Evolutionary history of post-fire debris flows in Ren’e Yong valley in Sichuan Province of China

Abstract

Post-fire debris flows have attracted considerable research attention because of their unique characteristics associated with forest fire. Most studies focus on debris flows that occur in the early post-fire years, the understanding of long-term debris-flow risk in burned areas was poor due to lack of evolutionary evidences that derive from field investigations. Through a tracking investigation of seven debris flow events with total seventeen debris flows during the six post-fire years in the burned area in Sichuan province of China, this study analyzed the evolutionary characteristics of post-fire debris flow. The results indicate that the frequency of debris flow events was the highest (five events per year) in the first post-fire year, which then decreased to twice in the following 5 years. Moreover, triggering and antecedent rainfall of these debris flow events increased over time. The material erosion in the initiation processes of debris flow followed the time sequence from runoff-initiated erosion to bank slide erosion to multi-stage bank slide and shallow landslide erosion. Burn severity correlated weakly to bank slides despite that it was directly proportional to runoff-initiated erosion. The particle size of debris-flow deposition gradually increased over time. More than 50% of the multi-stage landslides were undergoing deformation processes until the sixth post-fire year, acting as the potential source material for subsequent debris flows. Finally, a conceptual model with three material erosion steps and two consumptions was proposed to describe the evolution characteristics of post-fire debris flows. The results may provide scientific basis for the long-term debris-flow risk evaluation and the design of protection program in a recently burned area.