2021 Fiscal Year Final Research Report
Understanding of mega debris flow disasters around mounatainous area of central Japan based on sedimentological investigation
| Project/Area Number |
18K03762
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Multi-year Fund |
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| Section | 一般 |
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| Review Section |
Basic Section 17030:Human geosciences-related
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| Research Institution | Tokyo Metropolitan University |
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Principal Investigator |
Shirai Masaaki 東京都立大学, 都市環境科学研究科, 准教授 (50359668)
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| Co-Investigator(Kenkyū-buntansha) |
宇津川 喬子 立正大学, 地球環境科学部, 助教 (20822711)
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| Project Period (FY) |
2018-04-01 – 2022-03-31
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| Keywords | 自然災害 / 堆積物 / ハイパーコンセントレイテッド流 / 礫 / 南海トラフ地震 |
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| Outline of Final Research Achievements |
The sedimentological characteristics of hyperconcentrated flow (HCF) deposits formed by a slope failure event in the upper reaches of the Abe River, central Japan, were investigated. The lower major unit, composed of alternating pebble and cobble layers were interpreted as traction carpet deposits accumulating rapidly during a hyperconcentrated flow event based on the lithofacies, clast orientation, and matrix grain size distribution. Based on the extremely high mud content within the matrix of the middle unit, consisting of poorly sorted structureless muddy pebble gravel with cobble-boulder, this unit was interpreted as a suspension fallout deposit created during the waning phase of the hyperconcentrated flow event. Most of the cobble-boulder included in the middle unit likely consists of entrained basement blocks from the valley wall. The upper unit was interpreted as a debris flow deposit from which HCF was generated according to the dilution by water.
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| Free Research Field |
堆積学
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| Academic Significance and Societal Importance of the Research Achievements |
ハイパーコンセントレイテッド流 (HCF)は粒子濃度が土石流と通常の洪水流の中間的な存在と定義されているが，不明な点が多い．安倍川のHCF堆積物の大部分を占める成層礫層で，大礫層と中礫層それぞれの礫の長軸の方位と礫間を充填する砂泥の粒度組成に基づき，水底の礫層上部では強い水流により大礫が転動し，下部では中礫が土石流と同様に集合状態で移動・堆積することを明らかにした．
この中礫層と大礫層のオリエンテーションの組み合わせは特異的であり，小規模土石流の繰り返しや通常の洪水堆積物の繰り返しとHCF堆積物を判別するに極めて有用であり，今後HCF堆積物の特定およびHCFの理解に貢献が期待できる．
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