Intensity-duration threshold of rainfall-triggered debris flows in the Wenchuan Earthquake affected area, China

The Ms 8.0 Wenchuan Earthquake has greatly altered the rainfall threshold for debris flows in the affected areas. This study explores the local intensity-duration (I-D) relationship based on 252 post-earthquake debris flows. It was found that I = 5.25 D-0.76 accounts for more than 98% of the debris flow occurrences with rainfall duration between 1 and 135 h; therefore the curve defines the threshold for debris flows in the study area. This gives much lower thresholds than those proposed by the previous studies, suggesting that the earthquake has greatly decreased the thresholds in the past years. Moreover, the rainfall thresholds appear to increase annually in the period of 2008-2013, and present a logarithmic increasing tendency, indicating that the thresholds will recover in the future decades.