Field Reconnaissance: 20 November 2004

Reconnaissance of Komatsugura Landslide Dam

Randy Jibson, James Bay, Robert Pack, Naoyuki Inukai

GEER Beyond Reconnaissance Team

The reconnaissance team investigated the Komatsugura landslide dam and its impounded reservoir. The team approached Komatsugura from the east along Highway 291. The team hiked approximately 1 km to the site because of road closures and accessibility issues. The landslide dam is located at 37.303N, 138.905E.

Satellite image of reconnaissance route through Komatsugura Town to landslide dam (satellite image courtesy of DigitalGlobe, imagery acquired 24 October 04).

Fig. 1. Oblique air photo showing the landslide that formed a dam about 1 km west of Komatsugura. This photo was taken immediately after the earthquake before any significant amount of water was impounded. Small yellow arrows denote the main scarp of the landslide; large yellow arrow indicates movement vector of the landslide block; yellow line roughly outlines the landslide mass; locations and directions of photos in Figs. 2-6 are shown by red numbers and arrows. (Air photo from Kokusai Kogyo Co.)

Fig. 2. View looking at the upstream face of the landslide dam, about 1 km west of the town of Komatsugura. The landslide moved from beyond the left edge of photo toward the right. A spillway is being constructed in the center of the photo, and water is being pumped over the dam from the embayment in the right-center of the photo.

Fig. 3. View of the upper part of the landslide block that formed the landslide dam near Komatsugura. The bare slope beneath the yellow arrows is an uphill-facing scarp of a graben at the head of the slide. The slide block extends to the right of this bare slope. The landslide occurred on a dip-slope dipping 15-20^o west (to the right in photo).

Fig. 4. A makeshift drainage system consisting of multiple flexible hoses was deployed to try to reduce the rate of lake-level rise behind the dam.

Fig. 5. The downstream face of the landslide dam consists of loose, disrupted landslide material. The landslide dam was emplaced by slope movement from the right side of the photo to the left.

Fig. 6. At the time of our visit (20 November 2004), the reservoir behind the landslide dam was rising at a rate of about 15-20 cm/day, and a catastrophic breach of the dam would inundate downstream communities within 10 minutes of dam failure. Thus, work on pumping to lower the head and completion of the spillway were very high priorities.

Fig. 7. Close up of construction of spillway along upstream face of the landslide dam.

Fig. 8. Slope deformation instrumentation installed by the Japanese above the landslide dam.

Fig. 9. Barge carrying equipment across reservoir formed by the landslide dam.