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[MIS15-02] Debris flow caused in May 2021 on the snow cover of the Norikura Daisekkei, Nortern Japanese Alps
Keywords:debris flow, snow cover, snow melt period , alpine zone, Norikura Volcano
On June 7, 2021, we confirmed that debris flow deposits were spread over the snow surface in the Norikura Daisekkei of Norikura Volcano in the Northern Japanese Alps. Since the occurrence of this debris flow is different from that of ordinary debris flow, the occurrence and the mechanism of mudslide generation were investigated. Although the date of the debris flow is not clear, it is certain that it occurred before May 24. At the foot of Norikura volcano, the continuous rainfall from May 16 to May 22 was 352 mm, which suggests that the debris flow occurred during this period.
The debris flow occurred at 2694 m a.s.l. The debris flow occurred at 2694 m a.s.l. A transvers crack, 26 m long, was formed in the snow cover, and the debris flow blew out of the crack. The ground surface at this point is a ridge-like topography within the slope and it is higher than the surrounding area. Therefore, the snow depth on June 13 was about 2 m near the crack, while it was about 8 m on the surrounding slopes.
The debris flow deposits extended 275 m down slope from the source area. The maximum width of the deposit area is 44 m. The sediment consists mainly of granule and sand, with some silt and clay. The average layer thickness is about 10 cm, and it is thicker downstream. The maximum value of gravel is 40 cm. In some cases, gravels of 5 to 15 cm in size are densely packed on the surface of the debris flow deposit, lacking matrix.
A part of the end of the debris flow deposit rises to the ridge, while the other end reaches the road. The debris flow would not have stopped on a snow surface with the slope of 16° average degree. The large amount of precipitation occurred on the period of snowmelt season, therefore the channels under the snow layer could not carry the snowmelt water. It is thought that the water was dammed up under the snow layer. It is considered that the increased water pressure under the snow layer caused the water to gush out with sediment at 2694m, where the snow cover is relatively thin.
The debris flow occurred at 2694 m a.s.l. The debris flow occurred at 2694 m a.s.l. A transvers crack, 26 m long, was formed in the snow cover, and the debris flow blew out of the crack. The ground surface at this point is a ridge-like topography within the slope and it is higher than the surrounding area. Therefore, the snow depth on June 13 was about 2 m near the crack, while it was about 8 m on the surrounding slopes.
The debris flow deposits extended 275 m down slope from the source area. The maximum width of the deposit area is 44 m. The sediment consists mainly of granule and sand, with some silt and clay. The average layer thickness is about 10 cm, and it is thicker downstream. The maximum value of gravel is 40 cm. In some cases, gravels of 5 to 15 cm in size are densely packed on the surface of the debris flow deposit, lacking matrix.
A part of the end of the debris flow deposit rises to the ridge, while the other end reaches the road. The debris flow would not have stopped on a snow surface with the slope of 16° average degree. The large amount of precipitation occurred on the period of snowmelt season, therefore the channels under the snow layer could not carry the snowmelt water. It is thought that the water was dammed up under the snow layer. It is considered that the increased water pressure under the snow layer caused the water to gush out with sediment at 2694m, where the snow cover is relatively thin.