Seawall Performance Along Southern Coast of East Japan Impacted by the 2011 Tohoku Tsunami; A Note for the Reconstruction Process - Post-Tsunami Hazard: Reconstruction and Restoration - page 184

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

Shinji Sato

Abstract 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. Signifi cant fl ooding
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 fl ooding. 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 overfl ow. 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.

Keywords Tsunami inundation ￿ Seawall performance ￿ Tsunami disaster

mitigation

13.1 Introduction

Catastrophic tsunami disaster was developed by the 2011 Tohoku Tsunami, which
was generated by a M w = 9.0 earthquake with 300 km by 500 km large source zone.
Characteristics of the mega-tsunami and resultant coastal damage were investigated
by many survey teams under the coordination of the Tohoku Tsunami Joint Survey