Tsunamis pose a substantial threat to coastal communities around the globe. To counter their effects, several hard and soft mitigation measures are applied, the choice of which essentially depends on regional expectations, historical experiences and economic capabilities. These countermeasures encompass hard measures to physically prevent tsunami impacts such as different types of seawalls or offshore breakwaters, as well as soft measures such as long-term tsunami hazard assessment, tsunami education, evacuation plans, early-warning systems or coastal afforestation. Whist hard countermeasures generally aim at reducing the inundation level and distance, soft countermeasures focus mainly on enhanced resilience and decreased vulnerability or nature-based wave impact mitigation. In this paper, the efficacy of hard countermeasures is evaluated through a comprehensive literature review. The recent large-scale tsunami events facilitate the assessment of performance characteristics of countermeasures and related damaging processes by in-situ observations. An overview and comparison of such damages and dependencies are given and new approaches for mitigating tsunami impacts are presented.

Research on tsunami‐induced coarse‐clast transport is a field of rising interest since such deposits have been identified as useful proxies for extreme‐wave events (tsunamis, storm waves) that provide crucial information for coastal hazard assessment. Physical experiments are, beside in‐situ observations, the foundation of our understanding of how boulders are transported by tsunamis and provide clues to the development of empirical equations and numerical models describing the processes and fundamental mechanics. Nevertheless, investigating tsunami‐induced boulder transport is a comparatively young discipline and only a few experimental studies focusing on this topic have been published so far. To improve the knowledge on nearshore tsunami hydrodynamics, physical experiments utilizing real‐world boulder shapes have been carried out simulating three different shore types in a wave flume. Crucial insights were gained into boulder transport hydrodynamics and data resulting from the experiments were analysed in an empirical, statistical, quantitative and qualitative manner. The regular cuboid boulder – one of the specific shapes used in the experiments – showed the longest transport distances compared to a complex, natural boulder and a flat cuboid boulder, but also significant fluctuations regarding the total transport distance. The experiments indicate a strong influence of the shore shape on boulder transport behaviour. Experimental setups of increased mean transport distances also led to a higher spreading of results. This spreading was further amplified between the idealized‐shaped cuboid and the complex‐shaped boulder, which is associated with a lower drag coefficient. Due to the highly sensitive boulder reaction to divergent experimental setups, the need to recognize boundary conditions overcoming commonly considered parameters (e.g. roughness or Flatness Index) in field studies and numerical models is underlined. Beside the strong influence of initial boulder submergence and alignment, both the boulder shape and shore type influence the boulder transport pattern, increasing the total transport distance by more than 350% in some cases. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

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.

High-energy events such as tsunamis pose significant threats to coastal infrastructure and buildings, necessitating effective mitigation strategies to minimize damage. Compared to massive construction measures, buffer blocks are increasingly recognized as a potential solution for reducing the impact of such events. Understanding their effectiveness and optimizing their placement configurations is crucial for enhancing coastal resilience. The present study aims to experimentally evaluate the influence of buffer blocks on the depth of tsunami inundation. By comparing different configurations of buffer blocks, this study seeks to identify the most effective one for reducing the inundating flow depth. The laboratory tests revealed that the buffer blocks significantly influence flow depth and its characteristics. For ratios of flow depth to block height (R) below 1.5, the buffer blocks exhibited a delay in the arrival of the flow and effectively reduced the flow depths. However, for R values greater than 1.5, the effectiveness reduced as the flow began to overtop the obstructions. The increase in the flow depth at the buffer blocks was the least for a single-row configuration, while the multiple-row configurations with wider spacing offered higher flow resistance and increased the flow depths in front of the blocks. These findings suggest that strategically configured buffer blocks can substantially enhance coastal protection against high-energy flow events during natural coastal hazards. This study provides critical insights into the design and optimization of buffer block configurations, contributing to improved coastal infrastructure resilience and disaster mitigation strategies.

Heavy flash floods in Germany, which occurred in the aftermath of intense and long-lasting rainfalls (up to 150-200 l/m² within 48 h; CEDIM, 2021) in July 2021, led to serious damage to people, buildings and infrastructure. According to CEDIM (2021), the flash-flood caused at least 170 fatalities and 820 injuries. It is expected that it will take several years for buildings and infrastructure to recover from this catastrophic event.The Ahr valley on the border between North Rhine-Westphalia and Rhineland-Palatinate (Germany) was particularly affected by the flood and was therefore selected as study area for in-situ investigations. These were conducted approximately one month after the event (August 17-19, 2021), and served to systematically assess the flood-induced building damage in nine villages. Damage was recorded on the basis of photos, flood measurements and personal conversations with affected residents (Korswagen et al., 2022).In the aftermath of the survey, the information collected was used for describing the flood-induced damage in detail and to assess the processes that led to the structural failures. As key-findings, three main processes were identified as significantly accountable for flood induced damage to buidings:Hydraulic loads: The survey revealed that the water that penetrated the interior of the building neutralized the hydrostatic pressure acting from the outside. Nevertheless, hydrodynamic forces were present, leading first to damage to the facade and then to structural failure. Further damage was caused by different water levels around the buildings, which led to structural stresses due to an uneven distribution of the load. Scour and bank erosion exposed and undermined building foundations. Shallow-founded buildings adjacent to the river banks were the most vulnerable. Buildings on the outside and inside of river bends in particular need further attention in this regard. Debris: The effects of water-borne debris played a significant role during the flood, not only due to direct impact damage, but also because debris accumulated and became entangled, forming debris-dams upstream of buildings. Accumulations at building openings resulted in increased impact areas and higher water-level gradients inside and outside of buildings, leading to significantly higher structural loads on buildings. In particular, it was noted that buildings located on the upstream part of the villages were more affected by debris accumulation. Overall, the in-situ study highlights the importance of improving land use planning rules and guidelines, considering lessons learned from the July 2021 flash flood. Examples may include the need to reinforce buildings near river bends or in flood-prone areas with deep foundations, develop debris management plans that take into account the additional effects of debris during flash floods, and review, and if necessary, expand no-build zone regulations currently in place. CEDIM (2021). Hochwasser Mitteleuropa, Juli 2021 (Deutschland). CEDIM Forensic Disaster Analysis (FDA) Group – Bericht 1 "Nordrhein-Westfalen & Rheinland-Pfalz". Karlsruher Institut für Technologie, Karlsruhe, Germany. DOI: 10.5445/IR/1000135730Korswagen, P.A., Harish, S., Oetjen, J., Wüthrich, D. (2022). Post-flood field survey of the Ahr Valley (Germany): Building damages and hydraulic aspects. 4TU Research Data, Delft University of Technology, The Netherlands. DOI: 10.4233/uuid:3cafd772-facd-4e3a-8b1a-cee978562ff

This presentation gives an overview of the results of a five-year research project on tsunami-induced boulder transport. It stresses the importance of the exact determination of boulder shapes in contrast to simplified bodies (such as cuboids), especially with regard to the transport distance. It also provides insights about a newly developed numerical boulder-transport model based on Pudasaini (2012). Additionally, some ideas how experimental research on tsunami-induced boulder transport may be improved and coordinated in the future will be presented.The investigations by physical experiments are based on three boulder shapes of which one depicts the replica of an original boulder from the island of Bonaire (Caribbean Sea, Lesser Antilles). The experiments clearly reveal that the available impact area of the boulder has a great significance; however, this is so far insufficiently considered in analytical equations. In the given case, the comparison between the more streamline-shaped replica of the Bonaire boulder and an idealised cuboid boulder resulted in reduced transport distances of 30 %, in average. Additionally, statistical evaluations revealed that the entire process is highly sensitive with partly stochastic behaviour. Thus, we support the statement of Bressan et al. (2018) in this regard. We show, how important it is to calculate and communicate wave thresholds for mobilisation in terms of probability ranges instead of fixed values.Based on the results of our own physical experiments and the evaluation of published physical experiments, we developed a tool, which supports researchers in assessing the accuracy of analytical equations for specific in-situ settings (Oetjen et al., 2021). This tool encompasses the crucial parameters (e.g., bottom roughness, boulder shape), combines their influence on the transport process and finally gives an indication of whether the present conditions tend to amplify or hamper the boulder transport. The benefit and the usage of the above-mentioned tool will be demonstrated exemplarily.Furthermore, within the framework of the project a numerical Boulder-Transport-Model was developed which is based on the Immersed Boundary Method and the Two-Phase Flow Model of Pudasaini (2012). Insights into the functionality of the model and the importance of the increased flow density will be highlighted, while the further development steps will be indicated.As part of the project, we also dealt with the future development of research on tsunami-induced boulder transport (cf. Oetjen et al., 2021). One important suggestion is to establish a standardised reference setup for experimental investigations within the research community. It would enable researchers to compare the results of their own experiments and the effect of the investigated parameters with well-documented reference values and assist them to evaluate and classify their experimental results accordingly.  Bressan, L., Guerrero, M., Antonini, A., Petruzzelli, V., Archetti, R., Lamberti, A., Tinti, S. (2018): A laboratory experiment on the incipient motion of boulders by high-energy coastal flows. Earth Surface Processes and Landforms 43 (14), 2935–2947. DOI: 10.1002/esp.4461.Oetjen, J., Engel., M., Schüttrumpf, H. (2021): Experiments on tsunami induced boulder transport – a review. Earth-Science Reviews 220. DOI: 10.1016/j.earscirev.2021.103714.Pudasaini, S.P. (2012): A general two-phase debris flow model. Journal of Geophysical Research: Earth Surface 117, F03010. DOI: 10.1029/2011JF002186.