After 198 years of dormancy, Mt. Unzen began to erupt in November 1990, and continued the volcanic activity until the spring of 1995. Due to the volcanic activity, a large amount of volcanic ash and stone were deposited around the mountain which caused frequent debris flows along the rivers in the volcano. Especially, in the Mizunashi river, the debris flow occurred more than one hundred times from 1991 to 1997. In 1993, the debris flow inundated the down reach of the Mizunashi river and deposited 400 million cubic meters of volcanic sand and rock. But with a peak in 1993, frequency and deposits of the debris flows decreased with declination followed by termination of volcanic activity. In this paper, annual change in critical rainfall for occurrence of debris flow is investigated by using the threat score. A simplified runoff analysis method for debris flow is proposed and checked by the data at the Mizunashi river.
When channel work is built in the erosion control project, the river bank and/or the river bed is usually made by hard material. This created a large opened and monotonous space in a torrent, resulting in deterioration of stream ecosystem. In order to conserve an ideal environment, riparian trees, providing shade, spawning condition, litter, etc., should be conserved. Thus, the effective method to conserve and maintain the riparian trees in the erosion control project area should be developed. This study focused on the cross-sectional microtopographies as a foundation of growing vegetation and tha effect of the riparian trees in reducing the bank erosion during flooding time. Finally, the procedure to conserve torrential areas were proposed from the view points of both the disaster prevention and the environment preservation.
Distribution of woody debris and sediment was examined in a mountain stream, northern Kyushu, southwestern Japan. A densely-welded pyroclastic flow deposit dominates the study basin, and forms steep sideslopes and a flat valley floor. The watershed was disturbed by a powerful typhoon in 1991, which leveled many trees on the hillslopes. Subsequent storm-triggered shallow slope failures deposited timber along streambeds. Channel morphology (gradient, channel width and sinuosity), location and volume of woody debris, and streambed sediment characteristics were investigated three and half years after the typhoon. Log jams of various sizes occurred in the channel ; the largest log jam was 100m long, had a volume of 3, 100m3, and was located along the middle reach of the research section. Principal component analysis of channel morphology and volume of woody debris and sediment suggests that sediment deposition is controlled by tractive force decrease owing to broad channel width and gentle gradient. The analysis demonstrates that there is no conspicuous relationship between volume of woody debris and geomorphic factors such as gradient and sinuosity. Field observations reveal that most log jams occur behind large boulders in the research section. Our results indicate that accumulation of woody debris is influenced more by topographic barriers such as large boulders than by channel morphology. We believe that the supply of boulders from sideslopes greatly influences the storage of woody debris in the investigated stream.
A model of the process of bareland formation is presented. The model explains the whole history of devastation process of forestland to bareland in Japan. The history of the past century of devastation of mountains in Japan is deviled into three periods by the predonnant erosion type. i) Surface erosion type ‹Period of bareland› ‹1900-1965›. Barelands predominate sediment production, ii) Shallow landslide type ‹Period of surface soil slips and mudflow › ‹1966-1985› Poor forest covers by repeated cutting weakened the shear strength of surface soil, which accerelates the simultaneous occurrence of great number of surface slips and mudflow. iii) Deep landslide type ‹Period of natural erosion› ‹1986-› All forest except the former bareland areas has recovered. Erosion on forested hill-slopes has returned to natural process of erosion. Landslides of deep soil deposits and deeply weathered rocks predominate.
A very severe rainfall event occurred at Sado district in Niigata prefecture on August 4, 1998. About 23 houses were damaged, but no person was dead. The purpose of this study is to investigate how inhabitants were evacuated by themselves from slope failures and mud flows caused by the heavy rainfall, to learn useful lessons for developing proper warning and evacuation system.