Search

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.

Natural hazards (2022). doi:10.1007/s11069-022-05367-y, Published by Springer Science + Business Media B.V., Dordrecht [u.a.]

Severe storms, their extreme waves and surges pose the greatest natural hazard to the coasts of northwestern Europe, commonly resulting in infrastructural damages and high financial losses. Proxy records of past storminess are important for assessing future risks that may arise from storm surges and assessing whether storm activity has increased in recent decades. High-resolution records of North Atlantic storminess are generally limited to instrumental weather data or historical documentation of the past 50 to 200 years. Since the most destructive and severe storms passing over Europe originate in the North Atlantic, the Shetland Islands serve as a window to cyclogenesis in this region. In our research, we extracted lacustrine sediments of the coastal freshwater lake Loch of Flugarth on Mainland, Shetland Islands, that is separated from the ocean by a low sand and gravel barrier. A series of distinct sand layers intercalated in the otherwise fine-grained, organic-rich lake deposits and examined using particle-size analysis, microfossils, TOC and XRF, may represent storm overwash or aeolian transport mechanisms, both either pointing towards individual storm events or shorter phases of high storm activity. Based on radiocarbon data of some selected layers, the investigated sediment sequence covers ca. 1500 years and a Bayesian age-depth model is being established. In combination with a hydrodynamic wave model based on Delft3D-Flow, we simulate a critical threshold value at which waves may reach the lake to determine the sensitivity of the sedimentary archive. With the inclusion of historical documentation and observations, our multi-methodological approach aims at reaching a better understanding of the recurrence pattern of extreme storm events on the Shetland Islands over the last 1500 years. This implies further insights into the parameters driving extra-tropical storms in the wider region as well as the role and variability of the North Atlantic Oscillation across the targeted time frame.

s & presentations / EGU General Assembly 2021 EGU General Assembly 2021, vEGU21, online, 19 Apr 2021 - 30 Apr 2021; Göttingen : Copernicus Gesellschaft mbH EGU21-8583, 2 Seiten (2021). doi:10.5194/egusphere-egu21-8583, Published by Copernicus Gesellschaft mbH, Göttingen

This chapter summarizes background and setup of wave-flume experiments investigating boulder transport by tsunamis. It emphasizes both opportunities for better understanding boulder transport in natural settings and limitations, i.e., the sources of uncertainty, related to these experiments. It also reviews the most important findings published so far, explains linkages with numerical models, and draws conclusions on research directions and open questions, which need to be tackled in future studies.

Earth surface processes and landforms 45(9), 2118-2133 (2020). doi:10.1002/esp.4870, Published by Wiley, New York, NY [u.a.]

The interest in applying coastal boulders as a proxy for high-energy wave events, i.e., tsunamis and major storms, increased significantly during the last two decades. Studies include on-site investigations, numerical modelling or experimental studies. However, inferences on past inundation processes based on commonly used inverse modelling approaches are associated with significant uncertainties and the need for validation of relevant parameters has been expressed. Here, we review experimental studies on the transport of natural boulders (and artificial bodies) by tsunamis, which provide crucial insights into the physical parameters controlling the transport process. We summarize and categorize the differences and scopes of existing experimental studies and point out the importance of the boulders' wave-facing area and, thus, the drag force, which is highlighted in most studies. The boulder-transport mode (e.g., rolling or sliding), however, varies strongly throughout the publications. A considerable number of influencing parameters is not investigated systematically (e.g., the influence of boulder-boulder interactions) so far, and there is an agreement that existing hydrodynamic equations for mobilization thresholds tend to overestimate the necessary wave height and velocity due to the existing uncertainties. Therefore, and based on the current knowledge, working with parameter ranges (e.g., mobilization thresholds, transport-distance spans) is recommended instead of using exact values. However, we demonstrate that comparisons of the particular findings of the reviewed studies is not straightforward due to very individual experimental setups. Based on this, we propose a standardized research environment, which might simplify the knowledge transfer between studies and can possibly improve numerical models for tsunami-induced boulder transport.

Coasts worldwide experience considerable population pressure and the demand for reliable hazard management, such as of tsunamis, increases. Tsunami hazard assessment requires information on long-term patterns of frequency and magnitude, which are best explained by inverse power-law functions. In areas with a short historical documentation, long-term patterns must therefore be based on geological traces. The Caribbean tsunami hazard is exemplified by > 80 events triggered by earthquakes, volcanic activity, or mass wasting within the region or in the far-field during the last 520 years. Most of these tsunamis had regional or local impacts. Based on two numerical hydrodynamic models of tsunamis spawning at the Muertos Thrust Belt (MTB) and the South Caribbean Deformed Belt (SCBD), which are two scenarios only marginally considered so far, we show that pan-Caribbean tsunamis can also be taken into account. We furthermore review almost 60 sites for possible geological evidence of tsunamis in the Caribbean including fine-grained subsurface deposits and subaerial coarse clasts, and re-evaluate their implications for tsunami hazard assessment against state-of-the-art models of onshore sediment deposition by tsunamis and extreme storms. The records span the mid- to late Holocene, with very few exceptions of Pleistocene age. Only a limited number of reliable palaeotsunami records with consistent and robust age control were identified, hampering inter-island or interregional correlation of deposits of the same event. Distinguishing between storm and tsunami transport of solitary boulders is very difficult in most cases, whereas those clasts arranged as ridges or incorporated into polymodal ridge complexes, which line many windward coasts of the Caribbean, can mainly be attributed to long-term formation during strong storms, overprinting potential tsunami signatures. The quantification of tsunami flooding parameters such as flow depth, inundation distance or flow velocities, by applying inverse and forward numerical models of sediment transport is still very limited and needs to be extended in the future. Likewise, sediment-derived hazard implications still await implementation in spatial planning. As extreme-wave deposits are clearly understudied in the Caribbean, there is great potential for coastal hazard assessment to be developed and improved. Thus, further studies using common standards of high-resolution methods of bedform and stratigraphical documentation and robust chronological models with independent age control, combined with refined inverse and forward models of sediment transport and deposition are required to reconstruct reliable patterns of magnitude and frequency of palaeotsunamis in the Caribbean and to map hazard-prone areas. To date, known palaeotsunami deposits from the Caribbean probably represent only a fraction of actually happened prehistoric tsunamis and, therefore, do not reflect major tsunami inundations of the past adequately.

Coasts around the world are affected by high-energy wave events like storm surges or tsunamis. By focusing on tsunami impacts, we investigate tsunami-induced transport of boulders by an interdisciplinary combination of field observations, laboratory experiments and advanced numerical modelling. In phase 1 of the project we conduct physical laboratory experiments based on real-world data. Following the experimental phase we will develop an enhanced numerical boulder transport model (BTM) based on an existing two-phase model.

During the last years, an increased amount of damage has been observed on estuarine and riverine waterway structures such as groins or training walls in tidal waterways. The cause of these damages could be attributed to ship-induced long-periodic waves. Because of higher traffic volumes and increased ship dimensions these loads have risen. In contrast to short-periodic secondary waves, the long-periodic wave system is not taken into account in existing design approaches so far. In the framework of the project “Ship-induced long-periodic loads for the design of cover layers on maritime waterway structures” small-scale 2D physical model tests have been performed in order to quantify the specific overflow and overtopping rate taking into account different geometries, surface roughnesses and permeabilities of the structures as well as stationary overflow, short- and long-periodic waves. Furthermore, the stability of the structures under short- and long-periodic wave loads has been observed. These tests provide the basis for the design of cover layers on river structures in maritime waterways.