Tsunami Mitigation Effect and Tree Breaking Situation of Dahurian Larch Coastal Forest at Six Growth Stages under Thinning Management of Trees

Coastal forest can dissipate a part of tsunami energy but has a risk of producing driftwood. There are many studies about the effect 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 planning by the Bureau in Hokkaido Prefecture in Japan. The effectiveness of the hybrid defense system could be changed whether the trees are broken or not. In addition, it is important to know the management method of the coastal forest for effectively reducing the tsunami energy. Thinning management of coastal forest is regularly held according to the growth stage 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.

This study selects Dahurian larch (Larix gmelinii), one of the representative tree species used for wind protection forest in Hokkaido. The forest width was set as 10m, 40m, 160m, 320m, and 640m, and the tsunami height (TH) at shoreline was set as 5m, 10m, and 20m in numerical simulations.

After the thinning management of coastal forest, the forest density decreases, but the diameter of trees becomes larger as they grow. Therefore, the tree breaking rate is higher as the initial stage of coastal forest. In the final stage of coastal forest, tree breaking did not occur even when the TH was 10m. On the other hand, the tsunami mitigation effect estimated behind coastal forest in the initial stage is larger than that in final stage even including the effect of tree breaking, as the breaking pattern is not ‘tree trunk breakage (tree crown part is usually washed out)’, 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 in the initial stage is small. In addition, by using the parameter of forest thickness, the reduction effect of the tsunami run-up heights can be expressed by the same curves even when the forest stage is different.