Analysis of the rainfall threshold for post-fire debris flow initiation: A case study of the debris flow at Ren’eyong gully in Xiangcheng County, Sichuan Province

In the early summer of 2014, a wildfire ravaged the Ren’eyong valley in the central Mt. Hengduan region of southwestern China. Following the blaze, debris flows were triggered three times in branch No. 3 due to short-term, low intensity rainfall. A year later, in August 2015, a brief period of high-intensity rainfall generated debris flows not only in branch No.3, but also in branch No. 1 and No. 2, as well as several smaller basins in the vicinity. To investigate the rainfall response characteristics of post-wildfire debris flow, the distance correction method was used to process the rainfall data. By analyzing the rainfall patterns of four debris flow events, the reseachers were able to identify the effects of watershed characteristics on the initiation of debris flow and its influence on different rainfall thresholds in each branch. The study found that: 1） Post fire debris flows can occur at a low rainfall threshold, which tends to increase over time. 2） The Ren’eyong valley experience high-frequency post fire debris flows, which can be attributed not only to the amplification of slope runoff and erosion caused by rainfall after the destruction of natural vegetation due to the wildfire, but also to the geological and geomorphic conditions of the area. 3） The rainfall threshold in each branch is primarily dependent on the drainage area, as the magnitude of discharge controls the entrainment.