Analysis of Tsunami behavior and the effect of coastal forest in reducing Tsunami force around coastal dikes

Many coastal dikes in Iwate, Miyagi and Fukushima prefectures in Japan were damaged due to the 2011 Great Eastern Japan Earthquake Tsunami. The primary failure mode of these dikes was scour failure at the leeward toe by the overflowing tsunami. However, the forces exerted by the tsunami wave against the dikes were reduced to the presence of some obstacles (such as trees) located behind the dikes, increasing the tsunami inundation height immediately behind the structure and actually helping to reduce the force on the dike itself. In order to analyze how tsunamis affected coastal dikes, the authors carried out post-tsunami field surveys and laboratory tests using dikes and different coastal forest models. The critical failure mechanism of dikes was analyzed and co-related to tsunami overflowing pattern. In addition, the experimental results show how coastal forest can help to reduce the damage caused by the tsunami to coastal dikes by increasing water depth and changing the tsunami motion pattern. Thus, the placement of coastal forests behind dikes can help to increase their resilience against tsunami attack, and should be considered as part of a disaster management strategy for certain tsunami prone areas.