Temporal changes in the debris flow threshold under the effects of ground freezing and sediment storage on Mt. Fuji

[摘要] Debris flows are one of the most destructive sediment transport processes in mountainous areas because of their large volume, high velocity, andkinematic energy. Debris flow activity varies over time and is affected by changes in hydrogeomorphic processes in the initiation zone. To clarifytemporal changes in debris flow activities in cold regions, the rainfall threshold for the debris flow occurrence was evaluated in Osawa failure ata high elevation on Mt. Fuji, Japan. We conducted field monitoring of the ground temperature near a debris flow initiation zone to estimate thepresence or absence of seasonally frozen ground during historical rainfall events. The effects of ground freezing and the accumulation of channeldeposits on the rainfall threshold for debris flow occurrence were analyzed using rainfall records and annual changes in the volume of channeldeposits since 1969. Statistical analyses showed that the intensity–duration threshold during frozen periods was clearly lower than that duringunfrozen periods. A comparison of maximum hourly rainfall intensity and total rainfall also showed that debris flows during frozen periods weretriggered by a smaller magnitude of rainfall than during unfrozen periods. Decreases in the infiltration rate due to the formation of frozen groundlikely facilitated the generation of overland flow, triggering debris flows. The results suggest that the occurrence of frozen ground and thesediment storage volume need to be monitored and estimated for better debris flow disaster mitigation in cold regions.

[学科分类] 土壤学