In order to indicate effects of mountain slope distribution in a three-dimensional space, infiltrated water movement in a surface layer is modeled, in the study, by Darcy's law and a continuous equation in a cell which is called a square area determined by a mesh method. To predict site of probable mountain slope failure by evaluating three-dimensional influences of mountain slopes, infiltrated water levels obtained by the model are applied to the multi-planar sliding surface method which was proposed elsewhere. The results show that sites of the most dangerous sliding mass almost appeared at the same sites where failures occurred in the past time. Another predicting method, moreover, is proposed in the present study by using a digital land form model which was also proposed to calculate F values. Infiltrated water levels at each cell are applied to the infinite slope stability analysis method. Potential failure areas are mapped and are classified into various dangerous degrees by the time when the safety factor becomes less than unity under the assumptions that the depth of the surface layer is 1.2m and rainfall, 20mm/hr, continues 50 hours. More hazardous cells are found to appear at the sites where mountain slope failures took place in the past time.
Topographical change in the Kamikamihori valley on the Yakedake volcano was quantitatively explained in terms of the processes of debris production, transportation and deposition. Rack fall from the flat sidewall was most active in early spring when the air temperature rises rapidly crossing 0°C, while from the convergent sidewall, rock fall was most active under the condition of water convergence by rapid snow melting or by heavy rainfall. But the rock-fall rate was not directly promoted through the wall-foot scouring by debris-flow passing. The retreat rate of sidewall took the large value amounting to the order of 101cm/year. Depositional structure of debris-flow lobe assumed the inverse grading, and the swollen lobes in the upper fan consisted of the sandy boulder deposits conserving the original composition of moving material in the flow, while the flat lobe in the lower fan consisted of the boulderly sand deposits almost losing the larger boulders. Topographical analysis of transversal cross profiles of the fan along the survey lines of concentric circles around the fan apex, was very useful for the prediction of the shifting of running direction and depositional pattern of debris flows, in addition to the explanation of the fan evolution itself.