Journal of Natural Disaster Science

RUNNING-OUT PROCESSES OF THE DEBRIS ASSOCIATED WITH THE ONTAKE LAND SLIDE

Kazuo ASHIDA
Professor, Disaster Prevention Research Institute, Kyoto University
and
Shinji EGASHIRA
Associate Professor, Disaster Prevention Research Institute, Kyoto University

(Received 14 Nov., 1986 and revised form 8 Jan., 1987)

Abstract

An earthquake with a magnitude of 6.8 occurred at the southern side of Mt. Ontake, Western Nagano on the 14th of September in 1984. A catastrophic land slide, which was named the Ontake Land Slide, was triggered by this earthquake. The rock fall volume was estimated to be 36,000,000 cubic meters. The large amount of sediment set in motion at the slope area from 1900 to 2500 meters in altitude ran down through the Denjo River to the Nigori River and then into the Ohtaki River. The sediment motion brought about various kinds of disasters, including the loss of human lives.

The present study will discuss the debris motion triggered by the earthquake. General descriptions are made at first on the running-out processes of the slide debris, and our conclusions as to the dynamics of the movement of the side debris are presented. Theoretical calculations of fiuidization as well as the movement velocity of the debris avalanche are made and compared with the results obtained in field surveys.

Key words

sediment hazard, Ontake land slide, debris avalanche, debris flow, sediment runoff