Large Sediment Movement Caused by the Catastrophic Ohya-Kuzure Landslide
Satoshi Tsuchiya* and Fumitoshi Imaizumi**
*Faculty of Agriculture, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka-shi, Japan
**Ikawa University Forest, University of Tsukuba, 1621-2 Ikawa, Aoi-ku, Shizuoka-shi, Japan
The Ohya-kuzure landslide, one of three largest catastrophic landslides in Japan, is assumed to have been triggered by a strong earthquake and a large-scale debris terrace in a channel downstream from landslide. We verified the time of the landslide’s occurrence, its volume, and the amount of sediment supplied to the main river downstream. The landslide’s occurrence in 1707 was confirmed by historical documents and earthquake records of sediment disasters. The landslide’s size was estimated to be 94 million m3, from the geomorphic change in the debris terrace. Moreover, it was presumed that 33% of the sediment accumulating as a debris terrace (29 million m3) was eroded, and that a sediment volume of 17 million m3 was supplied to the upstream region of the main river. Small-scale debris flows have been triggered recently in the source head of the landslide during heavy annual rainfalls. In 2006, a debris flow in Ichinosawa tributary with the most vigorous debris production and transport in the landslide was recorded during a typhoon. Hydrographs of the debris flow quantified by ultrasonic sensor and hydraulic pressure sensor supplemented video images.
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