Tsunamis, impulse-waves and dam-break waves have affected humanity in recent decades and the construction of vertical shelters can provide safety to people. However, for non-critical infrastructures, typically residential houses of lower height, overtopping is accepted during such events. This study experimentally quantifies the effect of building overtopping, i.e. water flowing over the roof, on the resulting loading process. Both surges and bores were investigated and the impact against buildings with two different heights was assessed. Detailed measurements of forces and moments allowed key differences to be captured between the scenarios with and without overtopping. Results showed that overtopping induced higher downstream water depths, leading to lower horizontal forces and a reduced resistance coefficient. Furthermore, cantilever arm, moment and impulse values were constantly lower in case of overtopping. Finally, this study presents an innovative methodology to assess the main loading features of buildings subject to overtopping, supporting engineers to design safer resilient structures.

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Hydraulic jumps are commonly employed as energy dissipators to guarantee long-term operation of hydraulic structures. Thus, a comprehensive and in-depth understanding of its main features is fundamental. In this context, the current study focused on a hydraulic jump with a low Froude number (Fr₁ = 2.4) and a relatively high Reynolds number (Re = 1.83×10⁵). Experimental tests employed dual-tip phase-detection probes to provide a comprehensive characterisation of the main air-water flow properties of the hydraulic jump in terms of void fraction, bubble count rate and interfacial velocities. Importantly, this research focused on the air-water flow property distribution across the channel width, revealing lower values of void fraction and bubble count rate next to the sidewall as compared to the channel centreline. Such a spatial variability in the transverse direction questions whether data near the walls may be representative of the flow behaviour in the centreline, raising the issue of sidewall effects in image-based techniques. These findings provide helpful information to both researchers and practitioners for a better understanding of the physical process, leading to an optimised design of hydraulic structures.

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Tsunamis, impulse waves and dam-break waves represent extreme events that endanger human lives and generate damage to critical infrastructure. Recent events in the Indian Ocean (2004), in Japan (2011) and in Indonesia (2018) showed that constructive measures could be used to reduce the loads exerted on structures. Thus, the objective of this research is to study the loading process of different building configurations during wave impact. Through an extended experimental program, this research initially characterises the hydrodynamic behaviour of wet bed bores and dry surges in terms of their front celerity, water depths and velocity profiles behind the front. Then, the advantage of porous buildings in reducing inundation depths and loadings on the building is shown and discussed. Finally, this study provides simples formulae to predict the hydrodynamic load on a building, taking into account the effect of openings through an adapted resistance coefficient. These results reveal important information for the design of safer infrastructure that will act as vertical shelters in case of water related natural hazards.

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A breaking bore in a translating system of reference is mathematically comparable with a stationary hydraulic jump, whose dissipative nature is used in hydraulic structures to limit damage during floods. Although visually similar, analogies and dissimilarities between hydraulic jumps and bores have been discussed for decades. Recent developments in the investigation of unsteady flows allowed for a direct comparison between stationary and non-stationary flow motions. In this context, the present study provides a preliminary comparative analysis between a hydraulic jump and a breaking bore with the same Froude, Reynolds and Morton numbers in terms of roller toe characteristics and air-water flow properties. The results showed overall a good agreement between the two phenomena, with some differences associated with the non-stationary nature of breaking bores. The comparison between time-averaged void fractions in the hydraulic jump and instantaneous ensemble-averaged void fractions in breaking bore revealed similar patterns, in agreement with existing albeit limited literature. A quantification of analogies and dissimilarities is relevant to the energy dissipation processes, useful to practical engineers and researchers to optimise the design of hydraulic structures.@en

Steady free-surface flows around buildings occurring during flood or tsunami events can produce major damages and a quantification of the post-peak forces is essential for safety and resilience of coastal structures. The loading process is highly affected by the flow Froude number and the drag coefficient, commonly defined for highly subcritical flows, while field observations of tsunami flows reported Froude numbers close to one, visually appearing as a choked regime. The present experimental study addresses the hydrodynamic forces on emerging buildings in a subcritical choked regime, focusing on the effect of openings and orientation. Laboratory experiments indicated a substantial difference in flow depths between the up- and downstream side of the building for increasing Froude numbers. The presence of openings induced a flow through the building lowering the difference in flow depth, limiting the effect of the hydrostatic component of the loading process. The formulation of an empirical resistance coefficient C_R allowed a combination of both form drag and hydrostatic forces. Whilst for impervious buildings C_R was consistent with reference studies, for buildings with openings C_R was directly dependent upon porosity. Contrarily to the unsteady flow conditions, results showed that the sidewalls also played a role in the loading process. Buildings with a rotated orientation resulted in slightly larger surfaces exposed to the flow and larger horizontal forces. Nevertheless, these were applied at lower cantilever arms, thus reducing the tilting moment. Altogether, this study provides experimentally-derived parameters that will support hydraulic engineers in the design of coastal structures.

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Tsunami, impulse-waves and dam-break waves afflict humanity with casualties and damages. An insight into the flow mechanisms of these waves is important to provide safety and reduce reconstruction costs. This experimental study focuses on the effect of bed roughness on the main hydrodynamic properties of surges propagating on dry bed. In addition, the resulting wave impact forces on buildings with and without openings are studied. Results pointed out that dry bed surges on a rough bed had a lower front celerity and a higher flow depth, resulting into inundation depths during the impact around 20% higher as compared to the smooth bed. Furthermore, a rough bed induced a lower momentum flux during wave propagation, resulting into lower impact forces exerted on the building. The rough bed configuration also caused shorter impact durations, leading to lower impulse values transferred to the building. Results pointed out that even on rough bed, openings within the buildings linearly reduced impact forces, thus providing some helpful information for the design of safer coastal structures.@en

A comprehensive understanding of physical phenomena based on a hybrid experimental-numerical approach supports a safer design of critical infrastructures. However, for large-scale tsunami-like waves, the reliability of numerical models was insufficiently validated. Herein, we validate our numerical model of tsunami-like waves by comparing simulation data (obtained using a smoothed panicle hydrodynamics method based on a highly efficient parallel computing technique) with large-scale experimental data for both, dry bed surges and wet bed bores. The presented preliminary results are believed to aid the development of experimentally validated numerical tools for a better understanding of tsunami-like flows.

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Unsteady flows such as tsunamis, impulse waves and dam-break waves can lead to damages and human losses. Hence, specific research to limit casualties and reconstruction costs is needed. The complexity of the phenomena involved suggests that a hybrid experimental-numerical approach should be used to gain a more comprehensive understanding of the process. This paper presents an explorative study on the comparison of SPH numerical simulations using a highly effective parallel computing technique with large scale experimental data for dry bed surges and wet bed bores impacting free-standing buildings with and without openings. These preliminary results showed a relatively good agreement between the two approaches in the estimation of water depths around the buildings, which represents a key parameter for the design of vertical shelters. Nevertheless, some differences observed during the impact phase may be attributed to the incapability of the SPH numerical simulations to fully capture the turbulent nature of the process and its air-entrainment. These validation tests with detailed experimental data are a promising approach to improve the numerical models toward the development of reliable numerical tools for a safer design of resilient structures.@en

Chancy-Pougny is a run-of-river dam on the Swiss–French border constructed in the early 1920s. Since its inauguration, the operation of the four spillway gates was responsible for a progressive erosion of the stilling basin. Thus, a hybrid modelling was performed to study the scour potential and to determine adequate solutions to maintain future scour within acceptable levels. Visual observations on the physical model identified a strong recirculation in the nonsymmetrical basin that combined with the outflow from the spillway gate, lead to an enhanced specific discharge and to the formation of a turbulent vortex impacting on the rocky foundation. Some measures to limit the recirculation, including a free-standing wall and various configurations of concrete prisms for scour protection were tested on both the physical and numerical model. The pressure and water depth measurements resulting from the physical model were used within the numerical model to determine the corresponding scour potential for each of the tested mitigation measures. A solution containing a layer of randomly distributed concrete prisms laid on the basin’s current bottom was identified through this study, proving the importance of both numerical and physical approaches in hydraulic engineering.

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Uncontrolled scour is affecting multiple mobile dams. The plunge pool of the Chancy-Pougny barrage, located on the Rhone River bordering France and Switzerland, has recently been reported to develop in a potentially unfavourable direction after almost 100 years of acceptable scour development. In order to estimate the future scour potential of this asymmetrical plunge pool and validate and optimize possible solutions to limit its development, a physical model was built at LCH-EPFL. First-stage diagnosis tests showed the presence of recirculation surface currents interacting with the spillway outflow jets producing downward vorticity responsible for scour progression. In partnership with the responsible engineering company, different solutions were proposed and tested, starting with the installation of a vertical guide-wall to prevent the formation of the return currents. A solution with randomly-arranged concrete prisms was finally retained as the most favourable compromise between the effective protection of the riverbed and site implementation. Herein the robustness of this solution is analysed for two different, but plausible, operation scenarios. Both the closure of a spillway gate and the lowering of the water level proved the effectiveness of the chosen solution without endangering the stability of the prism-arrangement. The technical solution presented herein enlarges the range of alternative solutions for uncontrolled scour control not only at the Chancy-Pougny barrage but also for a large number of structures with similar layout.@en

Tsunamis, landslide-generated waves, and dam failures are rare, but highly destructive phenomena, associated with extreme loading on infrastructure. Recent events showed that specific measures must be taken to guarantee safety of both people and the built environment. This experimental study investigates the forces and moments exerted on free-standing buildings that are induced by both surges and bores. The hydrodynamic impact was characterized by high splash, subsequently followed by a quasi-steady flow around the structure. For dry bed surges, the time history of the horizontal force was proportional to the momentum flux per unit width. For wet bed bores, an attenuation of the peak force due to the presence of an aerated front was observed and the introduction of a reduction coefficient was necessary to achieve a realistic force estimation. Additional force analysis in terms of peak time, wave height at maximum force and impulse also pointed out some key differences between forces exerted by dry bed surges and wet bed bores. The occurrence of the maximum tilting moment on the building coincided with the maximum horizontal force and an evaluation of the cantilever arm was possible. These findings provide engineers with practical information for the design of safer coastal structures.

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Tsunamis, impulse waves, and dam failures are disasters that challenge humanity, often leading to massive casualties and extreme economic losses. The highly unsteady flow conditions generated by such events are often in the form of turbulent bores. The purpose of this study was to investigate and validate a new generation system for bores propagating over dry and wet bed conditions. There are multiple techniques to generate such waves experimentally, and the study focused on the generation of tsunami-like inundation conditions through the vertical release of a water volume. A detailed methodology to characterize the generated waves hydraulically, in terms of their wave heights and flow velocities, is presented, and good agreement with the classical dam-break case for both dry bed surges and wet bed bores was demonstrated. Because of the importance of estimating the impact forces induced by such waves, particular attention was given to the wavefront celerity and the velocity profiles measured behind the wavefront; these were found in agreement with Prandtl's power law for open channel flows, and in-depth measurements allowed for the definition of an expression to estimate flow deceleration behind the wavefront. Along with considerations of the Froude number and momentum, this paper provides relevant information to assist engineers in designing safer infrastructures in areas prone to such extreme loading.

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Previous studies and field surveys showed that specific structural designs can decrease the load on free-standing buildings along the coast, providing safer vertical shelters. This experimental study investigated the effect of openings in buildings (windows, doors and foyers) on horizontal forces and tilting moments induced by both dry bed surges and wet bed bores. Four configurations with seven porosity values ranging from 0% (impervious) to 84% (highly permeable) were systematically tested. Due to the presence of openings, the flow through the building reduced the upstream water depths. The porosity resulting from the presence of openings was shown to produce a linear reduction of the maximum horizontal force, when compared to the corresponding impervious building. The configuration with an impervious back showed results similar to those measured for the fully impervious buildings. The occurrence of the maximum tilting moment was shown to coincide with the maximum horizontal force and an estimation of the cantilever arm was therefore possible. The latter was constant for all configurations, independent of the geometry of the openings. Finally, two equations to predict the maximum horizontal force and the tilting moment were proposed, taking into account the effect of building openings within the resistance coefficient. These showed good agreement with experimental data and previous studies. These findings provide engineers with practical information for the design of safer vertical shelters in tsunami-prone areas.

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Waves impacting against structures can create damages and devastation. This topic regained interest after some recent catastrophic events and the present paper investigates the main phases of a wave impacting against a residential house commonly observed in areas subject to tsunami hazards. The project is based on an experimental approach and both dry bed surges and wet bed bores were tested. Through some visual observations and high speed cameras, important run-up heights in the vertical direction were observed for all configurations; these were more intense for wet bed bores. For the largest waves a full overflow of the structure was observed and the presence of the structure provoked a change of regime and the upstream propagation of a bore.@en

In the context of a comprehensive research project investigating the hydrodynamic loading on structures with openings, this paper focuses on the wave generation techniques currently used and the test results associated with it, proposing a particular tsunami-like wave using a vertical water volume release mechanism. The latter allowed a certain volume of water to flow gravitationally from an upper reservoir into a lower basin through a set of pipes. The propagation of the resulting wave took place in a horizontal channel where wave height and velocities were measured. Both dry bed surges and wet bed bores were investigated, however the present paper mostly focuses on dry bed surges. The study indicated that the surges generated with this mechanism had similar behavior to those resulting from a dam-break technique and a good agreement between the experimental points and the theoretical solutions of Ritter (1892) and Whitham (1955) was found. Lastly some differences between wet bed bores and dry bed surges are presented and briefly discussed herein.@en

The stepped spillway design has been used for more than 3,300 years. A simple structure is the gabion stepped weir. A laboratory study was performed herein in a large size facility. Three gabion stepped weirs were tested with and without capping, as well as a flat impervious stepped configuration. For each configuration, detailed air–water flow measurements were conducted systematically for a range of discharges. The observations highlighted the seepage flow through the gabions and the interactions between seepage and overflow. The air–water flow properties showed that the air concentration, bubble count rate and specific interface data presented lower quantitative values in the gabion stepped weir, compared to those on the impervious stepped chute, while higher velocities were measured at the downstream end of the gabion stepped chute. The re-oxygenation rate was deduced from the integration of the mass transfer equation using air–water interfacial area and velocity measurements. The aeration performances of the gabion stepped weir were lesser than on the flat impervious stepped chute, but for the lowest discharge. For the two configurations with step capping, the resulting flow properties were close to those on the impervious stepped configuration.@en

The appearance of the regular vegetated ridge patterns observed in some ephemeral rivers of semi-arid regions (Nanson, Tooth, & Knigthon 2002) has previously been explained by hydraulic arguments (optimization of the bed load transport capacity, see Huang & Nanson (2007)) without including the role of vegetation in the process. Those arguments provide neither the conditions under which the more efficient anabranching system can be realized nor a description of the dynamics leading to anabranching. As an alternative, we propose a simplified model accounting for the flow-mediated interactions between riparian vegetation located at different points of the river channel. Classically, the appearance of river morphologic features is explained by the equations of morphodynamics. However, due to the complexity of the action of vegetation on the flow and on sediment transport, a complete physically-based set of coupled ecomorphodynamic equations is not available yet. We propose an effective model for the interactions between vegetated obstacles. Depending on the relative position of the obstacles, one observes either positive or negative feedbacks: on the lee-side of a permeable obstacle, flood sheltering may occur and favor deposition that helps in turn the establishment of biomass, conversely scouring is increased laterally due to obstacle-induced flow diversion (Zong & Nepf 2010). In the situation where the hydrological timescale (flooding frequency for a given effective magnitude) and the biological timescale (vegetation development rate) are comparable, the spatially inhomogeneous feedbacks can result in the appearance of organized regular structures. Aerial photographs give us the characteristic morphological scale of patterns. We then perform a stability analysis of our model and derive a set of conditions under which the combination of hydrological, ecological and pedological factors allows the formation of anabranching patterns. We discuss the role of the different ingredients in relation to the fluvial environments in which such patterns typically emerge. Finally, we conjecture on the relevance of the proposed mechanism to explain ubiquitous vegetated scroll bars that are observed on the interior of meander bends.@en

In the last decades the design of stepped spillways regained some interest because of their suitability with new construction methods including gabions. The hydraulic performances of gabion stepped weirs were investigated experimentally in terms of the flow patterns, air-water flow properties, and energy dissipation. A laboratory study was conducted in a 26.6° slope (1V:2H) and 0.10-m step height facility, with both smooth impervious and gabion steps. The visual observations highlighted the seepage flow through the gabions, inducing a modification of the cavity flow especially in the skimming flow regime. In skimming flows, higher velocities were measured at the downstream end of the gabion stepped chute, associated with smaller energy dissipation rates and lower friction factors, compared to the smooth impervious stepped chute data.

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In the last three decades the design of stepped spillways regained some interest because of their suitability with new construction methods including gabion placement. In this study, the hydraulic performances of gabion stepped weirs were investigated experimentally in terms of the air-water flow properties and energy dissipation rate. A physical study was performed in a relatively large size facility with a 26.6° slope (1V:2H) and 0.10 m step height. For both gabion and impervious stepped weirs, a detailed comparison of the air-water flow properties was performed. The visual observations highlighted the seepage flow through the gabions, including a modification of the cavity flow dynamics, especially in the skimming flow regime. In skimming flows, higher velocities were measured at the downstream end of the gabion stepped chute, associated with lesser energy dissipation rates.@en

Several research investigations have explored the interaction between morphodynamic and vegetation growth processes from both the modelling and the experimental viewpoints. Results have mainly been concerned with morphologic analyses of the effects of vegetation on long term riverbed evolution without addressing the relative role of the timescales between such processes. This paper presents for the first time the statistics of uprooted biomass obtained while perturbing the vegetation growing in the river bed with periodic disturbances of constant magnitude. That is, we force the biological and hydrological processes to interact and study the related timescales in order to shed light on the role of flood disturbances in selecting the component of the biomass that has a higher chance of survival in relation to its growth stage. A simple interpretative stochastic model is then presented and thoroughly discussed in a companion paper (Biomass selection by floods and related timescales: Part 2. Stochastic modelling).

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