Abstract
Abstract. Debris flows are one of the most destructive sediment transport processes in mountainous areas because of their large volume, high velocity, and kinematic energy. Debris flow activity varies over time and is affected by changes in hydrogeomorphic processes in the initiation zone. To clarify temporal changes in debris flow activities in cold regions, the rainfall threshold for the debris flow occurrence was evaluated in Osawa failure at a high elevation on Mt. Fuji, Japan. We conducted field monitoring of the ground temperature near a debris flow initiation zone to estimate the presence or absence of seasonally frozen ground during historical rainfall events. The effects of ground freezing and the accumulation of channel deposits on the rainfall threshold for debris flow occurrence were analyzed using rainfall records and annual changes in the volume of channel deposits since 1969. Statistical analyses showed that the intensity–duration threshold during frozen periods was clearly lower than that during unfrozen periods. A comparison of maximum hourly rainfall intensity and total rainfall also showed that debris flows during frozen periods were triggered by a smaller magnitude of rainfall than during unfrozen periods. Decreases in the infiltration rate due to the formation of frozen ground likely facilitated the generation of overland flow, triggering debris flows. The results suggest that the occurrence of frozen ground and the sediment storage volume need to be monitored and estimated for better debris flow disaster mitigation in cold regions.
Highlights
Debris flows are one of the most destructive sediment transport processes in mountain areas because of their large volume, high velocity, and kinematic energy (Scott et al, 2004; Theule et al, 2018; Song et al, 2019)
In the shallower part of the ground (i.e., ≤ 0.25 m deep), diurnal freeze–thaw activity was frequently observed in autumn and spring, when the diurnal amplitude of the ground surface temperature was evident, whereas seasonally frozen ground was observed in winter (Figs. 4 and 5)
To determine the effects of frozen ground on temporal changes in the rainfall threshold for the occurrence of debris flows and slush avalanches, we estimated periods of seasonal ground freezing based on field monitoring conducted close to the initiation zone in the Osawa failure on Mt
Summary
Debris flows are one of the most destructive sediment transport processes in mountain areas because of their large volume, high velocity, and kinematic energy (Scott et al, 2004; Theule et al, 2018; Song et al, 2019). Because debris flows are usually triggered by heavy rainfall events (Scott et al, 2004; Sidle and Chigira, 2004; Dowling and Santi, 2014; Zhang et al, 2019), estimation of the rainfall thresholds triggering debris flows is essential for debris flow hazard mitigation measures, such as the installation of early warning systems (Pan et al, 2018; Hürlimann et al, 2019). The intensity–duration (ID) threshold is one of the most commonly used rainfall thresholds (Caine, 1980; Guzzetti et al, 2008; Staley et al, 2012; Zhou and Tang, 2014). Other factors, such as cumulative rainfall depth and the return time of rainfall, have been used to estimate
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