In the rail industry, sedimentary soil layer in the lower part of cut slopes formed from eroded material from weathered bedrock carried from the upper part of the slope have been known to collapse in heavy rainfall. Focusing on such collapse, stability of slope during past heavy rains is estimated by analysis of saturated-unsaturated seepage and slope stability analysis in this study. The rainfall characteristics that cause collapse of slope is studied by analyzing the relationship between slope stability and rainfall characteristics in a statistically method.

  For the reduction of debris-flow disasters spawned by heavy rainfall, the relationship among geomorphic quantities of the mountainous small drainage basin, the arrival of the debris flow to the basin outlet, and rainfall characteristics was examined by using the distribution maps of slope movements and digital elevation models (DEM) for seven disasters. The geomorphic thresholds for the debris-flow arrival become smaller with an increase in rainfall amount in granite areas, which increases the debris-flow risk in many basins. The geomorphic thresholds are shown to vary significantly with lithology. The threshold decreases in a slope area with the coarser soil and the soil-layer structure that soil water is restricted to moving deeper into the soil below the potential slip plane. The arrival ratio of debris flow to the basin outlet becomes larger with an increase in the drainage area.