Experimental study on the effects of local sediment accumulation on a debris flow surge in a steep channel

Debris flow surges can terminate in a steep channel of > 15°. However, the termination process and mechanisms remain unknown. This study conducted small-scale flume experiments to investigate the effects of local sediment accumulation on debris-flow surges in a steep channel. The experiments demonstrated that local accumulation of bed sediment terminates a debris flow surge owing to abrupt changes in bed gradients and infiltration of debris flow interstitial water. Subsequently, the mass of the terminated debris-flow surge and bed sediment began to move, triggering a larger debris-flow surge. This result suggests that predicting the scale of a debris flow arriving downstream requires measuring the distribution of bed sediment in the debris flow initiation zone

Initiation and runout characteristics of partially saturated debris flows in Ohya landslide scar, Japan

A partially saturated flow, which has an unsaturated layer in its upper part, has been monitored in the steep initiation zones of debris flows. Understanding the initiation and runout characteristics of partially saturated flows is essential for predicting the timing and magnitude of downstream debris flows. Monitoring performed using time-lapse cameras and water pressure sensors in the Ohya landslide scar in central Japan allowed us to obtain data on a series of partially saturated debris flow surges from initiation to termination on July 6, 2020, and July 13, 2021. Debris flow surges were mainly induced by repetitive mass movement of sediment deposit caused by the overland flow erosion of channel deposits, channel deposit slides, and water and sediment supply from channel banks and tributaries. The excess pore water pressure in a partially saturated flow on July 6, 2020, was clearly higher than that on July 13, 2021. Rainfall patterns, which control the water content in channel deposits, and the flow height likely affect the magnitude of the excess pore water pressure in partially saturated flows

Effects of deforestation and weather on diurnal frost heave processes on the steep mountain slopes in south central Japan

Freezing and thawing processes play an important role for the gravitational transport of surface materials on steep mountain slopes in Japan. The effects of deforestation on frost heave activity were observed through the 2012/2013 winter season in Ikawa University Forest, a southern mountainous area in central Japan (1180–1310 m above sea level). During periods without snow cover, needle ice development prevailed at a clear‐cut site, and the downslope sediment movement of upper soil was 10 to 15 cm through the winter season. At a non‐cut site, rise and fall in the ground surface level prevailed on a weekly scale, with no evident downslope movements at the surface; ice lens formation in the soil layer is assumed. Abrupt changes in the radiation budget, such as the strengthening of nighttime radiative cooling and increases in daytime direct insolation, induced frequent development/deformation of needle ice at the clear‐cut site. In snow‐free periods, the day‐to‐day variability in needle ice growth length and in nighttime averaged net radiation showed significant correlations; cloudy weather with warmer and moist air intrusion associated with synoptic disturbances prevented the occurrence of needle ice. Namely, day‐to‐day weather changes directly affected the mass movement of the upper soil after deforestation. Shallow snow cover occurred discontinuously through the winter and is likely an important factor in keeping the soil moisture sufficiently high in the upper soil layer for initiating needle ice during snow‐free periods. We also discuss contributions of coastal extratropical cyclone activities providing both snow cover and cloudy weather in the southern mountain areas of central Japan to the intra‐seasonal variability in frost heave and its indirect effect on soil creep and landslides on the deforested steep slopes

Initiation and runout characteristics of partially saturated debris flows in Ohya landslide scar, Japan

A partially saturated flow, which has an unsaturated layer in its upper part, has been monitored in the steep initiation zones of debris flows. Understanding the initiation and runout characteristics of partially saturated flows is essential for predicting the timing and magnitude of downstream debris flows. Monitoring performed using time-lapse cameras and water pressure sensors in the Ohya landslide scar in central Japan allowed us to obtain data on a series of partially saturated debris flow surges from initiation to termination on July 6, 2020, and July 13, 2021. Debris flow surges were mainly induced by repetitive mass movement of sediment deposit caused by the overland flow erosion of channel deposits, channel deposit slides, and water and sediment supply from channel banks and tributaries. The excess pore water pressure in a partially saturated flow on July 6, 2020, was clearly higher than that on July 13, 2021. Rainfall patterns, which control the water content in channel deposits, and the flow height likely affect the magnitude of the excess pore water pressure in partially saturated flows

Experimental study on the effects of local sediment accumulation on a debris flow surge in a steep channel

Debris flow surges can terminate in a steep channel of > 15°. However, the termination process and mechanisms remain unknown. This study conducted small-scale flume experiments to investigate the effects of local sediment accumulation on debris-flow surges in a steep channel. The experiments demonstrated that local accumulation of bed sediment terminates a debris flow surge owing to abrupt changes in bed gradients and infiltration of debris flow interstitial water. Subsequently, the mass of the terminated debris-flow surge and bed sediment began to move, triggering a larger debris-flow surge. This result suggests that predicting the scale of a debris flow arriving downstream requires measuring the distribution of bed sediment in the debris flow initiation zone

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

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