Author(s): Norio Tanaka; Yoshiya Igarashi
Linked Author(s): Norio Tanaka
Keywords: Bioshield; Multiple defense; Oscillation; Energy loss
Abstract: The 2011 Great East Japan tsunami has revealed the limit of hard solution when it is composed of only sea embankment. So the tsunami mitigation for level-2 tsunami (recurrent cycle is around several hundreds to one thousand years) is discussed by multiple defense system; not only by using a single sea embankment but by a combination of several hard and soft solutions. This study investigated energy reduction of tsunami overtopping flow from two embankment system in comparison with one embankment system. Flow type in between the two embankments are classified into four. Although the reduction of the two embankment system is mostly larger than that of one, the reduction becomes smaller when hydraulic jump just begins to create for small inland embankment or high overtopping depth. In that case, water table in between the two embankments oscillates and flow type is periodically changed. Coastal forest on the inland embankment has a role to change the transition of flow type by adding the drag force to the flow. The effectiveness of two embankment system has been discussed with changing the distance between seaward and inland embankment.