Chapter 17 - Destruction of Coastal Structures after the 2011 Great East Japan Earthquake and Tsunami

Coastal dikes, breakwaters, and seawalls are generally used to control floods, debris flows, erosion, and even massive catastrophic events such as tsunamis. However, these structures will only perform well if they are firmly constructed in terms of strength and stability of slopes and toes and can resist the dynamic forces imposed by the incident wave energy. During a Level 2 tsunami such as the 2011 Tohoku Earthquake Tsunami many concrete coastal defenses in the NE of the country could not withstand the forces imposed by the tsunami overflowing water and as a result failed either partially or completely. The authors carried out detailed field surveys in Miyagi and Fukushima prefectures during the period from 2011 to 2013 in order to explore the failure modes and mechanisms of coastal structures due to the 2011 Tohoku Earthquake Tsunami. Eight locations which were severely affected by this event were considered in the analysis. Subsequently, six major failure modes could be identified from the collected field data, namely: (i) leeward toe scour failure, (ii) crown armor failure, (iii) leeward slope armor failure, (iv) seaward toe scour and armor failure, (v) overturning failure, and (vi) parapet wall failure. At most surveyed locations the critical failure mode was identified as the scour initiated at the leeward toe. The present chapter discusses all six failure mechanisms, highlighting how the lessons learnt from them can help to improve the design of coastal structures under tsunami attack.