Characteristics of Snow-patch Collapse at the Hakuba Daisekkei Perennial Snow Patch

At Hakuba Daisekkei, a perennial snow patch in the northern Japanese Alps, three accidents resulting in casualties due to snow-patch collapses have occurred since the 1960s. The locations of the snow-patch collapses and their precursor phenomena between 2016 and 2021 are investigated using GPR surveys, topographical analyses with UAV-DSMs, UAV-ortho images, and field observations. The research reveals that a tunnel forms at the bottom of the snow patch in the same location annually. Measurements taken at ten points in the exposed tunnel at the ends of snow melts in 2016, 2019, and 2020 show that the tunnel was 9-16 m high and 20-31 m wide. Snow-patch collapses since 2016 are found to have occurred in the ice tunnel, specifically at sites with unstable cantilever bridges and at confluence points of tributaries. Precursor phenomena of snow-patch collapse are identified as the formation and expansion of cracks and local depressions on the snow patch. There is a tendency for many crack formations and snow-patch collapses to occur in light snow years.