the building altered the dynamics of the impact, improving the streamlines and lowering upstream water levels. In terms of loading, building rotation primarily influenced the initial impact phase, delaying and often reducing the peak forces compared to frontal impacts, in line with literature. Openings (e.g. windows, doors) allowed water to flow through the buildings, significantly reducing loads in the streamwise direction. However, for oriented structures, loads in non-streamwise directions become considerable and should be considered in the design process. To address this, simple empirical equations are introduced to predict forces and moments, providing engineers with practical tools to design safer and more resilient coastal infrastructure.

In this article, the Conflict of interest statement “Hubert Chanson has competing interest and conflict of interest with Matthias Kramer.” was removed. The original article has been corrected.

This study presents an analysis of debris accumulations at bridges and flume experiments, based on field data collected after the extreme flood event which hit Belgium and Germany in 2021. Post-flood photos were analyzed regarding bridge designs, debris accumulation volumes and debris compositions as well as flooding conditions. This showed that the voluminous debris accumulations contained a large share of anthropogenic materials characterized by various shapes. Based on averaged bridge data, prototype bridges were chosen for the experimental modelling, which was conducted in three laboratories in Belgium, Germany and the Netherlands. Thanks to this multi-lab approach, over 250 experiments were conducted, determining the effect of upstream hydraulic conditions, debris shape and bridge design on backwater rise. Compared to debris accumulations with only logs, backwater rise increased with larger shares of plates in the debris compositions, while decreasing with the same shares of cuboid elements. The number of piers and the geometry of the bridge deck showed a strong effect on the clogging behavior, and a closed handrail led to higher backwater rise compared to a porous or no handrail. As a result of various test set-ups and continuous comparisons, inter-lab differences could be determined and reduced, and therefore resulting in a more reliable dataset. On this basis, recommendations for future bridge design and operational flood protection measures were derived.

Dam-break waves are highly unsteady long-wave phenomena, characterized by a breaking front with a strong recirculating air–water mixture. While the air–water flow properties of steady flows have often been investigated, the understanding of dynamic processes in unsteady multiphase flows remains limited. In this experimental study, a new approach was implemented to analyze the air–water flow properties of highly unsteady flows in the form of dam-break waves using ensemble-averaging techniques to account for short-duration measurements. The new dataset includes four different flow conditions, providing novel insights into the relation between various hydrodynamic characteristics and key air–water flow properties, including bubble characteristics and void fraction. The void fraction profiles indicated the presence of a turbulent shear layer along with a recirculation zone close to the free surface, showing analogies with similar steady and unsteady flow phenomena. Variations in the Froude number were shown to strongly affect the number and size of air bubbles, particularly in the shear layer. Higher depth-averaged air concentrations were found with increasing Froude numbers, reaching up to 40% for Fr = 5.14. Overall, the results confirm the importance of considering the presence of air in dam-break waves and demonstrate the suitability of this new methodology for investigating air–water flow properties in highly turbulent flows. They offer a deeper understanding of the multiphase nature of dam-break waves, which is relevant for a wide range of processes in coastal and hydraulic engineering.