Effects of the coastal forests, sea embankment and sand dune on reducing washout region of houses at the tsunami caused by the Great East Japan Earthquake

Abstract

The tsunami caused by the Great East Japan Earthquake on 11 March 2011, broke most of the sea embankment and coastal forests, and caused dreadful damage to people and buildings in Tohoku and Kanto districts of Japan. This study hypothesized that the coastal forest had a tsunami mitigation effect even when the coastal vegetation was bent down, because most of the vegetation was not washed out, hence could acts as a dense roughness element. Therefore, the objective of this study is to evaluate the vegetation effect on reducing the washout region of houses under severe tree breaking phenomenon using numerical simulation and data from field investigation in April and May 2011. Numerical simulations estimated the effects of a 640m-coastal forest, sea embankment around 5.4m in height or sand dune (2m increase) on reducing the washout region of houses by around 100 m, 600m and 600m for 10m height tsunami at coast. It was observed/concluded that although the quantitative effect of coastal forest is smaller than sea embankment, the coastal forest and sand dune is not a negligible component of the mitigation measures when a large tsunami occurs and overflows the sea embankment.