Panoramic view of the Dozangawa Landslide
- Minamiyama, Ohkura-Mura, Mogami-Gun, Yamagata Prefecture, (38*38'N;
- Date of Slide
- Slide 1) June 28, 1966; Slide 2) April 1981; Slide 3) May 1994
- Size of Slide
- Slide 1): ? 2):Length:400 m; Width: 200 m; Area: 10 ha; Slide 3): slope
Length: 50 m; Width: 130 m; Area: 0.6 ha.
- In recent years, Slide 1)resulted in 25 deaths.
- Geology, Mechanism of Failure and Causes
- The site area is underlain by Upper Tertiary (Miocene to Pliocene Epoch)
black to dark gray mudstone in turn overlain by alternating beds of gray
sandy siltstone and sandstone. Further, the entire region is overlain by
up to 80 m of Quaternary volcanic eruptive materials (the eruption was
from the Hiziori Caldera about 10,000 years ago, and is located 4 km southwest
of the site). The eruption resulted in plateaulike topography. The north-south
running dozan River dissects the plateau forming very steep cliffs along
both sides of the river.
This region receives a large volume of precipitaion (2700 mm annually)
with snow fall in excess of 3 m. The thick accumulation of volcanic materials
functions as a ground water recharge area, causing numerous landslides
during the snowmelt seasons and early summer rainy spells.
Based on the site topographic characteristics, the dozangawa Landslide
is grouped into three areas: Sabuta Area, Minamiyama Area, and Yunotai
Area. However, the date of the sliding of each area is unknown.
The Sabuta Area is a very large landslide and comprises 79.19 ha. The slide
moves as a block with an average depth of 70 m.
The Minamiyama Area is a secondary landslide that occurred within the slide
degris (GL=10 to 20 m) of the primary landslie (Sabuta). The movement as
a whole is very slow, however, portions of theslide could move rather fast
during the snowmelt seasons.
The Yunotai Area represents the typical plateau topography underlain by
the volcanic eruptive materials. The plateaus are flat-topped with very
steep scarps. Scarp failures are repeated during the snowmelt seasons.
The slide planes are formed at GL=12 to 18 m. It is possible that the accelerated
activity is due to erosion at the toe and by the Furumizu River.
- Mitigation Measures
- 1) Sabuta Area: Because of the depth of the slide plane (up to 70 m),
ground water removal works were considered with a combination of drainage
tunnels and drainage wells.
- 2) Minamiyama Area: Due to the high ground water table, a combination
of deainage wells and drainage borings were planned, and pile works and
anchor works have been implemented.
- 3) Yunotai Area: In order to protect the toe area from erosion, dams
and bank protection works have been constructed. For future works, a combination
of drainage wells and drainage borings and pile works have been planned.
Conditions of Sbuta area immediately following the sliding
Cross section of the Sabuta Landslide