抄録
Field observations of large avalanches with extreme runout are crucial for enhancing our understanding of avalanche dynamics and for improving protective measures. Avalanche behavior is strongly influenced by meteorological and topographic conditions; therefore, most avalanche models rely on carefully selected parameters that are validated against historical events. However, in Japan, datasets capturing the runout extent and spatial deposition patterns of large avalanches remain scarce. This study focused on a large dry-snow slab avalanche that occurred on January 10, 2021 at Mt. Nodanishoji in Gifu Prefecture, where the runout zone contains numerous check dams for erosion and sediment control. We combined unmanned aerial vehicle (UAV)–based photogrammetry with ground snow surveys to estimate the spatial distribution of deposition thickness along with the total deposited volume and mass. Within the runout zone of 32.3 ha, the total deposition volume and mass were (5.4 ± 1.1) × 105 m3 and (2.9 ± 0.7) × 105 t, respectively, where the ± values reflect errors estimated from the deposition thickness and density measurements. Among documented events in Japan, this total deposition is second only to the March 27 Hidarimatadani Valley avalanche. Our observations further indicate that the deposition thickness decreased downstream along the flow path. The observed patterns may reflect deposition on low-slope angle runout zones containing numerous check dams, downstream valley widening, and the superposition of multiple avalanches.
