GIS Evaluation of the Impacts on the Built and the 'Natural' Environment of the 11 March 2011 Tsunami in Rikuzentakata, Iwate Prefecture, Japan

19 pagesThe 11 March 2011 a Mw 9.0 earthquake triggered a tsunami off the Tohoku coast (Japan). To the north of the infamous Sendai plain area, a series of small bays stretches from the North of Miyagi prefecture to mid-Iwate prefecture. At the border between both prefectures, the municipality of Rikuzentakata lies close to the river-mouth of the Kesen River. Both the built and the 'natural' environment of the municipality have been wiped off the map. In order to create more resilient communities, it is important to reduce the costs of reconstruction and restructuration by integrating lessons learned from the present tsunami. Therefore, this contribution provides a GIS evaluation of the different impacts in order to improve further planning. We have used remote-sensing based GIS - ESRI ArcGIS suite - using spatial data from July 2010 and March 2011. The GIS analysis has been completed using data collected in Japan during the period 2006 - 2010. The results show that the shoreline has retreated up to 501 m with the disappearance of the lagoon barrier. Scouring has also occurred along the banks and the point-bars of the Kesen River. The point-bars located within 1 Km to the coast were totally eroded. The scouring occurred on low-vegetation land but it also occurred in forested areas, with the total destruction of forests in some areas. The ~0.26 km2 well-known coastal forest of the Takata Matsubara park has totally disappeared. We have also observed forested lands that have been flattened down along the river. The forested areas at higher elevations saw very little damage. The built environment in Japan comprises different types of concrete buildings, metallic structures and timber-structures. The latter two building types, which rely on their low weight to resist earthquakes, have been totally destroyed within 3.5 km from the coast. Indeed the low weight increases the floatability and the light walls have poor resistance to the load of the tsunami waves. Therefore buildings that resisted the tsunami within 3.5 km of the shoreline are all concrete buildings. Nevertheless some concrete buildings also disappeared, because of the scouring around their base. These elements call for the reconstruction of a more resilient community, in order to decrease the cost of reconstruction in the event of another tsunami strike