Measuring Mass Movements of Gravel on Periglacial Smooth Slopes in the Hakuba Mountains Using UAV Aerial Photographs

Mass movements of gravel are investigated on periglacial smooth slopes at Mikunizakai, Mt. Hakuba, and Mt. Shakushi in the Hakuba Mountains, northern Japanese Alps, using unmanned aerial vehicle (UAV)-derived orthoimages. UAV orthoimages are generated from aerial photographs acquired with UAV between 2020 and 2022 using structure-from-motion multi-view stereo (SfM-MVS) photogrammetry. Based on field surveys and UAV-derived orthoimages, surface gravel areas are classified into three categories: matrix-filled fine-gravel (< 8 cm), matrix-free fine-gravel (< 8 cm), and matrix-free large-gravel (≥ 8 cm). Subsurface materials are examined to a depth of 50 cm, together with snow-cover conditions. From autumn 2021 to autumn 2022, annual mean distances of gravel mass movements are 11.7 cm at Mikunizakai, 7.9 cm at Mt. Hakuba, and 13.4 cm at Mt. Shakushi. Large mobility is observed in matrix-filled and matrix-free fine-gravel areas. In these areas, gravel mass movements are attributed mainly to daily freeze–thaw cycles in spring and autumn, with an additional contribution from wash processes induced by summer rainfall events.