Rainfall thresholds for debris flows occurrence in the Wenchuan earthquake area

Values of rainfall parameters like the cumulative rainfall, the mean rainfall intensity and the rainfall duration, depend on the definition of a single continuous rainfall event. In this study, a revised method is presented for defining a single continuous rainfall event. Using rainfall data collected during the occurrence of debris flows in the Wenchuan earthquake area, single continuous rainfall events and derived values of rainfall parameters are determined with the Jan's method and a revised method proposed in this paper. Empirically-based rainfall thresholds for debris flows occurrence (based on the cumulative rainfall, the mean rainfall intensity and the rainfall duration) are assessed following the two methods for defining single rainfall events. The threshold values obtained using the two methods are validated with debris flows events, which occurred in the Qingping town of Mianzhu city in the Sichuan province. Results show that the cumulative rainfall threshold values slightly differ for the two methods. The mean rainfall intensity threshold obtained by the revised method is generally smaller than that of the Jan's method. This is due to the fact that the revised method is able to improve the shortcoming of the Jan's method often resulting in a shorter duration of a rainfall event. The revised method proved to be both rational and feasible. The results show that the rainfall thresholds obtained by the revised method can be used to predict the debris flows in the Wenchuan earthquake area.