Geomorphic Settings and Mechanisms of Slope Failures on Fossil Periglacial Slopes in the Hidaka Mountains: Features of Slope Disasters Caused by Heavy Rains in August 2016

In recent years, an increasing frequency of heavy rainfall events has led to more frequent slope failures on fossil periglacial slopes in Hokkaido. These slope failures, observed in areas such as the Hidaka Mountains, can be classified into three types—deep-type, shallow-type, and gully-type—based on their scale, morphology, and depth of occurrence. Deep-type failures extend across the postglacial dissection front, from the upper sideslope to the lower sideslope. Shallow-type failures commonly occur in head hollows, whereas gully-type failures typically develop on upper sideslopes with smooth ground surfaces. At the lowermost part of the periglacial slope deposits, layers of gravel facies and the underlying heavily weathered bedrock beneath exhibit high permeability, which causes groundwater to accumulate in these zones. Deep-type slope failures are triggered when pore-water pressure increases near horizons with contrasting permeability. In contrast, the uppermost part of the periglacial slope deposits is generally composed of low-permeability massive silt facies, overlain by more permeable black soil. This stratigraphic configuration promotes the concentration of rainwater above the periglacial deposits, resulting in shallow-type slope failures mainly within the black soil layer. When piping erosion develops in the lower part of the highly permeable layer, it can cause a collapse of the overlying topsoil, resulting in gully-type slope failures. Given the increasing frequency of heavy rainfall events, understanding these failure mechanisms is crucial for disaster prevention. Accurate prediction of failure type based on the position within the periglacial slope can aid in risk assessment and mitigation efforts.