This research investigates the hydrodynamics of a physical boundary transition from free slip to no slip, which usually occurs in ice-jams, large wood and debris accumulation in free-surface flows. Using direct numerical simulation coupled with a volume penalisation method, a series of numerical simulations is performed for an open-channel flow covered with a layer of floating spherical particles, replicating the laboratory set-up of Yan Toe et al. (2025 J. Hydraul. Eng., vol. 151, 04025010). Flow transition from the open channel to the closed channel induces a new boundary-layer development at the top surface, accompanied by a flow separation and an increased bottom shear stress that enhances particle mobility at the bottom. Analysis of a fully developed flow in an asymmetric roughness channel (rough surface at the top boundary and smooth surface at the bottom boundary) also shows that the vertical position of maximum velocity is higher than the position of zero Reynolds shear stress, which supports the experimental observation of Hanjalić & Launder (J. Fluid Mech., vol. 51, 1972, pp. 301–335), demonstrating the shortcoming of traditional turbulence closure models such as the k−ε model. Finally, the stagnation force acting on a particle at the leading edge of the accumulation layer is compared with the analytical prediction of Yan Toe et al. Understanding the flow transition improves the prediction of the stability threshold of the accumulation layer and design criteria for debris-collection devices.

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.

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.

Plastic debris can accumulate at hydraulic structures and waste-collection devices, leading to a so-called floating carpet formation. Understanding the accumulation of plastic debris at structures is pivotal in the prediction of increased flood risk and design of waste-collection devices. In this research, we studied the stability of plastic carpets under different flow conditions using laboratory experiments, and we developed analytical models to predict critical velocities that led to two instabilities: (1) squeezing—particles inside the carpet are pushed downward due to cumulative compressive force, and (2) erosion—particles at the upstream edge of the carpet mobilize completely. Velocities of the fully developed flow were measured under a stable carpet to estimate boundary shear stress, which was applied to calculate the compressive force of the particles. Using measured flow velocity data and particle’s properties, the critical flow velocities that led to instabilities were calculated. Overall, this research supports a better understanding of physical processes associated with plastic accumulation, supporting the development of optimized plastic removal strategies.

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.

During summer of 2021, devastating river floods occurred in Western Europe as a result of extreme rainfall. At numerous bridges, debris accumulations were observed, exacerbating flooding upstream by impeding waterflow and sometimes contributing to bridge failure. Due to widespread building damage and flooding of settlements along the rivers, these accumulations differed markedly from classic logjams, revealing substantial amounts of man-made objects. A new database of clogged bridges in Belgium and Germany (described in a separate data descriptor) was analyzed to characterize bridge clogging and determine the effect of bridge design, bridge location and hydraulic conditions. Results showed that nearly half of the debris volume consisted of man-made materials, including building rubble, anthropogenic wood and vehicles. This created remarkably dense accumulations, highlighting the importance of further studying debris accumulations of mixed composition. Examination of the relations between bridge design and accumulation volumes found that bridges with narrow pier spacing (≤10 m) are more susceptible to extreme clogging. Blocking by the deck and railing also played a prominent role, in conjunction with blocking by the piers, as peak water levels at 85% of the analyzed bridges reached or exceeded the deck. Altogether, these findings can help to better understand bridge clogging effects on flood conditions, to design bridges with lower debris accumulation risks, and to inform future flood hazard assessments, flood risk mapping, and disaster response strategies, especially in urbanized regions.

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.

Coastal dikes have been built for millennia to protect inhabited lands from exceptional high tides and storm events. Currently, many European countries are developing specific programs to integrate the construction of new dikes (or the raising of existing ones) into the built environment to face sea level rising. Technical difficulties in succeeding in this operation are questioning the paradigm of protection for the long term, pointing out the need for alternative strategies of adaptation that are not yet fully explored. This paper elaborates on innovative models to deal with coastal flooding, presenting the results of an interdisciplinary research and design process for the case-study of Southend-on-Sea (UK). Detailed numerical simulations are used to develop a spatial strategy to accommodate water during extreme events, introducing different prototypes of dike designs that include seawalls, enhanced roughness through rock and stepped revetments, as well as vegetation. The overall goal is to push forward the traditional approach of planning water protection infrastructure within the solely field of civil engineering. It elaborates on the integration of the disciplines of spatial design and engineering and presents novel advances in terms of spatial design for the revetment of overtopping dikes.

Recent catastrophic events caused by tsunamis, storm surges, flood waves, and the failure of dams (e.g. in Ukraine and Libya) have shown to be a significant threat to densely populated coastal communities. Interactions with built environments can lead to violent wave impacts that may have severe consequences, including significant infrastructural damage and potential loss of life. The frequency of these water-related disasters is increasing globally due to climate change and sea level rise, resulting in a rising demand for deeper knowledge related to the physical processes of such hazards.

The dynamic behaviour of these type of wave phenomena is described by long-period, high translatory waves, where the on-shore propagation or inland inundation is associated with sudden free-surface deformations. This results in a steeping of the slope at the leading edge, causing non-linear flow behaviour to prevail and inducing the wave to collapse. The breaking process generates a breaking roller at the wave front, containing a rapidly fluctuating mixture of air and water, associated with a strong recirculation. The high degree of air-water interaction in these unsteady flows has a significant impact on the flow properties as it influences many dynamic processes, including viscous and surface tension effects at air-bubble level, as well as larger scale gravitational effects associated with the turbulent flow and eddy formation (Brocchini and Peregrine, 2001). New innovative measurement techniques have allowed experimental studies to more precisely quantify the air-water interactions in multiphase flows. However, most experimental research focused on air-water flow properties in hydraulic jumps and other steady flows (e.g. spillway flows, plunging jets). Currently, limited research is available for unsteady flows and mostly based on small datasets and limited flow conditions. This lack of availability and diversity of experimental data restricts the understanding of how these multi-phase flows behave under different conditions, hence the need for future research.

Knowledge of plastic debris transport mechanism in open waters and its interaction with hydraulic structures (i.e. accumulation and clogging) is of paramount importance for effective waste-removal strategies and sustainable management of plastic debris. To the author’s best knowledge, current models for prediction of plastic debris transport assume a highly simplified geometry, while making use of parameterization of the physical processes, therefore pointing out the need for further research. Herein, the effect of shape and buoyancy on the motion of a single particle was studied employing point-particle approach while the background flow is solved using RANS approach. It is observed that the particles with the same amount of plastic mass but different shape and density showed substantially different behaviors, resulting in different trajectories. Since parametrization and point-particle approach were used, even if the particle size is larger than the mesh size, these preliminary results showed that further validation is required for prediction of accurate trajectory by means of resolved-particle approach.

This study investigates the effect of driftwood on submerged culverts through scale experiments, focusing on their accumulation and the hydrodynamic processes occurring underneath. Examining temporal evolution and velocity measurements, this research delves into the implications of driftwood accumulation, including its geometry, hydraulic conditions and associated backwater rise. Findings reveal that accumulation shape is strongly influenced by hydraulic conditions, with higher Froude numbers pulling logs toward the bottom and thus yielding more compact accumulations. This effect holds implications for submerged culverts, where the opening near the bottom diminishes the importance of surface flow resistance. Accurate prediction of accumulation lengths is achieved using wood volume and initial flow velocity. The study also provides valuable data for developing quantitative design equations for backwater rise from driftwood accumulation at culverts. Additionally, detailed measurements of velocity profiles and Reynolds stresses under the accumulations highlight a slightly lower flow velocity, prompting the need for future research to discern its generality and implications for driftwood-induced scour at submerged culverts.

During the European flood of 2021, large debris accumulations were observed at numerous bridges, causing backwater rise, increased upstream flooding, and extended damage. To date, debris accumulation studies mainly focused on debris consisting of logs, at bridge piers or debris racks. However, during the 2021 flood, debris contained a large share of man-made materials in various shapes, often reaching the bridge deck and railing. Therefore, flume experiments on debris accumulation at bridges were conducted at three laboratories in Belgium, Germany and the Netherlands. Hereby, we investigated how backwater rise depends on flow conditions and on debris composition – using debris mixtures of 75% logs with either 25% cubes or 25% plates. Results showed that mixtures with plates caused 1.8 – 2.9 times more backwater rise than those with cubes. This means that previous studies on natural log accumulations may substantially underestimate backwater rise at debris accumulations with e.g. building rubble during flood events. Almost no backwater rise occurred below approximately Fr = 0.2, after which backwater rise increased with the Froude number. Comparison of results between labs agreed relatively well, with backwater rise under the same conditions varying often by 10% to 35%. However, the results of a single series of experiments were higher by up to a factor 2.5. This implies that any multi-flume or multi-lab study should ensure sufficient overlap between test conditions, rather than a pure workload split. Moreover, the observed inter-lab variability implies that multi-lab setups can increase confidence for the generalization of test results to real-world conclusions.

"Plastic pollution is a threat for all ecosystems due to its effects on people, animals, and environment. Rivers are estimated to transport around 0.5 millions tons of plastic per year. When plastic enters a river system, it is transported downstream towards the sea but it is also likely to accumulate at specific cross sections and locations, including hydraulic structures, eventually increasing the risk of floods. Gates, locks, weirs, and bridges are commonly present in rivers and canals and have several functions, including water level regulation, flood safety, and inland water shipping. These can also be found in water treatment plants, hydropower stations as well as debris/plastic collection systems. Riverine plastic accumulation is also known to cause geomorphic changes. In-depth knowledge on how plastic particles accumulate upstream of hydraulic structures is therefore crucial to understand the processes that affect plastic transport, its influence on the safety and functionality of hydraulic structures and their effects on the hydro- and morphodynamic conditions of the flow. In this research experiments were performed using simplified plastic particles to analyse the processes that lead to the instability of accumulated particles upstream of a simple gate."

The crest level of seawalls is often based on estimates of the amount of wave overtopping. Methods to estimate the mean overtopping discharge have been provided in several guidelines. One of the important parameters affecting wave overtopping is the wind. However, the effects of wind have not been accounted for in detail in present design guidelines although some guidance for coastal structures with crest elements is provided in literature. For onshore wind the expected wave overtopping discharge at coastal structures with a crest element can be up to a factor 5 larger than for situations without wind. In the present study the maximum influence of wind on wave overtopping at impermeable seawalls with crest elements has been studied based on physical model tests. The result of the study is a guideline to estimate the maximum influence of wind on wave overtopping at seawalls with crest elements.

In the context of today’s climate change, with extreme events becoming more frequent and more intense, highly unsteady flows are a threat that can no longer be ignored in hydraulic and coastal engineering, since these can lead to human casualties and extensive damage. Impulse waves, storm surges, flash floods and tsunamis are among these unsteady flows, with tragic examples in the last years, including the Indian Ocean tsunami in 2004, Japan Tohoku in 2011 and Indonesia in 2018. These events showed that a deeper knowledge of the underlying physical phenomena is necessary to ensure safety to people and minimize expenses associated with recovery. Due to rarity and complexity of these flows, experimental approaches are often required and in laboratories unsteady flows can be reproduced using dam-break waves (Ritter 1892, Stoker 1958). However, most laboratory tests are conducted on (unrealistic) smooth inverts, hence rising the question on how the bed roughness affects the propagation and the hydrodynamic properties of these flows. Previous studies, including Dressler (1952), Wüthrich et al. (2019) and Nielsen et al. (2022) provided relevant information, but more research is needed to gain a better understanding. In particular, little knowledge is available on the behaviour of these highly unsteady flows propagating through Rigid Stagged Vegetation (RSV), which is representative of forests and other natural areas surrounding built environments.

Based on a large experimental campaign, this research studied the propagation of dam-break waves on rough beds, in the form of various configurations of RSV. Waves were generated in a 14 m long and 0.4 m wide horizontal flume, where a d0 = 0.4 m impounded reservoir was released through the sudden opening of a gate, as shown in Figure 1. The waves propagated in the downstream horizontal flume, where different roughness configurations are installed. More specifically, the study analysed a smooth plywood configuration and 4 Rigid Stagged Vegetation (RSV) configurations (Figure 2), reproduced using nails with various grid densities and lengths, as detailed in Table 1. Tests were conducted on dry bed, as well as on an initial still water level h0, which ranged between 7.5 and 50 mm (i.e. 0.0188 < h0/d0 < 0.125). Tests on dry bed were repeated 5 times, while tests on wet bed were repeated 10 times. Data were analysed using ensemble-average values. Six Acoustic Displacement Meters ADM (Microsonic TM mic+35/IU/TC, Dortmund Germany) were used to capture the wave profiles in time as well as the wave front celerity C between various ADMs. In this study only the celerities between ADM 5 and 6 are considered, since at this location the bore was fully developed (Buitelaar 2022). Wave propagation was also documented using videos and SLR high speed photographs.

The July 2021 flood heavily affected many inhabitants, buildings and critical infrastructure throughout Germany, Belgium and the Netherlands. Specifically, the Ahr Valley (Germany) showcased the destructive power associated with these extreme events. Hence, this region was the focus of a field survey, aiming at describing the flood-induced damage to buildings and assessing the possible underlying processes that led to structural failures. The field assessment revealed a close connection between building failures and (1) local flow depths and velocities, (2) building location, (3) distance from the riverbank and (4) construction type. Although it is difficult to identify the exact causes that induced failures, the detailed assessment revealed that damages mainly originated from local scour and hydraulic loads, often unevenly distributed around buildings. Importantly, many buildings were significantly affected by (large) floating debris impacts and damming, both responsible for additional loads, highlighting their importance in flood-resistant building design. Furthermore, data showed that buildings near the riverbanks and in the upstream part of villages were more severely damaged. Altogether, data provide a better understanding of the flood processes that lead to building failures, fostering future research towards the development of safer protection measures and more effective flood risk management strategies.

In a time of climate emergency due to global warming, nature-based coastal defence systems are attractive solutions for flood mitigation and adaptation. Coastal forests such as mangroves have received a growing interest for their disaster mitigation effectiveness such as water flow energy dissipation, hence helping communities to become more resilient (Iimura & Tanaka, 2012). The role of coastal forests as a defence measure was highlighted in the aftermath of the 2004 Indian Ocean Tsunami, which claimed the lives of more than 200,000 people and displaced millions more across fourteen countries. Post-disaster damage observations indicated that forests, particularly mangroves, reduced the impact of the tsunami wave in some locations. As a result, significant international relief and reconstruction efforts focused on extensive forest replantation of coastlines (Satake, 2014).

The role of coastal vegetation in reducing the severity of tsunami waves has been studied since. Several studies using physical modelling and computational approaches have provided insights into the wave attenuation provided by coastal vegetation, in terms of relationships between incident hydrodynamic conditions, forest configurations and wave height decay. However, there are still many gaps in knowledge, particularly in quantifying the efficacy of coastal forests in reducing inland hydrodynamic conditions (Tomiczek et al., 2020). It is therefore essential to improve the understanding on how wave heights, velocities and runup are influenced by the characteristics of the “obstacles”, e.g. the forest density, as well as the incident hydrodynamic conditions, e.g. the wave period. This study aims to address these questions conducting physical experiments using the novel pneumatic Tsunami Simulator (TS) developed by HR Wallingford together with UCL (Rossetto et al., 2011).

Follett et al. (2020a, https://doi.org/10.1029/2020gl089346) developed an analytical model to predict backwater rise by log jams, using the size and packing density of logs and the jam length, as well as river slope and bed roughness. We show that the model formulas can be rewritten using the Froude number instead of river slope and roughness, thus improving their applicability in engineering practice. The equation terms and results of Follett et al. (2020a, https://doi.org/10.1029/2020gl089346) are found to be similar to those of the empirically derived formula by Schalko et al. (2018, https://doi.org/10.1061/(asce)hy.1943-7900.0001501). However, some differences are identified, calling for further study. Most notably, these distinctions pertain to the effect of accumulation porosity, with additional minor differences in the exponent of the Froude number. Lastly, model implications for some broader applications are explored, showing a methodology to calculate the representative log size for log mixtures, and the expected effect of log orientation on backwater rise.