Departmental Bulletin Paper 2014年広島災害における表層崩壊の発生メカニズムと降雨閾値
Triggering Mechanisms and Rainfall Threshold of Shallow Landslides: Cases in the Hiroshima Disaster on 20 August 2014

松四, 雄騎  ,  渡壁, 卓磨  ,  鄒, 青穎  ,  平田, 康人  ,  千木良, 雅弘

Triggering mechanisms and rainfall threshold of shallow landslides were discussed based on field and laboratory surveys for the disaster by heavy rainfall on 20 Aug 2014 at Hiroshima, southwest Japan. Shallow landslides occurred in areas of maximum 3 h-rainfall >160 mm/3h with hourly rainfall intensity >70 mm/h. Hillslopes with bedrock of granite and hornfels slid severely, while areas underlain by rhyolite suffered only sparse landsliding. Three types of landslide were identified in field observation: 1) planer translational landslide, 2) gash-out failure, and 3) wedge sliding. Lithological and subsurface structural factors seem to affect the occurrence of these different types of landslide. Majority of landslides in granite hillslopes and about half in hornfels were planer translational landslides. This type forms a sliding surface within soil layer along mechanical or hydraulic discontinuity at a depth of ~1 m. Other half of landslides in hornfels area was gash-out failure with scars of 1–3 m deep, outcropping an openwork gravel layer at the bottom. Water supply exceeding drainage capacity of this conduit may generate excess pore-water pressure, leading to fluidization of overburden. Wedge sliding was a minor type, with a shallow gullying of soil or a deeper slip surface in bedrock (~5 m), occurring along geologic discontinuity such as joints, faults and dykes. Prediction of potential sites, timings, and magnitude of such landslides requires detailed investigations of thickness and mechanical strength of potential sliding material, observation and modeling of subsurface hydrological processes.

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