Variability in rainfall threshold for debris flow after Wenchuan earthquake in Gaochuan River watershed, Southwest China

Wenchuan earthquake occurred 7 years ago, and more than 100 landslides and rock avalanches were triggered by the earthquake in Gaochuan river watershed, in Southwest China, including the Daguangbao landslide, which is the largest one triggered by Wenchuan earthquake, having a volume around 1.15 billion m(3). The massive landslide deposits provided uncountable material source for debris flow in the study area, resulting in 25 debris flow events occurred in the past 6 years (2009-2014). The purpose of this paper is to analyze the variability in the rainfall threshold for debris flow after the earthquake in the study area. Results suggest that the critical rainfall threshold is closely related to the rainfall intensity (I) and the antecedent accumulated precipitation (P). There was a sudden drop of the rainfall threshold when the earthquake happened. During the 6 years, the rainfall threshold value kept increasing gradually in a linear trend. However, it is now still much lower than the original rainfall threshold prior to the earthquake. The threshold is expected to reach the original level around 2022 based on the analysis of existing data. Our study indicates that the increase of rainfall threshold is related to the decrease of sediments. And there are still lots of loose deposits in the gullies. Thus, the debris flow can still happen in a high possibility in the next few years.