Koichiro Ohira

Classification of Tsunami and Evacuation Areas

On March 11, 2011, a large earthquake that occurred offshore the north-east coast of Japan generated a large tsunami which devastated extensive areas of the Tohoku coastline. Despite Japan being considered a country well prepared for these types of disasters, large casualties were recorded, with numerous discussions amongst the Japanese coastal engineering community ensuing. As a result, two different levels of tsunamis have been proposed and now recognized in Japan, depending on the frequency of such extreme events. The idea that hard measures can protect the lives of inhabitants of coastal areas has been abandoned, and these measures are only considered to be effective in protecting properties against the more frequent but lower magnitude events. Soft measures should always be used to protect against the loss of lives, and to this respect, the authors of the paper propose the introduction of a Classification of Evacuation Areas, to show which of these should be prioritized by residents as they seek to evacuate.

Stability of Breakwater Armor Units against Tsunami Attacks

The design of breakwater armour units against tsunami attacks has received little attention in the past because of the comparative low frequency of these events and the rarity of structures designed specifically to withstand them. However, field surveys of recent events, such as the 2011 Great Eastern Japan Earthquake Tsunami and the 2004 Indian Ocean Tsunami, have shown flaws in the design of protection structures. During these extreme events, many breakwaters suffered partial or catastrophic damage. Although it is to be expected that most normal structures fail due to such high order events, practicing engineers need to possess tools to design certain important breakwaters that should not fail even during level 2 events. Research into the design of critical structures that only partially fail (i.e., “resilient” or “tenacious” structures) during a very extreme level 2 tsunami event should be prioritized in the future, and in this sense the present paper proposes a formula that allows the estimation of armour unit damage depending on the tsunami wave height.

Implicaciones del tsunami de Tohoku del año 2011 para la gestión de desastres naturales en Japón

El 11 de Marzo del 2011 un terremoto de magnitud 9.0 en la escala Richter genero un devastador tsunami que arraso una extensa area de la costa noreste de Japon. El Gran Terremoto y Tsunami del Este ha sido descrito como un evento que solo sucede una vez cada varios miles de anos aproximadamente, y como consecuencia de este es necesario que la filosofia sobre Gestion de Desastres Naturales en Japon sea revisada para tener en cuenta varias lecciones aprendidas a raiz de este desastre. En este articulo los auotres presentan los resultados de investigaciones de terreno en el area de Tohoku. Se detalla el dano en varias localidades y se analizan los mecanismos de dano sobre diversos tipos de estructuras. Los autores tambien presentan una serie de cambios que estan ocurriendo en la filosofia de gestion de riesgos en Japon. Esencialmente, la idea que solo los metodos de infraestructura fisica son siempre capaces de proteger las vidas de la poblacion local ha sido descartada, y aunque se mantiene la idea de que estos metodos pueden resultar utiles para la proteccion de las propiedades, en ultima instancia los metodos de planificacion deberian formar la pieza mas importante del sistema de medidas para la proteccion de las vidas de las personas en estas areas.