A three-dimensional multi-planar sliding surface method is proposed in this paper to predict an area of surface failure which appeared during a heavy rainfall. The safety factors calculatoed by the method show similar to those obtained by different methods (Hovland and simplified Janbu). A dangerous area is predicted by an application of the method to a digital land form model.

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Establishing an appropriate method to predict the reach of avalanches is becoming an urgent need with the calamities caused by the Maseguchi Avalanche occurred in January 1986. The one-dimensional Voellmy type model, which handles the movement of the avalanche as a fluid in steady flow, is well known as a model to simulate the reach of the avalanche. By applying the said model to the RANDOM WALK MODEL (hereinafter abbreviated as RWM), which is used in the simulation of the inundation reach caused by debris flow, the authors have succeeded at creating a two-dimenesional avalanche reach simulation model. Since RWM is a technique making use of the probability theory, in conventional cases it is able to identify the risk at the various points which the avalanche passes through but on the other hand it is unable to identify the volume of the avalanche. Such being the case, an attempt was made to build a more rational model, based on the RWM and able to calculate the passing-through and the accumulation volumes of the avalanche as well as its reaching time. Satisfactory results were obtained through simulations referring to the calamities caused by the Maseguchi Avalanche and the Samutani Avalanche that were carried out by making use of these techniques.

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Debris flow peak discharge is one of the most important factors when designing control structures. The method to predict the peak discharge, however, has not been established. In this study, field data of debris flow were analized to obtain the method. Through the data processing, it was found that there is a good corelation between peak discharge and magnitude (total volume of debris flow). As the total volume of debris flow can be predicted to some extent based on the volume of sediment available upstream or the volume of supplied water calculated from amount of rainfall, the peak discharge can be predicted by the empirical equations proposed.

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After the last explosions of the 1988-1989 Tokachi-lake Volcano Eruptions, the volcano returned to dormant condition. However, the western slope to the summit of the volcano has been covered by the block- and ash-flow deposits which ejected by the 21 times eruptions. The deposits are interbedded with thick snow deposits because that the eruptions occurred during winter time.
On the summit areas of the Tokachi-dake, considerably heavy rainfalls have been recorded on August, 22nd and 23rd, 1989. In particular, the rainfalls on August 23rd triggered debris flows along the upper branch stream of the Furano-gawa. Water running on the surface of the deposits of the summit slope has eroded those flowing along the western slope, accerelated by melting of snow interbeds between the block- and ash-flow deposits.
Consequently, several gullies have been formed on the upper western slope. Debris flows have been generated and deposited a small-scale debris (1, 000m³) on a gentle slope about 7° in degree. Mean diameter of emplaced blocks is about 0.5-0.8m and the maximum diameter is about 1.1m.
The velocity of the gully erosion and debris flow has been measured by the cut down of the cable for seismometers set on the western slope. If the seismometer might have caught the time of commence of the debris mass movement on the summit and that of the generating of the debris flows on the lower slope, the data suggest that the velocity of the debris flow was 2-4m/sec.
Application of the actual data, such as size, deposition volume, slope inclination, to several experimental formula, has revealed that the velocity of the debris flow is about 5m/sec, which is similar to the value estimated by the seismometers.

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