Boulder rich type of debris flow is known to generate strong ground tremor. Discharge of 34 debris-flow surges and ground tremor due to the surges have been observed at the Kamikamihori Gully of Mount Yakedake, Japan. Analysis of the relationship between peak discharge of the surge and peak acceleration of the tremor shows a positively high correlation with a correlation coefficient of R=0.90. This relationship results from the fact that a larger momentum of boulder may be expected with the larger peak discharge of the surge, and that the larger the momentum, the higher the intensity of the ground tremor. Another relationship between the volume obtained by integrating the discharge over the duration time of a surge and the integral of acceleration amplitude of the tremor over the same period shows a positively very high correlation with a correlation coefficient of R=0.98. This relationship may be roughly understood by a conceptual analogy, in which the volume should be a substitute of momentum of a surge, and the integral of tremor-acceleration amplitude should be an alternate of impulse which is equivalent to momentum. The result suggests that peak discharge of the surge can be roughly estimated from peak acceleration of the tremor, and volume of the surge can be precisely estimated from the integral of acceleration amplitude of the tremor.
Natural revegetation in slope failure sites in University Forest in Chiba, The University of Tokyo, where more than 700 slope failures occurred in July 1970, was investigated using aerial photography. Four research areas were chosen to analyze the 25-year trend in vegetation. Aerial photographs of each research site taken in 1966, 1971, 1976, 1984 and 1995 were overlaid to prepare an orthophotograph and matching scale using graphic software (Adobe Photoshop). The following results were obtained. 1) There is a good correlation between the initial area of slope failure and the length of elapsed time until a denuded slope is completely covered by vegetation. 2) An approximately 150 square meter slope failure became indistinguishable after ten years, and a 1500 square meter failure became indistinguishable after 20 years. 3) Long and narrow slope failures recover faster than other shapes. Even though slopes have been completely covered by vegetation in aerial photography, slopes with failures can still be identified by ground truth data because of shallow soil and old scars.
Heavy rain due to the activities of the front has attacked Kochi Prefecture on 24-25 September in 1998. One hour rainfall more than 70mm has continued almost 6 hours. Consequently, many slope failures and road and riverbank breakage have occurred. Collapse of natural slopes of weathered rock along the joint system, fall of fill slopes and fall down of slopes which have been utilized for the tombs or fruit garden were recognized. In this report, author has done the comparative investigation of the rainfall in 1998 with that in 1975 and 1990. The comparison of the rainfall indexes has proved the rain of last September to be very severe. The heavy rain of last September can be plotted on the right position in the disastrous zone on the diagram showing the relationship of the effective rainfall and one-hour rainfall which has been proposed by the author. The critical line by which the alarm for the evacuation can be given on the diagram was also effective. To express the time dependent reducing effect of the rainfall in use of a coefficient of the reduction of rainfall and a half-life of rainfall, one-hour rainfall was used as Yano has proposed. Considering the relationship of the snake-line and the critical line, the value of a half-life for the case of Kochi was proposed to be 36 hours.
Typhoon 7 hit Honshu on September 22, 1998. This storm brought about a certain amount of local damage in the path of the typhoon. Trees in the southern part of Nara Prefecture were blown down. Field survey and analysis of aerial photographs taken after the storm were carried out to determine some of the characeteristics of the trees blown down. Along narrow gorges the blown down trees on the steep slopes lay scattered in the direction of the downward grade. The blown down trees on slopes in the open areas lay in the same direction as the blast of the wind.The most trees blown down entirely from their roots and a few trees were broken at the middle of their trunks. The structure that check the outflow of the downed trees and the incidental landslides or debris flow sediment was proposed to be built from the logs of rather large diameters available at the sites.