Abstract
Coastal vegetation is widely recognized to reduce tsunami damage, but coastal forests in large areas of the Tohoku and Kanto districts of Japan were destroyed by the tsunami after the Great East Japan Earthquake on 11 March 2011. A field survey in the affected area elucidated the critical breaking conditions of Japanese coastal pine trees and identified the regions of large-tree debris production. Most broken trees remained in the vegetated region, but scoured regions behind the sea wall or on the down-slope regions of sand dunes produced large pieces of tree debris. In contrast, the pine trees in the inland zone were found to trap much debris. This trapping function should be utilized more in future designs. A numerical model that includes the breaking phenomena of trees estimates the capacity of a coastal forest to reduce the washout region of houses by around 150 m. This is not a negligible component of the mitigation measures when a large tsunami occurs and overflows the sea embankment.
