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