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.

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Plastic pollution is a threat for all ecosystems due to its effects on people, animals, and environment (Mai et al., 2020). Rivers are estimated to transport around 0.5 millions tons of plastic per year (Strokal et al., 2023). 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 (Al-Zawaidah et al., 2021), 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 reg-ulation, flood safety, and inland water shipping. These can also be found in water treatment plants, hydropower stations as well as debris/plastic collection systems (Honingh et al., 2020). Riverine plastic accumulation is also known to cause geomorphic changes (Al-Zawaidah et al., 2021). 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 morphody-namic conditions of the flow (Yan Toe et al., 2022). 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.@en

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.

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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.@en

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.

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The Authors thank the Discussers for their interest in the abrupt drop hydraulic jump study. The Discussers provided constructive comments on two aspects of this study, namely, the characterization of the bottom roller dynamics, and a design modification for the bottom-drop or negative-stepped stilling basin to improve bottom velocity reduction and hydraulic jump stabilization. [...]@en

This paper presents a dataset documenting 71 floating debris accumulations at bridges following an extreme hydrological event that hit Belgium and Germany in July 2021. Data were collected from various sources including public authorities’ documents, public online databases, post event pictures and field visits. The dataset covers bridges geometry, flood conditions and debris accumulation. In particular, it systematically details accumulations dimensions and quantifies accumulations components, which contain a significant portion of anthropogenic objects, in addition to driftwood. This dataset constitutes a unique set of invaluable data to better understand debris accumulation at bridges but also to analyze their impact on both the affected structures and flood conditions.@en

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.@en

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.@en

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.

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Transient motion, turbulence and bubble dynamics make any velocity quantification extremely difficult in unsteady gas–liquid flows. In the present study, novel Eulerian and Lagrangian techniques of velocimetry were developed, using both intrusive and non-intrusive measurements. The selected unsteady gas–liquid flow was a breaking bore, featured with a transient motion, air entrainment and coherent structures. Intrusively, Eulerian probe measurements resulted to the development of a single bubble event detection (SBED) technique in unsteady air–water flows. Non-intrusively, the motion of air–water pattern was detected using a novel particle tracking velocimetry (PTV). Both velocities obtained using SBED and PTV techniques were validated against the established optical flow (OF) results, achieving consistent velocity data among the three techniques. The filtering criteria of the SBED and PTV techniques were discussed, showing the best options in the breaking bore. It is concluded that the most robust velocity measurements in gas–liquid flow are achieved with consistent velocity data between the SBED, PTV and OF techniques.

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Continuing from the part 1 (Shi et al., 2022) this paper presents an experimental investigation of transient void fraction and bubble statistics in a highly turbulent breaking bore with Fr₁=2.4. The measurements were conducted using a combination of dual-tip phase-detection probes and an ultra-high-speed video camera. The enclosed bubble detection technique (EBDT) used the synchronised probe and camera signals to provide the contour of instantaneous void fraction in the bore roller. The ensemble-averaged void fraction was derived, and compared to analytical solutions of air diffusion models. The bubble statistics were characterised by the bubble clustering properties, pseudo bubble count rate and bubble size spectrum. The clustering data showed the non-random bubble grouping in the shear layer, and the bubble size distributions N(r) followed a commonly adopted bubble break-up model: N(r)∝r^−m, where r was the equivalent bubble radius in the present study. The comparison indicated that, in the breaking bore, its air diffusion process was similar to that in a stationary hydraulic jump, and the bubble break-up process was comparable to that in breaking waves.

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Venice is representative of the result of anthropocene acts on a vulnerable delta system. Therefore it is an excellent study case for a multidisciplinary team of TU students and tutors, to perform interdisciplinary research and design. Two visions were developed on the shared knowledge base to investigate ‘how do flood defense systems influence the spatial aspects of the territory in the context of a high dynamic landscape in the Anthropocene? ’The plan for the Perfect Lagoon is focused on tackling all of the current and upcoming problems where the emphasis lies on preserving and perfecting the lagoon in its natural assets by introducing a new dike system that enforces the natural ecosystem. It makes smart use of the sediment flows and tackles erosion. The interdisciplinary design is especially worked out in the section of the dike, where dimensions and functionality are designed from a shared engineering and landscape perspective. The plan Symbiotic System emphasizes the interconnectedness of the Veneto region in a sustainable way. The new transport/dam system will make the urban system more smart, it will be less burdensome, and give more space to the natural ecosystem of the lagoon. Both visions are a testimony of how flood defense systems have a major influence in the spatial aspects of the territory. Using the multidisciplinary approach, an integral design can be made for the flood defense, in which the opportunities for the territory can be first explored and then designed together with hydraulic infrastructures.@en

In high dam construction projects in China, stilling basin design with an abrupt bottom drop is sometimes introduced to reduce the bottom velocity and pressure loads by generating A-type hydraulic jumps. Although the stilling basin design is not new, A-type hydraulic jumps have not been studied taking into account the air entrainment and evolution of internal air–water flow structures. This paper presents an experimental study of self-aerated A-type jumps in terms of bubble transport and free-surface fluctuations over the bottom drop. Four Froude numbers from 4.1 to 10.3 are tested for three drop heights, in addition to the flat-bottom case. Compared to the classic hydraulic jumps, A-jumps are observed with longer jump lengths and weaker free-surface fluctuations. The downward deflection of the jet-shear flow and formation of a bottom roller in the step cavity require a modification to the analytical expression of velocity and void fraction distributions. The relationship between the bubble diffusivity and jump spreading rate differs from that in classic hydraulic jumps, suggesting a faster expansion of the bubble diffusion layer than the turbulent shear flow downstream of the drop, especially for large drop heights. At large approach velocities, the reattachment of the deflected jet-shear flow to the lowered bed may cause a local rise in bubble counts downstream the bottom roller. Further increase in drop height results in a W-jump with overwhelming bottom roller over the surface roller and an arced surface jet, which is beyond the scope of this study.

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Wave transmission at low-crested coastal structures has been studied, based on physical model tests with trapezoidal impermeable, permeable and perforated structures. The differences between wave transmission at impermeable and permeable structures are relatively limited. For a perforated hollow structure with an impermeable vertical screen in the middle, the wave transmission is significantly less than for perforated structures without an impermeable vertical screen; the blocking of the orbital motion by the screen significantly reduces wave transmission. The effectiveness of an impermeable vertical screen to block the orbital motion and consequently reduce wave transmission assists designers of artificial reefs to design structures that reduce wave transmission. Empirical expressions based on a hyperbolic tangent function have been derived to describe the test results. For permeable structures also available data for emerged structures has been used in the analysis, and the newly introduced expression appears to be accurate for both submerged and emerged permeable structures.

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This experimental study investigated the air-water flow properties and bubble characteristics in hydraulic jumps with Froude numbers Fr₁ = 2.4 and 6.3, using four dual-tip phase detection probes with sensor sizes from 0.25 mm to 0.64 mm. The hydraulic jumps were characterized by a fully breaking roller with substantial air entrainment and turbulent two-phase flow patterns. The measurements encompassed distributions of void fraction, bubble count rate, interfacial velocity and bubble clustering properties and the data sets were consistent with previous studies. The comparison of the different probes showed a small impact of the tested sensor sizes on the air-water flow properties, in terms of the trends, magnitude and maximum values. Some differences were observed in terms of the bubble count rate, with the larger probes detecting a lesser number of bubbles. The trend was further confirmed through a comparison with the data set of Chanson and Brattberg (2000) [10] with a smaller probe sensor size (Ø₁ = 0.025 mm), in which the maxima of bubble count rates were almost twice that of the present dataset for identical flow conditions. The present results confirm that the traditional signal processing techniques can be used for relatively small probe sizes, although different approaches might be needed for larger probes which cannot detect sub-millimetric bubbles. Overall, the findings should facilitate the development of sturdier phase-detection needle probes and help breaching the gap between laboratory and prototype.

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