Spatial features of debris flows and their rainfall thresholds in the Wenchuan earthquake-affected area

The 2008 Wenchuan earthquake greatly altered the thresholds for rainfall-triggered debris flows within the affected area. Debris flows were widely and densely distributed, and they exhibited a range of differing local rainfall thresholds. This study looked at 518 debris flow events that occurred post-earthquake in order to analyze their spatial characteristics. The duration D (in h) and the average rainfall intensity I (in mm/h) that triggered the debris flows were determined for 252 of these events in order to analyze the spatial features of rainfall thresholds. Results show that 49 % of debris flows occurred in the highest-intensity seismic zone, 58 % occurred within 10 km of active faults, and 49 % occurred in areas with humid climate. Rainfall thresholds in these three regions were persistently lower than others. Moreover, debris flows were most frequent in watersheds smaller than 5 km(2), and rainfall thresholds tended to decrease with a decrease in watershed size. Given the abundant loose materials available throughout the study area, 11 extreme debris flow-prone sub-regions were selected to illustrate the spatial features of rainfall thresholds in relation to local climate conditions. The lowest and highest I-D thresholds in the sub-regions examined were I = 5.94D (-0.70) (2 < D < 53) and I = 21.4D (-0.58) (3 < D < 50), respectively. The lowest and mean rainfall intensities needed to trigger debris flows were power-related with the local maximum 1- and 24-h rainfall. By normalizing the rainfall intensity (I) by mean annual precipitation (MAP), the I (MAP)-D thresholds were determined. Normalized results showed that the lowest and highest I (MAP)-D thresholds were I (MAP) = 0.0034D (-0.55) (2 < D < 53) and I (MAP) = 0.0090D (-0.40) (3 < D < 51), respectively. Such results are useful for debris flow forecasting based on empirical rainfall thresholds and have implications for hazard and risk assessment in this region.