It is empirically known that debris flows are more likely to occur during and after volcanic eruptions. This study conducted numerical simulations to examine the effects of reduced infiltration capacity and increased sediment supply owing to volcanic activity on the rainfall threshold that causes debris flows. Rainfall runoff analysis was performed using various rainfall intensities as constant input conditions. The occurrence of debris flows was evaluated based on theoretical and experimental formulas. The mechanism by which sediment in river channels transforms into debris flow was discussed in terms of two processes: sediment sliding as a mass and sediment being eroded by surface flow. This study clarified the critical role of infiltration capacity in debris flow occurrence. When infiltration capacity is low, surface runoff becomes more dominant, easily entraining sediment and triggering debris flows—even during short or weak rainfall events. Quantitative analysis revealed that when infiltration capacity drops from 100 mm/h to 10 mm/h, the cumulative rainfall required to trigger debris flows decreases to only a few percent of the pre-eruption requirement. However, when volcanic activity supplies large volumes of sediment to river channels, the mechanism can shift to mass sliding. In such cases, debris flows may occur even under relatively weak but prolonged rainfall. The results indicate the importance of both sediment erosion and mass sliding processes in debris flow occurrence.

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