Long-term evolution of debris flow activities at the epicenter of Wenchuan earthquake

The 2008 Wenchuan earthquake triggered numerous landslides in the stricken area. The loose landslide materials on mountain slopes and in gullies are prone to reactivation and may transform into debris flows in the rainy season. Nine years after the 2008 Wenchuan earthquake, landslide activities are still active in the study area near the epicenter of the earthquake in Sichuan, China. The debris flows blocked the Minjiang and Yuzixi river and caused severe damage to nearby villages and reconstruction sites. How did the mechanisms evolve in these repeated debris flows? We evaluated the loose deposit volumes in catchments of the triangle study area before and after each heavy storm since the earthquake, and analyzed the triggering rainfall intensities, initiation mechanisms and runout characteristics of these debris flows. The preliminary results of a case study in Gaojiagou Ravine show that the rainfall threshold for debris flows increased with time, the initiation mechanisms evolved from landslides to channel-bed failure and subsequently to channel-bank erosion. The mobility of the debris flows decreased from 2010 to 2016 as the initiation positions moved lower and the particle size of the runout materials became coarser.