Dozangawa Landslide

Location

Minamiyama, Ohkura-Mura, Mogami-Gun, Yamagata Prefecture, (38*38'N; 140*12'E).

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.

Damages

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.