Chapter 16 - Stability and Disaster Mitigation Effect of Wave-Dissipating Concrete Blocks of Detached Breakwaters Against Tsunami Waves

Abstract

In Japan, detached breakwaters assembled with wave dissipating concrete blocks such as Tetrapods have been constructed along wide stretches of the coastline. However, neither their effectiveness to protect against tsunamis nor their stability against such waves has been reviewed in detail. Only a few studies on wave run-up reduction were carried out after the Nihonkai Chubu Earthquake Tsunami in 1983. To further understand how to mitigate tsunami disasters in coastal areas the effectiveness of detached breakwaters and the stability of their wave-dissipating concrete blocks should be properly evaluated. The effectiveness and stability of a detached breakwater on reducing a solitary tsunami wave run-up and wave pressure on a seawall located behind them has received increasing attention following the 2011 Tohoku tsunami. Wave run-up and wave pressure onto a seawall can be reduced when a detached breakwater is placed in front of it. The present chapter also provides details on hydraulic model tests that have been carried out to study the stability of the wave-dissipating blocks in a detached breakwater against solitary tsunami waves. Tetrapods on the rear side of a breakwater tend to be damaged by solitary waves, with the collapse mechanism being similar to that of a land slide. Damage by tsunami is greater than that from ordinary wind waves, and the present chapter also proposes countermeasures for improving stability, based on the damage process of a detached breakwater. Much research is still needed on tsunami disaster mitigation and the stability of detached breakwaters, but the present chapter represents a novel first step on such topics.