Influence of coastal vegetation on the 2004 tsunami wave impact in west Aceh

Abstract

In a tsunami event human casualties and infrastructure damage are determined predominantly by seaquake intensity and offshore properties. On land, wave energy is attenuated by gravitation (elevation) and friction (land cover). Tree belts have been promoted as "bioshields" against wave impact. However, given the lack of quantitative evidence of their performance in such extreme events, tree belts have been criticized for creating a false sense of security. This study used 180 transects perpendicular to over 100 km on the west coast of Aceh, Indonesia to analyze the influence of coastal vegetation, particularly cultivated trees, on the impact of the 2004 tsunami. Satellite imagery; land cover maps; land use characteristics; stem diameter, height, and planting density; and a literature review were used to develop a land cover roughness coefficient accounting for the resistance offered by different land uses to the wave advance. Applying a spatial generalized linear mixed model, we found 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 by an average of 5%. In contrast, dense vegetation behind villages endangered human lives and increased structural damage. Debris carried by the backwash may have contributed to these dissimilar effects of land cover. For sustainable and effective coastal risk management, location of settlements is essential, while the protective potential of coastal vegetation, as determined by its spatial arrangement, should be regarded as an important livelihood provider rather than just as a bioshield. Indo-Australian and southeastern Eurasian tectonic plates 150 km off the coast of west Aceh, Indonesia, triggered one of the largest seismic events in the last four decades (1). The seaquake generated a tsunami with disastrous consequences in the region.Soon after the 2004 event, the possible effects of coastal vegetation regarding the impact caused by tsunamis (mitigating or aggravating) were researched into, especially under scenarios with initial water heights below 10 m (2-5). In Sri Lanka and India, coastal communities located behind tree cover were reported to be less affected than those directly exposed to the sea (2, 3, 6). Parameters such as stem diameter and height as well as a "bioshield" width were identified as key vegetation characteristics with a bearing on impact mitigation (6). However, several studies advocating bioshields have been criticized for lacking empirical evidence to support the protective function of vegetation, some even suggesting that bioshields may give a false sense of security to coastal populations (7-10). The role of vegetation in tsunami impact mitigation still remains a controversial issue (11-13). In the coastal regions of western Aceh in 2004, the potential for mitigating tsunami impacts appeared limited as a result of the massive energy released by waves with heights exceeding 20 m (13). Mangroves along this...

Will funding to Reduce Emissions from Deforestation and (forest) Degradation (REDD+) stop conversion of peat swamps to oil palm in orangutan habitat in Tripa in Aceh, Indonesia?

“…According to a Consensus Forest Land Use Plan (Tata Guna Hutan Kesepakatan or TGHK) (Ministry of Forestry and Estate Crops 1998), the legal status of Tripa was "other uses [non-forest] area" (Area Penggunaan Lain or APL). However, in 1985, the first legal right for oil-palm establishment (Bayas et al 2011) although in Tripa only the communities adjacent were affected. All the communities further inland were effectively protected owing to the efficient buffering action of Tripa itself.…”

“…However, awareness of the protection that the forest had provided during the tsunami (Cochard et al 2008;Bayas et al 2011) plus new evidence of the area's biodiversity values, initiated actions by local and international NGOs to conserve the remaining forest. In addition, the carbon emissions from conversion of peat-swamp forest to oil palm became an issue of international concern but also, more specifically, in Aceh Province.…”

Resolution vs. image quality in pre-tsunami imagery used for tsunami impact models in Aceh, Indonesia

“…The models previously developed by Laso Bayas et al (2011) (henceforth called the "initial study"), described the role of coastal vegetation in impact mitigation of a tsunami event. Those models statistically related tsunami impacts on land to several characteristics.…”

“…), LCR T -s = cumulative land cover roughness in the transect up to the maximum flood distance, D-s = distance from the settlement to the shoreline (m), E F -s = maximum elevation at the settlement level (m a.s.l. ), LCR F -s = weighted average land cover roughness from the Bayas et al, 2011). settlement to the shoreline and LCR B5 -s = weighted average land cover roughness from the settlement up to 500 m behind.…”

“…In the initial study, 13 different land cover classes were determined with their specific characteristics (Laso Bayas et al, 2011, Table 1). By including planting density, the vegetation resistance coefficient (VR) at stand level was defined as:…”