Coastal vegetation is widely recognized to reduce tsunami damage to people and buildings, and it has been studied recently because it requires relatively little capital investment compared with artificial measures, provides human-friendly beach fronts, and enhances inter-relationships with other ecological systems. However, the tsunami caused by the Great East Japan Earthquake at 14:46 JST on March 11, 2011, with a magnitude of 9.0 and epicenter 129 km east of Sendai, broke most of the sea wall (tsunami gates, large embankments) and caused catastrophic damage to coastal forests in the Tohoku and Kanto districts of Japan. A field survey was conducted to elucidate the critical breaking condition of Japanese coastal pine trees. Tree-trunk breakage was observed when the sea embankment was washed out or when there was no sea embankment and the tree was under strong inertia force or impact force by debris. Even though the trunk bending and breaking phenomena are different, statistical analysis showed that the critical diameters for trunk bending and trunk breaking were not very different. The overturning phenomenon is a little more complex than trunk breaking because the resistive force is a function of the substrate and root anchorage. An equation to determine the critical diameters for trunk bending, trunk breaking, and overturning was derived as a function of tsunami water depth, soil-root strength, and the hydrodynamic parameter (H <Subscript> D </Subscript>) formulated by Froude number, drag coefficient, and the ratio of impact force to drag force considering the physical mechanisms to resist the tsunami. Trunk bending and breaking were closely related to tsunami water depth and the hydrodynamic parameter (H <Subscript> D </Subscript>), but tree overturning was found to be more site specific, and the root-soil strength greatly affected the critical value. The proposed critical diameter equation and its coefficient are useful for the design of an inland forest of pine trees that can trap large trees, cars, debris, etc., to its breaking limit. The trapping function should be utilized more in the future designs of inland forests, if possible, on embankments. Copyright Springer Science+Business Media B.V. 2013
