Journal of the Japan Society of Erosion Control Engineering, Vol.63,No.4,2010

Prompt investigation report on the sediment]related disaster on July 16, 2010 in Shobara]City, Hiroshima Prefecture

Masahiro KAIBORI, Shigemitsu SUGIHARA, Shinji NAKAI, Yoshinori ARAKI, Takao YAMAKOSHI, Shin]ichiro HAYASHI and Yuuichi YAMASHITA


Abstract

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 PU, QOPO. Damaged area was located within only a square of about Tkm. 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 ODT]PDTm. 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 aboutQSOyears of return]period to two]hour]rainfall, and about TCVOOyears to three]hour]rainfall in this district. Areal ratio of slope failures steeper than RO to this same watershed area was VDO for Shinodo]area, and SDV for Saki]Oto]area.


Key wordsFSediment]related disasters, heavy rainfall, return]period, rhyolitic rocks, Shobara]City


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