We analyzed the occurrence of torrential rain in low rainfall areas by comparing sediment disasters between low and high rainfall areas. Additionally, to clear conditions where torrential rain causes slope failure in mountain soil surfaces, we investigated hydrological processes, including rainfall initiation, storage, runoff and drainage. From this investigation we have shown the usefulness of the same one tank model for effective rainfall. We chose heavy precipitation (daily rainfall R1/2), which has a probability of occurring once every two years, as a factor to represent resilience against sediment disaster, and considered this daily rainfall R1/2 to be a local peculiarity rainfall (R1/2). The results can be summarized as follows. Mountains in both high and low rainfall areas involve the same mechanisms for slope failure. Following this assumption, we concluded that for mountains in high rainfall areas, hydrological basement drains well. However, mountains in low rainfall areas have poor drainage. Based on these results, we propose a “model of constant critical effective rainfall amount" concerning slope failure. This model indicates that the minimum amount of accumulated precipitation in low rainfall areas, which is where disasters occur, is the most appropriate factor for determining the critical effective rainfall amount (Dc). Additionally, our results indicate that Dc is 270mm. We calculated the runoff and drainage factor (b) of the one tank model, for each disaster area based on the hourly precipitation and the effective rainfall amount leading up to the time of the disaster. The results suggest a linear relationship between the runoff and drainage factor (b), which is represented by the one tank model for runoff and drainage, and local peculiarity rainfall (R1/2).

Key words�Fsurface failure, local peculiarity rainfall, tank model, surface soil layer, basement