Simulated effectiveness of coastal forests on reduction in loss of lives from a tsunami

Although a substantial body of research has investigated the impacts of coastal vegetation on tsunami propagation over land, the effectiveness of coastal forests in reducing the number of expected casualties behind them has rarely been evaluated. Thus, this study investigates the effectiveness of coastal forests in reducing tsunami-related casualties using an agent-based evacuation simulation model. A tsunami propagation and inundation simulation model that considers vegetation-induced resistance to the flow was first developed and used to simulate the inundation observed at Shobutahama Beach in Miyagi Prefecture, Japan, during the 2011 Tohoku Tsunami. The results confirmed that the model is able to simulate the effects of coastal forests with relatively good accuracy. Then, this agent-based tsunami evacuation model was used to simulate evacuation that considers the implementation of coastal forests and other countermeasures (e.g., coastal dykes and evacuation signs) to reduce casualty rates. When coastal forests alone were considered the simulated casualty rates were reduced by a maximum of around 54% (compared with the case when no countermeasures were contemplated). However, a more significant reduction in casualty rates (a maximum of around 97%) was achieved when a coastal dyke capable of withstanding tsunami overflow was constructed. Furthermore, considering a combination of coastal forests and the construction of a dyke was shown to be more effective than either countermeasure alone, indicating the importance of implementing combined strategies to minimize the number of casualties due to a major tsunami event.