Slope failures from rain disaster top 7,000 in southern Hiroshima, including mountain tops: survey

JIJI

The rain disaster in western Japan has caused slopes to fail at over 7,000 locations in southern Hiroshima Prefecture, a Hiroshima University survey found.

The failures, which include landslides and flows of both mud and rock debris, occurred widely across the region, including areas where the soil and rocks are usually not prone to collapse, such as mountain summits.

“Weathered layers seem to have collapsed due to massive precipitation,” an expert said.

For the survey, the Hiroshima University team compared before and after shots of 2,000 sq. km of southern Hiroshima taken by the Geospatial Information Authority.

Slope failures were found at a minimum of 7,448 locations.

By municipality, Higashihiroshima logged the largest number of failures at 2,730, followed by Kure with 1,460 and Mihara with 1,077.

The disaster involved 72 hours of precipitation that broke records at more than 20 places in the prefecture, indicating that heavy rain triggered the collapses.

Southern Hiroshima is rich in granite, which decomposes and forms fragile soil if weathered, and in rhyolite, which is solid and impervious to water.

But slope failures occurred even in areas formed from rhyolite.

Collapses occurred in these areas apparently because unusually large amounts of rain fell on ground where fragile layers of rhyolite had formed after being weathered over a long period, said Hideaki Goto, an associate professor who led the team.