Author(s): Yoshiya Igarashi, Norio Tanaka
Keywords: Overturn; Tree trunk breakage; Houses damage probability; Fluid force index; Run-up height of tsunami;
Abstract: Coastal forest can dissipate a part of tsunami energy but has a risk of producing driftwood. There are many studies about the effect and limit of coastal forest by numerical simulation and/or flume experiment when the trees are not broken. To mitigate tsunami energy, a hybrid defense system consisting of a coastal forest and any artificial structure like an embankment are planned by the Hokkaido Bureau in Japan. The effectiveness of the hybrid defense system could be changed whether the trees are broken or not. The thinning management is regularly held according to the growth situation for making woods quality good. Therefore, the objective of this study is to investigate how the thinning management affects the mitigation effect and tree breaking situation against six growth stages of coastal forest by using two-dimensional depth-averaged tsunami flow model. The results show, the breaking rate of the young coastal forest is higher than that of a well-grown coastal forest due to the growth of tree diameter and tree crown height. On the other hand, the tsunami mitigation effect behind coastal forest of young forest is larger than that of well-grown forest even including the effect of tree breaking, as the breaking pattern is not ‘tree trunk breakage (tree crown part is broken off)’, but ‘overturn (tree crown part is submerged under water)’, and the resistance can be kept after breaking even though it becomes smaller. This is because that the tree crown height of young forest is small, and the density is high.