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
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摘要:
2014年6月1日，中国西南中横断山区仁额拥沟遭受了一次森林大火，火后的短时低雨强降雨在3#支沟内激发了3次泥石流；2015年8月的一次强降雨在仁额拥沟的1#、2#和3#支沟及其他附近的多个更小的流域内均激发了泥石流。为了解火后泥石流的降雨响应特征，文章采用距离修正的方式处理降雨数据，对4次泥石流进行降雨过程分析，查明了流域特征对泥石流启动的作用及其对各支沟泥石流不同降雨阈值的影响。研究发现：（1）泥石流降雨阈值在火后非常低且有随时间推移而增大的趋势；（2）仁额拥沟火后泥石流具有高频率特征，其原因除了在火后天然植被的破坏后降雨对坡面径流和侵蚀效应的放大，流域本身的性质也有极大的贡献；（3）各支沟内泥石流降雨阈值差异的主要原因在于流域面积的差异，泥石流侵蚀受制于径流量大小。
Abstract: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.
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表 1 1#—3#支沟基本信息
Table 1. Geographic information of branches No.1−No.3
编号 流域面积/km2 沟长/m 平均坡度/（°） 1# 0.37 544 27 2# 0.73 755 26 3# 2.30 836 15 
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表 2 1#—3#支沟火烧强烈程度及面积统计
Table 2. Summary of fire severity classification and affected areas of branches No.1−No.3
编号 火烧面积/km2 轻度火烧/km2 中度火烧/km2 1#支沟 0.31 0.21 0.10 2#支沟 0.64 0.45 0.19 3#支沟 2.11 0.6 1.51
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表 3 泥石流基本情况
Table 3. Basic property information of the debris flows
序号 时刻 位置 冲出量
/（104 m3）前期3 d
降雨量/mmDF1 2014年6月8日16:08 3# 0.8 1.11 DF2 2014年6月30日16:00 3# 1.4 14.84 DF3 2014年7月10日23:20 3# 0.5 0.81 DF4 2015年8月24日19:43 1#、2#、3# 2.2 39.97
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表 4 3处雨量站基本情况
Table 4. Summary of the basic property information of the three rain gauges
序号 气象站名 海拔/m 距泥石流沟
距离/km精度/mm 
1 正斗站 2858 4.10 0.1 0.577 2 热打站 3363 5.56 0.1 0.313 3 阿都站 2783 9.40 0.1 0.110
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