Browsing by Subject "Tsunami"

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Modeling the influence of coastal vegetation on the 2004 tsunami wave impact
(2014) Laso Bayas, Juan Carlos; Cadisch, Georg
A tsunami causes several effects once it reaches inland. Infrastructure damage and casualties are two of its most severe consequences being mostly determined by seaquake intensity and offshore properties. Nevertheless, once on land, the energy of the wave is attenuated by gravity (elevation) and friction (land cover). Despite being promoted as ‘bio-shields’ against wave impact, proposed tree-belt effects lacked quantitative evidence of their performance in such extreme events, and have been criticized for creating a false sense of security. The current study analyzed some of the land uses in sites affected by the 2004 tsunami event, especially in coastal areas close to the coast of Indonesia, more specifically on the west coast of Aceh, Sumatra as well as on the Seychelles. Using transects perpendicular to the coast, the influence of coastal vegetation on the impact of the 2004 tsunami, particularly cultivated trees, was modeled. A spatial statistical model using a land cover roughness coefficient to account for the resistance offered by different land uses to the wave advance was developed. The coefficient was built using land cover maps, land use characteristics (stem diameter, height, and planting density), as well as a literature review. The spatial generalized linear mixed models used showed that while distance to coast was the dominant determinant of impact (casualties and infrastructure damage), the existing coastal vegetation in front of settlements also significantly reduced casualties, in the case of Aceh, by an average of 5%. Despite this positive effect of coastal vegetation in front of a settlement, it was also found that dense vegetation behind villages endangered human lives and increased structural damage in the same case, most likely due to debris carried by the backwash. The models initially developed in Aceh were adapted and tested for the effects that the same tsunami event caused in the Seychelles, where the intensity of the event was a tenth of that in Aceh. These new models suggested no direct effect of coastal vegetation, but they indicated that vegetation maintained dunes decreased the probability of structural damage. Additionally, using satellite imagery with higher resolution than that of the first study and/or from different years before the tsunami, corresponding land roughness coefficients were developed and tested with the existing models. The new models showed no signs of further increase of goodness of fit (AIC). Nevertheless, weather conditions at the acquisition dates as well as coverage and lack of image availability diminished the predictive power of these models. Overall, more than advocating for or against tree belts, a sustainable and effective coastal risk management should be promoted. This planning should acknowledge the location (relative to the sea) of settlements as the most important factor for future coastal arrangements. Nevertheless, it should also consider the possible direct and indirect roles of coastal vegetation, determined by its spatial arrangement as shown in the study models. Sustainability of these measures would only occur when coastal vegetation is regarded as a livelihood provider rather than just as a bio-shield. Practical examples could include, e.g. rubber plantations or home gardens in front of settlements, while leaving escape routes or grasslands and coconut plantations behind these. Therefore, the enforcement of educational programs, the setup and maintenance of effective warning systems and the adequate spatial allocation of coastal vegetation bringing tangible short and mid term benefits for local communities, as well as its adaption to local customs should be considered.