Effectiveness of a compound defense system of sea embankment and coastal forest against a tsunami

The Great East Japan tsunami revealed the limits of using only a sea embankment for defense. Therefore, a compound defense system combining a coastal forest and sea embankment for a tsunami overflowing the embankment is proposed to mitigate damage. This study was conducted to clarify the changes in overflow volume from an embankment due to the location and thickness of a coastal forest. The effect of tree overturning was also investigated in relation to changes in wave height. A flume with a gate quickly lifted by air pressure to create an unsteady tsunami-like bore was used to investigate the changes in overflow volume and the reflection characteristics of various compound-defense models. The results show that a landward forest has the advantage of decreasing the fluid force behind the forest, and seaward forest can reduce overflow volume landward. Even when the sea-side forest is thin, the change in flow pattern in front of the embankment increases its total reflection and decreases the overflow volume by approximately 10%, although the forest itself does not increase the reflection. A thin seaward and overturned forest also decreases the overflow volume when the approaching tsunami height was less than the embankment height.