Rainfall Warning Model for Debris Flow Occurrence after Extreme Rainfalls: Recent Events in Chenyoulan River, Taiwan

Due to the high uncertainty surrounding extreme rainfall events and debris flow occurrence, combining probability and risk is a commonly used approach for predicting debris flows. The Chenyoulan River Watershed (CRW) in central Taiwan has experienced a major earthquake and multiple extreme rainfall events, making it a suitable area for this study. A rainfall warning model based on records of multiple debris flows and long-term rainfall data was used in this study. The model reflects the probabilistic characteristics of debris flow occurrences following earthquakes and extreme rainfall, and it has been successfully applied to predict recent debris flow events. Although the CRW experienced several severe debris flows in the past, it has not faced any major debris flow disasters for over a decade. However, in 2024, multiple debris flows were observed, many originating from the same location, with high recurrence. Fortunately, these events did not result in any injuries or fatalities. This study evaluates the model's adaptability to the rainfall events of 2024 and proposes an improved method for predicting rainfall thresholds, warning times, and risk levels for this type of debris flow. The findings offer valuable insights for future debris flow monitoring and prediction efforts.