Multiply factors driving continual post-wildfire debris flows with varied rainfall thresholds in the Reneyong Valley, southwestern China

Abstract. In early June of 2014, wildfire struck the Reneyong Valley in the central Hengduan Mountains of southwestern China. Three days after the wildfire, the first debris flow was triggered in branch No. 3, followed by 2 other debris flows that same year. In August 2015, another debris flow occurred in branches No. 1, No. 2 and No. 3, respectively. Rainfall data from three nearby rain gauges and rainfall totals speculated from debris flow volume suggest the three debris flows in 2014 were generated by isolated convective rainfall. Later, we found that varied rainfall thresholds existed among the branches and that these thresholds might be related to the geological and geomorphic characteristics. The results show that (1) the thresholds of post-fire debris flows tend to increase as time passes; (2) post-fire debris flows in the Reneyong Valley occur with high frequency not only because of the loss of the natural canopy, the occurrences of an ash layer and dry ravels and an increase in soil water repellency but also because of the geology, drainage area, channel gradient and regional arid climate, which may not be affected by wildfire; and (3) the varied rainfall thresholds among the different branches are dependent on the drainage area, as entrainment is controlled by the magnitude of discharge.