Effective management of Japanese black pine (Pinus thunbergii Parlat.) coastal forests considering tsunami mitigation.

Coastal forests have been recognized to considerably reduce the energy of tsunamis and act as a measure against them. However, broken trees due to tsunamis produce woody debris and cause secondary damage to people and buildings. Thus, coastal forests have both positive and negative effects on tsunamis and people. To elucidate the relationships between these effects and forest management, three experimental forest management types were considered in this study: the sparse type (ST: 0.6 ? Ry ≦ 0.7), middle type (MT: 0.8 ? Ry ≦ 0.9), and dense type (DT: unthinned), where Ry is the relative yield index. In addition, numerical simulations were performed for the forest types considered. The effect of fluid force reduction depended on the growth stage of the forest stands and the occurrence of forest damage caused by the tsunami. For a tsunami height of 5 m, DT and ST exhibited the highest and lowest effectiveness in reducing the fluid force, respectively. However, when the tsunami height exceeded 15 m, the three management types exhibited almost the same effectiveness in reducing the fluid force. Moreover, tree damage increased with an increase in the tsunami height. All trees were damaged under all management types for a tsunami height greater than or equal to 15 m. Modes of tree damage included tree overturning and trunk breakage. The modes of tree damage for MT and ST were overturning only, and DT exhibited a combination of overturning and trunk breakage. Therefore, ST and MT are more suitable than DT for preventing trunk breakage. Thus, forest management against tsunamis should take estimated tsunami height into account for each region.