Flume experiments on optimal arrangement of hybrid defense system comprising an embankment, moat, and emergent vegetation to mitigate inundating tsunami current

Abstract

A flume study using a surge-type flow produced by quickly lifting a gate was conducted to simulate a tsunami the height of which is larger than level 1 (prevention level) by a hybrid system comprising a forest, an embankment, and a moat while changing the layout of the components. The overflow volume, tsunami arrival time, fluid force, and moment by fluid force were measured, and the tsunami reduction effects in each model were compared.The hybrid defense system in the order of vegetation, a moat, and an embankment (Case V40ME) from the sea side showed the highest reduction in overflow volume, moment index, and tsunami delay effect. In terms of the fluid force index, Case V40ME performed best up to a certain tsunami height. For higher tsunamis, a hybrid system in the order of vegetation, an embankment, and a moat from the sea side (Case V40EM) performed better, although Case V40ME had a large advantage in total when there was a space landward of the existing forest. Suitable structures when there is space seaward and on both sides of a forest were also clarified for different wave heights through the mechanism of reflection, hydraulic jump, and vegetation resistance.