粘性土石流の発生,流動,堆積のメカニズムと対策

Abstract

中国雲南省の蒋家溝を対象に, 粘性土石流のメカニズムと土石流対策に関する観測調査を行って, 土石流が降雨の発生時刻からかなり遅れて発生すること。土石流が, 多数のサージが間欠的に流下するという一連のイベントからなっていて, その間欠性は, 降伏応力を用いたモデル, 天然ダムの決壊モデル, 流れの不安定性に基づくモデルを用いて説明が試みられていること。砂礫の濃度が著しく大きいにもかかわらず, 土石流が流れ易いこと。流動中の土石流には栓流が存在しないこと。さらに, 蒋家溝の中下流域での, 土石流発生に伴う地形変化が災害発生ポテンシャルを高めていること。禿げ山化した山地の緑化が進められた結果, 土石流発生などの土砂流出抑制効果が認められたが, そのメカニズムの検討が課題であることを述べた。

The observational study on the viscous debris flows at Jiang-jia Creek in Yunnan Province, China clarified the following points. The time delay of one to five hours from the beginning of rainfall to the onset of debris flow may be ascribed to an accumulating process of slurry from tributary gullies to the main channel bottom at the source reach of the debris flow. The intermittence of debris-flow surge may be explained through the mechanism either of or a complex of an yield strength model, a model of a series of breaching of small natural dams, or a model of an instability of the flow itself. The absence of plug-flow phenomena in the viscous debris flow indicates the yield strength of the flow is unexpectedly small, and a new alternative model is necessary to explain the mechanism of viscous debris flow. The field survey of the countermeasures against debris flows in Dongchuan County over five years showed the tree-planting project at mountain slopes in these several years may have been effective in increasing the slope stability, suppressing sediment discharge and debris flows. Further study is needed to evaluate this effect of countermeasure quantitatively.