Challenges in reconstruction and strategies for prevention and mitigation of coastal disasters five years after the GEJE

Abstract

After the 2011 Great East Japan Earthquake (GEJE) and tsunami, the Central Disaster Management Council of Japan newly introduced the concept of two different levels of tsunamis for disaster prevention and mitigation strategies. Under this concept, in general, the height of the coastal dike is designed to prevent the inundation against the level-one tsunami. Against the higher level tsunami, on the other hand, the dike may allow overflow but should be designed to avoid fatal collapse and thus to reduce the damage level of the area behind the dike. The quantitative evaluation of such damage-reduction effects of dikes is, therefore, one of the key tasks because it affects land-use planning and other disaster-prevention or mitigation measures in the inundated area. This chapter first discusses to what extent coastal dikes can reduce the extent of various coastal hazards against a tsunami flowing over the dikes. Based on a numerical study, the overall performance of coastal dikes is investigated by comparing the following three parameters: (i) volume of the overflow, (ii) inundation area and (iii) “damaged area”, i.e., the area where the expected number of collapsed houses exceeds that of surviving houses. It was found through the analysis that coastal dikes show better performance in the reduction of the damaged area rather than in the other parameters and that these effects are practically diminished as the relative height of the dike decreases to below half of the tsunami height in front of the dike. The stability of the dike against an overflowing tsunami is also an important factor to ensure the damage-reduction effect of the dike; thus, a slope-type dike becomes one of the preferred designs for the dike. Such slopes of the dike, however, may reduce the dissipation effect of an overflowing tsunami and may thus sacrifice the damage-reduction effect in the inundated area. The influence of such a trade-off relationship is examined through laboratory experiments. The experimental results showed that the sloping-type dike clearly increases the overflow of a tsunami and has a significant impact on the damage in the inundated area. Finally, this manuscript outlines some other problems for the quantitative estimations of inundation characteristics, hazards and risks under various uncertain factors and discusses possible multi-disciplinary approaches to overcome these difficulties.