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 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 about240years of return‐period to two‐hour‐rainfall,
and about 5,700years 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.
Key words:Sediment‐related disasters, heavy rainfall, return‐period, rhyolitic
rocks, Shobara‐City