In the western hills of the northernmost Hidaka Mountains, gentle relict periglacial slopes formed byfreeze-thaw processes during the last glacial period are widely distributed. The green areas in the photograph are pastures, demonstrating the effective land use of the piedmont slopes. In August 2016, Typhoon No.10 struck Hokkaido and the Tohoku region, bringing a record-breaking rainfall of 400-500 mm to the Hidaka Mountains. This extreme event triggered more than 700 landslides in the surrounding area. This photograph, captured by a UAV, shows two adjacent cocoon-shaped landslides. Since numerous landslides also occurred in the upper reaches of the small stream flowing through the center of the image, sabo dams have been installed as part of debris flow countermeasures.
These landslides were initiated across the transport and depositional zones of the slope deposits at the foot-slope. While the river in the foreground now follows a gentle arc due to post-disaster restoration work, the original channel was bent sharply. Lateral erosion of the valley wall at this attack slope triggered a succession of failures in the relict periglacial slopes behind it, which then expanded upslope. The thickness of the periglacial deposits covering the gentle slope reached ca. 4 m. It is estimated that multiple slumps occurred along a sliding surface near the base of the deposits, caused by the downcutting of the stream within the landslide scar.
Landslides on gentle periglacial slopes were once extremely rare. However, due to recent climate change—including shifting typhoon tracks and more frequent heavy rainfall—there has been an increasing number of failures on slopes that were previously considered stable. Since the conditions of the occurrence sites and the failure mechanisms of these periglacial slopes remain largely poorly understood, further case studies and detailed investigations are required.
(Photograph: Ken'ichi KOSHIMIZU, Photographed on June 3, 2020; Explanation: Satoshi ISHIMARU)
