Graduate School of Science and Engineering, Saitama University, Japan; International Institute for Resilient Society, Saitama University, Japan.
Forestry Research Institute, Hokkaido Research Organization, Hokkaido, Japan.
J Environ Manage. 2018 Oct 1;223:925-935. doi: 10.1016/j.jenvman.2018.07.006. Epub 2018 Jul 10.
The effectiveness of coastal forests to mitigate a tsunami has received increased attention. However, many trees are broken, overturned, or washed out in large tsunami events like the 2011 Great East Japan tsunami (GEJT). For quantitatively estimating the advantages and disadvantages of a coastal forest, it is important to reproduce the forest breakage, especially paying attention to the production of driftwood and the trapping ability of standing trees. The objective of this study was to analyze the tree-breaking mode in detail, considering the stand structure of trees, to demonstrate an energy reduction even when trees are broken, and to utilize the information about the breaking pattern to design and manage a coastal forest properly. In this regard, one location, Misawa, the forest of which was affected by the GEJT, was selected for model validation, and coastal forests in two locations, Shiranuka Town and Taiki Town, in Hokkaido, Japan, were selected because a large tsunami is expected to occur there in the future. A numerical simulation of two tsunami magnitudes at the two Hokkaido sites demonstrated that a tree whose crown is far from the ground tends to be broken at the tree trunk. Dahurian larch (Larix gmelinii) and Daimyo oak (Quercus dentata) tend to be overturned and broken at the tree trunk, respectively. However, the tendency changed with the tsunami magnitude. In addition, even when trees with a dense crown were overturned, they contributed to tsunami resistance to some extent. The fluid force was reduced not only with the forest thickness but also with the tree species and the destruction mode. To maintain the fluid-force reduction and to reduce secondary damage by driftwood, mixed planting is recommended in which tree stand structures are different and large diameter trees at the landward side of forest are planted to trap the driftwood produced from the sea side.
沿海森林在减轻海啸方面的有效性已引起越来越多的关注。然而，在像 2011 年东日本大地震海啸（GEJT）这样的大型海啸事件中，许多树木会被折断、倾倒或冲走。为了定量评估沿海森林的利弊，重要的是要重现森林的破坏情况，特别是要注意浮木的产生和立木的捕获能力。本研究的目的是详细分析树木折断的模式，考虑到树木的林分结构，即使树木被折断也能降低能量，并利用有关断裂模式的信息来合理设计和管理沿海森林。在这方面，选择了受 GEJT 影响的三泽市的一个地点进行模型验证，并选择了北海道的两个地点，Shiranuka 镇和 Taiki 镇，因为预计未来那里会发生大型海啸。在这两个北海道地点的两种海啸规模的数值模拟表明，树冠远离地面的树木往往在树干处折断。兴安落叶松（Larix gmelinii）和大野橡木（Quercus dentata）分别倾向于在树干处被推翻和折断。然而，这种趋势随着海啸规模的变化而变化。此外，即使树冠茂密的树木被推翻，它们也会在一定程度上有助于抵抗海啸。流体力的减小不仅与森林的厚度有关，还与树种和破坏模式有关。为了保持流体力的减小并减少浮木造成的二次破坏，建议采用混合种植的方式，其中林分结构不同，在森林向陆一侧种植大直径树木，以捕获从海侧产生的浮木。
