72 Comparison of Failure Mechanisms of Coastal Structures due to the 2004 Indian Ocean and 2011 Tohoku Tsunami Events G. Lim 1 , B. Premaratne 1 , R. Jayaratne 1* , M. Marriott 1 and T. Shibayama 2 1 University of East London, London, UK 2 Waseda University, Tokyo, Japan *E-Mail: r.jayaratne@uel.ac.uk, TP: +44 (0)208223 2536 Abstract: By analysing and comparing the results of post-disaster field studies and literature regarding the mechanisms by which coastal structures failed due to the 2004 Indian Ocean Tsunami and the 2011 Tohoku Tsunami events (the focus being on defence structures where applicable), trends were identified and examined. This paper highlights the most commonly occurring / major failure mechanisms identified in the various locations affected by the two tsunami events. The failure modes found in over twenty locations throughout the Fukushima, Iwate and Miyagi Prefectures of Japan were categorised into seven failure modes: a) leeward toe scour, b) crown armour failure, c) leeward armour failure, d) parapet wall failure, e) overturning, f) seaward toe scour, and g) sliding. Leeward toe scour was found to be the major failure mechanism in seawalls and dikes, and sliding was found to be the major failure mechanism in concrete breakwaters. The failure modes found throughout regions affected by the Indian Ocean Tsunami were categorised into five failure mechanisms: a) scouring of foundations, b) beam/column failure, c) joint failure, d) wall failure, and e) total disintegration. The ‘total disintegration’ caused by seismic forces, debris collision and hydrodynamic forces was the major failure mode throughout the studied regions. Some of the major tsunami induced forces found to have been among the causal factors of structural failure included hydrostatic and hydrodynamic forces. Flow velocities as high as 13.4m/s were found in areas of Japan, and flow velocities of up to 10.4m/s were found in regions affected by the 2004 Indian Ocean Tsunami. Potential strengthening measures were suggested for structures such as seawalls and coastal dikes, which were most vulnerable to scouring at the toe. By producing armoured components to protect the toe of the structures, they would become less susceptible to toe scour failure. Keywords: Coastal structures, Failure mechanisms, 2004 Indian Ocean tsunami, 2011 Tohoku tsunami, Toe scour, Total disintegration. 1. Introduction Recent extreme events such as the 2004 Indian Ocean Tsunami and the 2011 Great East Japan Earthquake and Tsunami (2011 Tohoku Tsunami) have been the cause of excessive amounts of damage not only to coastal structures, but also to the lives and economies of many. In order to be able to prevent such large-scale disasters in the future, it is imperative that research on the probable failure modes of the coastal structures is carried out. It is also very important that the relevant findings are used to design, strengthen and refine existing and future coastal structures to resist against such events. The purpose of this study is to compare the various failure mechanisms present in two tsunami events and in turn, to identify correlations between the structures and failure modes present in different locations. This paper also focuses on identifying some of the major destructive tsunami-induced forces that are responsible for the various failure mechanisms observed. By examining the identified trends, this study aims to investigate the vulnerabilities of the coastal structures and suggest viable solutions to strengthen them. 2. Identified coastal structures, failure mechanisms and tsunami-induced forces The various coastal structures and failure mechanisms identified in this section, for both tsunami events, were obtained by examining and comparing field notes, reports, case studies, photographs and various literature documenting post-tsunami field surveys. Field studies that were analysed and compared included the works of Jayaratne et al. [1, 5], Kato et al. [2], Chock et al. [3], Saatcioglu et al. [4] and Shibayama et al. [6], which collectively covered field surveys that were carried out in over twenty locations throughout the Fukushima, Iwate and Miyagi Prefectures of Japan