Author(s): Norio Tanaka; Yasushi Sasaki
Linked Author(s): Norio Tanaka
Keywords: Pandanus odoratissimus; Casuarina equisetifolia; Tsunami protection; Coastal vegetation; Forest structures
Abstract: This study explored the effects and limitations of coastal vegetation for tsunami protection based on field observations carried out in Sri Lanka and Thailand after the Indian Ocean tsunami on 26 December 2004 and in Indonesia after the Java tsunami on 17 July 2006. The impact of vegetation structure on drag forces was analyzed using the observed characteristics of the reference tree species, Cocos nucifera, Pandanus odoratissimus, and Casuarina equisetifolia. The drag coefficient, including the vertical stand structures of trees, Cd-all, and the vegetation thickness in a unit area, dNu (d: reference diameter of trees, Nu: number of trees per unit area), varied greatly with the species. Based on the field survey and data analysis, dense C. equisetifolia and P. odoratissimus grown in beach sand were found to be especially effective in providing protection from tsunami damage due to their density and complex aerial root structure, respectively. The breaking moment of trees was investigated as a function of tree diameter. The breaking moment equation of P. odoratissimus represents well the damaged situation of the trees in the two tsunamis. The threshold tsunami water depth for bending or breaking of P. odoratissimus was observed at about 80% of the tree height. The threshold height was analyzed by comparing the breaking moment equation with the calculated drag moment on a tree, considering the stand structures. The calculated height also represents well the observed threshold water depth for breaking. Considering the limitations of P. odoratissimus for tsunami water depth and the other roles of coastal vegetation, a forest with two layers in the vertical direction with P. odoratissimus and dense C. equisetifolia is useful for increasing drag and trapping floating debris.