2:30 PM - 2:45 PM
[U07-13] Landslide disaster induced by the 2014 Hiroshima rainstorm
Keywords: landslide, debris flow, rainstorm, disaster
We identified three types of landslides. The first is a planar type, in which a planar soil layer with a depth of 1 m or so slid. The second is a gush-out type, in which water gushed out from slopes to make holes; the base of a hole is occupied by rock fragments with many openings and less amounts of fine fractions. The third is a long, gully like slide with a wedge-shaped profile sided by an along-valley joint or a fault. In the torrent of the Ken-ei-jyutaku in Yagi 3-chome, debris avalanches went down along the river entraining large rock blocks separated from the bedrock by sheeting joints and high-angle joints. Similar phenomenon occurred in the upstream torrent of Midorii 8-chome. Other than these two torrents, large blocks in debris flows were mainly fine-grained granite, which is resistant to weathering. In hornfels areas on the other hand, gush-out type landslides were predominant.
Landslides and debris flows in granitoid areas have characteristics dependent on weathering extent. In the affected area of the Hiroshima disaster, granite in higher elevations is weathered to form micro-sheeted rock, which is incised deeply to expose fresh rock beneath it. The fresh rock body was separated into large rock blocks by sheeting joints and high-angle joints along the river. Consequently, the debris of landslides in higher elevations came down along a river, entraining the large blocks, getting destructive energies, and hitting the residential houses on fluvial cones. Another cause of the destructive debris flows were large blocks of fine-grained granite, which is more resistant to weathering than medium-grained granite. Gush-out type landslides in hornfels areas are assumed to have occurred when the rainfall intensity became too strong for the openings in the zone of rock fragments to drain the groundwater flow, which finally blew out.