A large landslide dam, around 100m high, resulted from a deep]seated landslide that followed heavy rainfall during Typhoon No. 12 in September 2011 in the Kuridaira area, Totsukawa Village, Nara Prefecture. Although the Kii Mountain District Sabo Office had completed construction of a temporary overflow channel as a part of urgent countermeasures, the channel was damaged by erosion caused by the heavy rainfall (230mm/day) during Typhoon No. 17 in September 2012. Following this event, we studied the damage mechanisms by analyzing CCTV images and concluded that the flow in the temporary channel, with a head difference of over 90m, scoured the foot of the channel slope. As result, the temporary channel was fatally damaged. Considering this finding, we have reassessed the countermeasures for large]scale landslide dams with high head differences and steep slopes. We conclude that, following the formation of landslide dams, it is necessary to construct not only an overflow channel, but also a sabo dam at the foot of the overflow channel to prevent scouring at the foot of the channel. It is also necessary to reduce the height of landslide dams by excavating the crown to reduce the water level.

Key wordsFdeep]seated landslide, landslide dam, overflow channel, scouring