Mechanisms of Coastal Dike Failure Induced by the Great East Japan Earthquake Tsunami

Abstract

Based on the results of field surveys, coastal dike failures caused by the Great East Japan Earthquake were classified into eight patterns. The results of hydraulic model experiments related to major failure patterns reinforced the proposed failure processes. In addition, the aggregated length of each failure pattern showed that failure from scouring at the landward toe is the dominant failure pattern.

Stability of Breakwater Armor Units against Tsunami Attacks

“…However, it is also clear that 508 some important structures might have to be designed so that they fail in a non-catastrophic way. These 509 were described by Kato (2012) as "tenacious structures", representing a structure that would slowly fail 510 over the course of the event while retaining some functionality (this idea is similar to what has been 511 described by other authors as "resilient" structures, which would indicate a structure that would suffer 512 limited damage even if its design load was greatly exceeded). The difference between "tenacious" and 513 normal structures is shown by the failure of the breakwaters at Kamaishi (which could be regarded as a 514 "tenacious structure", as it suffered great damage but somehow survived the event) and that at Ofunato 515 (which was completely destroyed).…”

“…A prolonged overflowing effect would generate a very intense current, and many 478 structures along the Tohoku coastline appeared to have failed due to erosion of the landside toe of the 479 structure. This has led some researchers (Kato et al, 2012, Sakakiyama, 2012, Hanzawa, 2012 to state 480 that the failure mode is directly related to this overflowing current. Nevertheless, the initial impact of 481 the wave also has an effect on the breakwater armour, and it would appear logical that once this initial 482 wave shock has been absorbed, the overflowing current would have no effect on the armour units.…”

Risk Assessment and Design of Prevention Structures for Enhanced Tsunami Disaster Resilience (RAPSODI)/Euro-Japan Collaboration

“…Overturning of a seawall as observed on the Ryoishi Coast, Iwate Prefecture, during the 2011 Tōhoku tsunami may occur if the overturning moment induced by the wave load on the seawall during tsunami run-up or drawdown is larger than the resistance moment due to the weight of the seawall [Kato et al, 2012].…”

“…Tsunami drawdown also leads to parapet failure, in particular because parapets are designed only for loads of incident waves, and not for drawdown [Kato et al, 2012].…”

“…During the 2011 Tōhoku tsunami, seaward flow during drawdown caused scouring at the seaward toe of the coastal dike and revetment. Scouring at the leeward toe affected the stability of the seaward armor, resulting in seaward armor floating away and breaching of the dike [Kato et al, 2012].…”