Recent downpours in western Japan have inundated numerous communities with flash floods, with nearly 200 people losing their lives to drowning or landslides. A particular fear that became clear in this month’s disaster is giant boulders, washed free from hillsides and hurled along by the torrential waters. Potential countermeasures proposed include levees and wire nets to keep rocks from wreaking havoc in the event of flooding.

Massive Rocks a Major Cause of Damage in Western Japan

Western Japan is currently suffering from the effects of violent, heavy rains that struck earlier this month. Much attention has focused on the flooding brought on by the downpours, but one of the biggest culprits behind the damage was actually large rocks, especially those referred to as “core stones,” washed free from hillsides and send tumbling down into communities by landslides.


Victims of the rocks in Kumano, Hiroshima Prefecture, state that these core stones could be heard rumbling and seen rolling through the havoc brought on by the recent storms. Some core stones can be as big as two meters across and weigh several tons.

Where do these boulders come from? Core stones are large chunks of granite from nearby hillsides that, through the process of weathering, break off from the landscape as large chunks of rock. Measuring up to two meters across and weighing several tons, core stones are a prominent feature in ravines in the Chūgoku and Kinki regions of western Japan.

Preventing Further Boulder Damage

Incidents involving core stone damage have proved quite serious in Japan. An incident in 2011 in Wakayama Prefecture caused the deaths of over 80 people during torrential downpours on the Kii Peninsula. A Ministry of Land, Infrastructure, Transport, and Tourism center tasked with crafting landslide countermeasures stresses the need to quick evacuating areas where damage could result as soon as heavy rains begin to fall. In areas where core stones exist, other preemptive measures are needed, such as installing levees or covering these rocks with wire netting to keep them from being washed downhill by the rains.

(Originally broadcast in Japanese on FNN’s Prime News Days on July 11, 2018. )

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