4:30 PM - 4:45 PM
[HDS07-23] Reports on the damage caused by the slush flows that occurred on the northwestern slopes of Mt. Fuji on April 9, 2024
Keywords:slush flow, Yukishiro, Lahar, landslide disaster, Mt. Fuji, UAV
On April 10, 2024, various media outlets reported that soil and gravel were flowing onto the road around the first and fourth stations of the Fuji-Subaru Line, which connects the northern foot of Mt. Fuji with the fifth station of the Yoshida route. In order to understand the overview and the damage of this disaster, we conducted deposit surveys, aerial photography surveys using UAVs, and record confirmation and waveform analysis of earthquake observation networks and gravity observation. This presentation will focus on the results of the deposit surveys and aerial photography surveys using UAVs that we conducted around the deposit inflow site at the fourth station of the Fuji-Subaru Line from April 11 to 15th.
The deposit that flowed onto the road from the east covered the entire hairpin bend with a layer of 1 m to several meters thick, and some of it flowed down to the west. The deposit consists mainly of a mixture of snow and ice and volcaniclastic particles such as scoria and ash, which are typical of deposit associated with slush flows. The deposit also contains trees and artificial objects, such as concrete fragments and pieces of wire rope. Two valleys on the west slope of Mt. Fuji converge at the east side of the hairpin bend. Six training dikes have been installed at the confluence to prevent deposit flowing down the “Namesawa”, one of the two valleys from entering the hairpin bend. However, this time, it has been confirmed that part of the slush flow that flowed down along the Namesawa hit the training dikes, collapsing two of the dikes on the south side and partially damaging a third. As a result, the slush flow which breached the training dikes entered the hairpin bend. The slush flows along the two valleys merged and then flowed down a single valley to the west, ultimately overriding six sabo dams located about 600 m west of the hairpin bend and continuing to flow downstream. On the other hand, observation using UAV-mounted telephoto lens towards the summit revealed that slush flows appeared to be running down from multiple locations at elevations of 3,200 to 3,300 m, approximately 1.1 to 1.3 km northwest to west-northwest from the summit crater center, into several valleys on the northwest slopes.
The gravimeter installed at the fourth station recorded a distinctive waveform, going off-scale, around 10 AM on April 9th, which seems to be due to some geophysical phenomenon related to the slush flows. However, for more detailed discussion based on the waveform analyses of geophysical observations, please refer to the presentation entitled “Study on the geophysical observations of the slush flows that occurred on the northwestern slopes of Mt. Fuji on April 9, 2024,” which is the same session as this one.
The atmospheric temperature recorded by the AMeDAS at the summit rose by about 10℃ from -10℃ to -0.5℃ between 12 AM on April 6th and 12 AM on the 8th. It then remained near 0℃ until 12 AM on the 9th, with little change in temperature. Rainfall was recorded starting around 0 AM on April 9th at rain gauges located at the first, fourth, and fifth stations along the Fuji-Subaru Line. The precipitation increased throughout the morning, culminating in over 30 mm of rainfall at the rain gauges at the fourth and fifth stations between 8 and 9 AM. These weather conditions seem consistent with the conditions for the generation of slush flows that have been discussed in previous studies (e.g., Anma, 2007).
The deposit that flowed onto the road from the east covered the entire hairpin bend with a layer of 1 m to several meters thick, and some of it flowed down to the west. The deposit consists mainly of a mixture of snow and ice and volcaniclastic particles such as scoria and ash, which are typical of deposit associated with slush flows. The deposit also contains trees and artificial objects, such as concrete fragments and pieces of wire rope. Two valleys on the west slope of Mt. Fuji converge at the east side of the hairpin bend. Six training dikes have been installed at the confluence to prevent deposit flowing down the “Namesawa”, one of the two valleys from entering the hairpin bend. However, this time, it has been confirmed that part of the slush flow that flowed down along the Namesawa hit the training dikes, collapsing two of the dikes on the south side and partially damaging a third. As a result, the slush flow which breached the training dikes entered the hairpin bend. The slush flows along the two valleys merged and then flowed down a single valley to the west, ultimately overriding six sabo dams located about 600 m west of the hairpin bend and continuing to flow downstream. On the other hand, observation using UAV-mounted telephoto lens towards the summit revealed that slush flows appeared to be running down from multiple locations at elevations of 3,200 to 3,300 m, approximately 1.1 to 1.3 km northwest to west-northwest from the summit crater center, into several valleys on the northwest slopes.
The gravimeter installed at the fourth station recorded a distinctive waveform, going off-scale, around 10 AM on April 9th, which seems to be due to some geophysical phenomenon related to the slush flows. However, for more detailed discussion based on the waveform analyses of geophysical observations, please refer to the presentation entitled “Study on the geophysical observations of the slush flows that occurred on the northwestern slopes of Mt. Fuji on April 9, 2024,” which is the same session as this one.
The atmospheric temperature recorded by the AMeDAS at the summit rose by about 10℃ from -10℃ to -0.5℃ between 12 AM on April 6th and 12 AM on the 8th. It then remained near 0℃ until 12 AM on the 9th, with little change in temperature. Rainfall was recorded starting around 0 AM on April 9th at rain gauges located at the first, fourth, and fifth stations along the Fuji-Subaru Line. The precipitation increased throughout the morning, culminating in over 30 mm of rainfall at the rain gauges at the fourth and fifth stations between 8 and 9 AM. These weather conditions seem consistent with the conditions for the generation of slush flows that have been discussed in previous studies (e.g., Anma, 2007).