Probabilistic Estimation of Debris-Flow Occurrence after Extreme Rainfalls

This article studied the debris-flow occurrence and the corresponding extreme rainfall events, as well as a severe earthquake, the Chi–Chi earthquake (CCE), in Chenyulan watershed, Taiwan. By examining the associations between the rainfall index (RI), defined as the product of the maximum 24-h rainfall and maximum hourly rainfall of a rainfall event, the return period (T), and the probability (P) of debris flow after an event, an empirical model of the P–T relationship was developed and this model was applied to evaluate an extreme rainfall event, Typhoon Morakot. Results of this study showed that debris flows could be triggered at lower RI values, corresponding with lower T values, for rainfall events after extreme rainfalls or after the CCE. The extreme events mostly had T values exceeding eight years. The T value for the critical RI affected by the CCE was approximately one year, much smaller than that affected by extreme events. P rose significantly after an extreme rainfall event or the CCE at the same T. The P value affected by CCE was markedly higher than that affected by extreme rainfalls. In addition, field data of debris-flow occurrence and rainfall between 2012 and 2014 were collected to assess the proposed model. The model was successfully applied to evaluate the probability of debris-flow occurrence after extreme rainfalls.