Abstract
Damage to coastal forests in southern Miyagi Prefecture caused by the mega tsunami associated with the 2011 earthquake off the Pacific Coast of Tohoku was surveyed nine months after the disaster. Specifically, the relationships among tsunami inundation depth, tree diameter, tree height, and whether the tree was standing or fallen were examined in coastal forests. The findings revealed that the relationship between the tsunami inundation depth and the tree diameter (tree age) was critical in determining whether trees remained standing or whether they fell over. A tilt-condition expression for trees was introduced using two moment-equations of the theoretical moment of the drag force due to tsunami flow and the moment of the pulling-down test for trees based on forestry methods. In the tilt-condition expression for trees, values of CD=1 and Fr=0.53 for the drag coefficient and Froude number, respectively, agree well with the critical condition identified for whether trees remain standing or fall over in the field.
