13. Seawall Performance Along Southern Coast of East Japan Impacted by the 2011 Tohoku Tsunami; A Note for the Reconstruction Process

This article describes the performance of seawalls in the 2011 Tohoku Tsunami on the basis of tsunami surveys along the southern coastline of East Japan. In Chiba and Ibaraki Prefectures where incident tsunami was slightly higher than the height of seawalls but lower than the backshore dune height, the tsunami was blocked by the presence of the dune as well as the seawalls. Significant flooding damage was developed only in the harbour area as well as in the area around the river mouth. The presence of the water gate at the river mouth appeared to be effective to minimize the flooding. In the south of the Fukushima Prefecture where incident tsunami was 1–3 m higher than the height of seawalls, a clear contrast was observed in the damage of seawalls as well as in the inland damage behind collapsed and survived seawalls, which provided valuable hints for tenacious seawall structure that enhances durability against tsunami overflow. These observations helped to establish a new strategy for tsunami disaster mitigation and thus to promote the reconstruction process on the basis of proper understanding of the limitation and effectiveness of seawalls.