Inland forest as a trapping function against tsunami-borne large broken or washed out trees

Abstract

After the 2011 Great East Japan tsunami, field observations have shown the possibility of an inland forest trapping tsunami-borne large driftwood (TLD). This research was conducted to understand the driftwood trapping capability by a forest patch downstream of a shoreline, considering a model scale driftwood with attention to the crown and roots of a tree. The results demonstrated that there are four types that driftwood gets trapped by an inland forest, with each trapping type exhibiting a different force on the trapping tree. A trapping type where the driftwood got trapped at the front face of an inland forest model (IFM) in a cross-stream position by one or more trunk(s) (Type FC) exhibited high force on the trunk, because of the high cross-sectional area of the driftwood against the flow. However, when the upstream trees’ (potential to being TLD) trunk length was small, the occurrence of this trapping type decreased. The results showed that the trunk length to tree height ratio of 0.18 had a trapping occurrence of 33%, while trunk length to tree height ratio of 0.06 had a trapping occurrence of 16%. The driftwood condition, in this case, was driftwood with roots. This study further demonstrated the behaviour of driftwood trapping by an inland forest and proposed a further improvement to tsunami mitigation, by elucidating its ability to trap tsunami-generated large driftwood as a function of trunk diameter at breast height.