The Pine-plantation on seaside sand dunes sustained serious damage caused by the 3.11 tsunami. The old-growth pine trees were fallen in-line in the area. This phenomenon could be an influence on the depth of water caused by a slight unevenness on the ground surface. An aerial photo of alder tree distribution was used to estimate the unevenness. Consequently, it was confirmed that an infinitesimal concavity in the Pine-plantation on seaside sand dunes let tsunami invade the area and the wave induced the in-line fallen forest.