Critical rainfall for typhoon-induced debris flows in the Western Foothills, Taiwan

Typhoon Morakot devastated Taiwan during August 6–11, 2009, with a maximum total rainfall of 2884 mm, causing > 22,000 cases of slope failures. Rainfall thresholds for debris flow initiation are investigated using the hourly intensity vs. duration (I–D), hourly intensity vs. cumulative rainfall (I–R), and daily rainfall vs. antecedent daily rainfall (ADR) index methods. Among the 39 well-documented cases, 32 sites with known date and time of debris flow initiation were focused. Results reveal that debris flows and debris avalanches occur under a very wide range of rainfall characteristics, with return periods ranging from several years to 200 years, indicating that the empirical rainfall threshold alone is insufficient for an effective debris flow prediction system. A consistent pattern of all I–R paths for the investigated sites shows the combined effect of rainfall characteristics and inherent stability features of the slopes, suggesting that the I–R path is potentially useful in facilitating debris flow predictions. It is also suggested that empirical rainfall thresholds should be calibrated using physical models against specific slope conditions to increase the accuracy of debris flow predictions.