2010 Volume 63 Issue 4 Pages 30-37
A very rare and heavy rain-storm for a few hours in Shobara-City, Hiroshima Prefecture, induced a lot of slope failures and debris flows in the afternoon on July 16, 2010. Damaged area was located within only a square of about 5km. This mountainous area is geologically covered by Takata rhyolitic rocks, Kisa andesitic rocks, and Bihoku sedimentary rocks. Because the surface soil layer was not thick, most of failure-depths were 0.5-1.5m. Countless small holes and fissured bed-rocks, through which water-flow pushed out surface soil layer, were observed around most of slope failures. Almost displaced materials turned to debris-flows or a kind of liquefied state and roared down into main stream, and then flowed down as a large hyper-concentrated flow. The rainfall as a cause of this disaster was statistically about 240 years of return-period to two-hour-rainfall, and about 5,700 years to three-hour-rainfall in this district. Areal ratio of slope failures steeper than 30° to this same watershed area was 7.0% for Shinodo-area, and 4.7% for Saki-Oto-area.
