Influence of Coastal Vegetation on the 2004 Tsunami Wave Impact in West Aceh

Proceedings of the National Academy of SciencesPNAS, 15 November 2011 vol. 108 no. 46 18612-18617

Juan Carlos Laso Bayas, Carsten Marohn, Gerd Dercon, Sonya Dewi, Hans Peter Piepho, Laxman Joshi, Meine van Noordwijk, and Georg Cadisch

“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.

Schematic transect showing the variables used in the models.

Schematic transect showing the variables used in the models. MD = maximum flood distance (m), CASU = casualties (%), STD = structural damage (%), IWH = initial water height (m), D = distance from the shore line to the settlement (m) ET = maximum elevation over the whole transect (m a.s.l.), EF = maximum elevation at the settlement level (m a.s.l.), LCRT = weighted average land cover roughness in the transect (up to the maximum flood distance), LCRF = weighted average land cover roughness in front of the settlement and LCRB5 = weighted average land cover roughness from the settlement up to 500 m behind.

“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.”