This study investigates the effectiveness of coastal forests in mitigating the tsunami damage
using the field data of forests. A filed survey was conducted on Casuarina equisetifolia forests that
established after the Indian Ocean tsunami on 26 December 2004, at eastern coast of Sri Lanka. Tree
and forest characteristics were measured in order to analyze the effectiveness of the forests in mitigating
the tsunami damage. In addition, a numerical simulation was carried out to find out the optimum
conditions of the C. equisetifolia forests. Results revealed that the spacing between the trees had a
positive correlation with trunk diameter where larger diameter trees required greater spacing. Moreover,
drag coefficient was varied along the tree height and it was affected considerably by the branches and
the leaves. A numerical simulation was performed for evaluating the quantitative effect for tsunami
reduction and damage. It found that the tsunami force was reduced largely and the tsunami velocity and
depth were reduced slightly subsequent to the forest. The most appropriate tree density was found as 0.3
trees/m2
