Developing Fragility Functions Based on Aquaculture Raft and Eelgrass due to Tsunami Damage: A Case Study of Mangokuura Lake

抄録

 The 2011 Great East Japan tsunami caused large scale devastated damage in both offshore and inland areas. There are many previous researches that were focusing on the relationship between tsunami characteristics and damage or loss such as human loss, building damage and fishing boat damage. Similarly, great damage and loss of aquaculture and marine ecosystem was also caused by the 2011 tsunami. However, relationship between the tsunami and their damage has not been studied. To understand such relationship, this study has mainly three components namely 1) Reproduction of the 2011 tsunami, 2) Damage investigation and 3) Developing fragility functions. The source models of the 2011 tsunami were verified and adjusted so that the tsunami characteristics were able to accurately reproduce in the target area. Damage ratios of the aquaculture raft and eelgrass were investigated using satellite images taken before and after the 2011 tsunami through visual inspection and binarization. The tsunami fragility functions were then developed based on the relationship between the simulated maximum flow velocity and the calculated damage ratio. It was found that the damage ratio of higher than 0.5 is equal to the maximum flow velocity of 0.8 m/s (aquaculture raft) and 1.0 m/s (eelgrass). Whereas, the damage ratio of higher than 0.9 is equal to the maximum flow velocity of 1.3 m/s (aquaculture raft) and 3.0 m/s (eelgrass). It was found in this study that the tsunami amplitude had no relationship with the damage ratio. In addition, the proposed fragility functions for the aquaculture raft are getting along well with the previously proposed damage criteria. The proposed fragility curves will be useful for marine product and environmental damage assessment but also suggestion for the zoning of aquaculture raft to mitigate impact from tsunamis in the future.