In order to operate the debris flow alert system, the critical rainfall value is needed. Therefore a shortterm rainfall forecast method is developed in author's formar study (1990) . Here in this paper, two remining problems in it, i.e. 1) the determination of the normal rain conversion coefficient between base station and any un-obserbed pint in the basin, 2) analysis of the relation among effective rainfall amount, rainfall intensity and occurrence probability of debris flow are studied. The normal conversion coefficient at any point is expressed by an equation made up of topogrophic factors and corrective term. These factors in 1km grid square is given respectively to simulate the aim rain. The occurrence probability of debris flow or other sediment disaster is shown by diagram consists of effective rainfall factor and 1 hr peak rainfall factor. some sample critical lines are determined on it. Consequently the operation of debris flow predicting system can be carried out using the results obtained through this series of studies.
This paper presents an experimental approach for the shear resistance of fill materials in the steel framed Sabo structures. The experiments have been performed by applying the horizontal load to the rigid frame with or without fill materials, respectively. The pure shear resistance of fill materials in the case of rigid frame test has been compared with the one in the case of the simple shear frame test. It is found that the pure shear resistance of fill materials in the rigid frame test is greater than the one in the simple frame test. The effects of the upper load, particle size and the frame rigidity are also examined on the relation between the shear resistance and horizontal displacement.
Mt. Tokachi located in central Hokkaido has erupted since December 16, 1988 after 26 years dormancy. The volcanic eruptions continued intermittently, and counted 21 times until March 5, 1989. Pyroclastic flows, pyroclastic surges and small-scale volcanic mudflows initiated by snow-melting occurred with eruptions. As a result of these events, pyroclastic materials were distributed over the hillslopes of Mt. Tokachi. Field researches were carried out to know the area, the configuration and the grain sizes of volcanic deposits. The lower end of the deposits reached 1 km downward from the crater and the volume was estimated 3 x 104m3. The pyroclastic- and mud-flow deposits contain a large number of boulders, the maximum size of which was over 80 cm. In comparison with the eruption in 1926, which initiated a huge mudflow and killed 144 people, the amount of the pyroclastic materials of this event was about 1/70 of that in 1926. Although the depth of the snow that covered the hillslopes of Mt. Tokachi from 1988 to 1989 was up to the annual average, the volume of melted snow was extremely less to develop the mudflow.