Tsunamis in the Caribbean Sea : implications from coarse-clast deposits and the importance of their shape

Oetjen, Jan; Schüttrumpf, Holger (Thesis advisor); Brückner, Helmut (Thesis advisor)

Aachen : RWTH Aachen University (2021, 2022)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021

Abstract

On many coasts around the world, tsunamis pose a serious threat to the population, causing direct damage (e.g., drowning deaths) and indirect damage (e.g., damage to infrastructures and impairment of drinking water supplies). Recent tsunamis, such as the 2011 Tōhoku-Oki tsunami in Japan and the 2004 Indian Ocean tsunami in Thailand, have shown that there is a lack of sufficient understanding of the specific local tsunami hazard in many regions. Thus, although geological evidence of high-energy tsunamis existed prior to 2011, tsunami management plans and mitigation measures in many regions of Japan were based only on the strongest events of the past 100-200 years and were, therefore, inadequate to cope with the effects of the Tōhoku-Oki tsunami. However, because the available geologic deposits already indicated such intense tsunamis, Japan’s authorities decided to explicitly consider geologic evidence of undocumented historical and prehistoric tsunamis (paleo-tsunamis) in the design of mitigation measures and in management plans, in the aftermath of the 2011 tsunami disaster in Japan. Consideration of such geological evidence has the benefit of providing information on tsunamis that have not been recorded or observed and have long-standing occurrence intervals or very low probabilities of occurrence (recurrence intervals of 1000 years or more). Thus, the analysis of geological deposits allows a coast-specific estimation of whether tsunami events can occur which exceed previous observations or estimates. Thus, risk mitigation strategies and management plans can be adapted accordingly. In this dissertation, the analysis of geological deposits is applied to the Caribbean Sea and, in particular, the island of Bonaire (ABC Islands, Lesser Antilles). While for the Caribbean Sea a large number of tsunami observations from the past approximately 520 years exists, of which only a fraction is verified, no concrete records are available for Bonaire and the entire ABC islands. On Bonaire in particular, however, there is a large number of geological deposits that indicate high-energy wave events. The study of the possibly tsunami-induced transport of some of these deposits on Bonaire by experimental and numerical models forms the key aspect of the present dissertation and is accompanied by a holistic analysis of the tsunami hazard in the Caribbean region. In this context, the first step was to evaluate the inventory of tsunami deposits throughout the Caribbean Sea and attempt to assign these deposits to the respective cause of displacement (storm waves or tsunamis). For this purpose, numerical tsunami simulations were performed for the Caribbean Sea based on verified and assumed earthquake scenarios as tsunami triggers. In the course of these simulations, tsunami heights were calculated along all coastlines in the Caribbean Sea, but special attention was paid to the Boka Olivia and Spelonk areas along the northern coast of Bonaire. Comparing the simulated tsunamis with results from empirical block transport equations, it became clear that the simulated tsunamis did not produce wave intensities sufficient to relocate the largest boulder on the island of Bonaire. The main identified reasons for this discrepancy depict insufficient accuracy of the empirical transport equations (e.g., as a result of oversimplifications) and inadequate selection or reproduction of earthquake scenarios. Under the impression of these results and in order to assess the accuracy of empirical boulder transport equations, a comprehensive analysis of published physical experiments on tsunami-induced block transport was carried out and their weaknesses were identified. As a major weakness the neglect of boulder shapes beyond idealized cubes or parallelepipeds was identified, which was, therefore, chosen as the core element to be investigated in the subsequent physical experiments. During this analysis, it also became clear that the currently available and widely used empirical equations for the description of block transport by tsunamis are not suitable for determining exact values of possible tsunami intensities. Instead, it is recommended to work with probability ranges for boulder mobilization and thus to limit public statements also to ranges of possible tsunami intensities. Another element of the study was the development of a tool that allows the user of the empirical transport equations to assess whether the application of the equations tends to underestimate or overestimate the actual transport-causing tsunami, taking into account local boundary conditions. For the physical model experiments, the largest wave-transported boulder on the island of Bonaire was selected for extensive investigations, especially with respect to the influence of the shape, but also considering the influence of the coastal shape, the submergence of the boulder as well as the boulder orientation on transport processes and to assess their effect on the application of empirical boulder transport equations. For this purpose, this boulder was photogrammetrically recorded and investigated at a scale of 1:50 in physical experiments and compared to idealized block models. Based on the experimental results, it could be shown that the consideration of shape in calculations of the relationship between boulder transport and necessary tsunami intensity is essential. Compared to idealized blocks, streamline-shaped blocks are later mobilized by the impacting wave energy and are displaced over shorter distances in the subsequent transport process. Further investigations included experiments with different coastal models, boulder submergence as well as their orientation to the wave. Finally, based on an existing numerical two-phase flow model aiming on the wave generation by landslides, the first version of a numerical boulder transport model was developed, which should prove the significance of the block shape on the one hand and the influence of sediment load in the wave on the transport on the other hand. First results from the application of the current model version prove the importance of the shape and its orientation to the tsunami and indicate that a higher mean density of the attacking wave has a transport-promoting effect.

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