Tsunami Impact on a Detached Breakwater: Insights from Two Numerical Models

Abstract

Tsunami waves are often very energetic and therefore pose a significant threat to coastal structures. However, most coastal defenses such as detached breakwaters are designed to protect against storm waves, with the consequence that tsunamis often lead to a catastrophic failure of these structures. This numerical study was inspired by the wave scenario, which occurred at Soma Port in Japan, during the 2011 Tohoku tsunami where a combination of tsunami-type waves and shorter period undular bores caused severe damage to a detached caisson breakwater. The objective herein is to analyze the role that undular bores play on breakwater stability in the case of a tsunami propagating over a gentle slope into shallow water. Two complementary wave models, one of Boussinesq type and the other of Reynolds-averaged Navier-Stokes (RANS) volume of fluid (VOF) type, are used to compute the propagation of a representative tsunami composed of multiple waves components and their impact on a detached breakwater. The presence of an undular bore is controlled by the balance between wave nonlinearity and dispersion. As the draw-down from the first wave reduces the water depth around the breakwater and leaves the structure almost completely exposed, only the second wave transforms into an undular bore. This causes wave breaking far offshore from the breakwater where a large amount of energy is dissipated ultimately resulting in much less-destructive wave impact at the structure. The undular bore influences the wave loading on the breakwater, though the bulk of the wave loads and the excessive bearing pressures were computed for the long-lasting overtopping process of the first wave. These findings facilitate the damage assessment of detached breakwaters as they relate some specific tsunami features to structural stability and failure mechanisms. The results also provide information regarding the applicability and usefulness of different numerical modeling approach for the analysis of the stability of detached breakwaters under tsunamis. For this particular case, even depth-integrated numerical models provide conclusive solutions. To the best of the authors’ knowledge, this is the first numerical study addressing breakwater stability with respect to the impact of undular bores under tsunami conditions.