Field experiments conducted on coastal forests after the 2011 Tohoku earthquake tsunami revealed that the tree size influenced the tree damage rate and its status (uprooting or stem breakage). In this study, to examine these results from the physical standpoint, the critical flow velocities for uprooting and stem breakage were calculated using a simple dynamics model and using field data from Misawa, Aomori Prefecture, Japan. The calculations showed that the damaged trees in the field experiments had relatively low critical flow velocities. In particular, trees with crown base heights of ≤ 6 m suggested relatively low critical flow velocities. According to the predicted status obtained using calculations, most of the trees were uprooted. This result was consistent with those of the field experiments. The influence of slenderness ratio on the critical flow velocity was not apparent. This approach can facilitate the comparisons of tree damage mechanisms in respective forests.