Three factors, inclination of a slope, depth of a surface layer and catchment area, are considered to be important ones in considering the causes of failures which took place during heavy rains on granite mountainslopes. In this report, inclination and catchment area which can be measured on a topographical map are choised to evaluated a degree of a potential failure. In order to measure these topographical factors exactly and quickly, a topographical map is transformed into a map with gridded data. A grid spacing is selected 10 meters. The topographical factors are calculated by computer with the grid map. Grid points which are regarded to appear on stream are determined objectly by using a computer. The program for calculation of slope inclination (tan θ) and of catchment area (A) at these grid points are deviced and developed by using informations of the digital map. An index of a geomorphic driving force (F=tan θ ×Aa, a is constant value) which have been proposed by another researcher is also calculated at these grid points on valleys and the indexes of failed points are compared to ones of non-failed points. We found that the index is larger at the grid points where failures had taken place. Therefore, we can point out a failure potential site availing the indexes that are obtained from the topographical factors such as slope inclination and catchment area. If a slope has some failure potential sites, we regard the slope as a failure potential slope. If we know a depth of a surface layer and its distribution on the slope that is discriminated as a failure potential slope by the above mentioned method, we could foretell the site of a potential failure more exactly by using the method of multiplanar sliding surface which have proposed elsewhere.
We investigated the actual conditions of heavy rain disasters which occurred on August 1981 at Hidaka and Mt. Yohtei in Hokkaido, and studied the deposit shapes formed by debris flows and landslides, partially with some laboratory experiments. The results are as follows: 1) The plane shape of deposit was expressed by the length, width and angle of dispersion (Fig. 3, Table 1). 2) It was considered that the deposit shape was influenced by the channel plane shape (Fig. 7. 8). 3) Comparing the results of actual investigations and experiments, it was clarified that the deposit shape was more influenced by the channel plane shape than by the channel longitudinal one (Fig. 9, 11, 12). 4) The values of angle of dispersion (tan θ) were 0.36-2.90 in Hidaka, 0.20-0.73 in experiments and 0.11-1.07 in Shodoshima. The values of the ratio of deposit width to length (Bd/L) were calculated at 0.20-1.23 in Hidaka, 0.12-0.53 in experiments and 0.11-0.56 in Shodoshima. 5) The model of deposit shape was expressed by Bd/L and tan θ in Fig. 13, and was formulated in the following equation: log (Bd/L)=0.690 log (tan θ)-0.165 6) We discussed the length and area of deposit by the scale and form of movement, and the effects of retardation works to the debris flows which had occurred in the Aokinosawa River at Mt. Yohtei (Fig. 14, 15). 7) Some effects of retardation works were recognized and the planning method was discussed with regard to the effects (Fig. 16, 17).