Effects of Coastal Forest on Tsunami Hazard Mitigation — A Preliminary Investigation

In order to utilize coastal forest as tsunami countermeasures, quantitative evaluation for hydrodynamic effect of coastal forest and the clarification of prevention functions against tsunamis were examined in this paper. Field survey and review and summary on the control forest conditions were carried out to evaluate the effect of coastal forest and to find the condition of coastal forest. A relation between the forest density and the diameter of trunk is obtained through the field survey of control forest and some previous researches, which is useful information for the evaluation of the tsunami reduction effects. Numerical simulation including the control forest effect is performed for evaluating the quantitative effect for tsunami reduction and damage. It is found that an increase of forest width can reduce not only inundation depth but also current and hydraulic force behind the coastal forest. By using these results, effects of reducing tsunamis can be quantitatively evaluated in a relation to the tsunami and forest conditions. Finally, review and summary is done to recognize the mitigation effect of tsunami disaster by the coastal forest in the past. Four main functions to reduce tsunami disaster are found; 1) to stop drifts or ships carried by a tsunami, 2) to reduce tsunami energy, 3) to form sand dune protecting tsunamis as well as high waves, and 4) to catch persons carried back by a tsunami to the sea. However, it should be reminded that the coastal forests would be destroyed by a huge tsunami and floating material would cause secondary damage.