Mechanism of the post-fire debris flow of the Xiangshui gully in “3·30” fire area of Xichang,Sichuan Province
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摘要:
2020年3月30日，西昌市经久乡发生森林大火，响水沟流域植被被林火大面积烧毁，同年雨季，响水沟流域内多条沟道暴发泥石流，其中1#、2#、3#沟毗邻居民房屋和耕地，影响较为严重。通过野外调查、遥感解译和室外试验，以响水沟1#、2#、3#沟为研究对象，分析了不同林火烈度下，渗透特征、坡面侵蚀和沟道侵蚀的差异，从而揭示响水沟火后泥石流的成灾机理。结果表明，林火是泥石流暴发的重要诱因，火后泥石流的降雨阈值会明显降低。林火干扰导致坡面土壤的渗透系数表现出不同程度的降低，林火烈度越严重的区域，渗透系数越小，降雨更大比例地转化为坡面径流参与到坡面侵蚀。随降雨次数的增多，轻度、中度、重度火烧区域的坡面土壤侵蚀深度均增加；中度、重度林火烈度的侵蚀深度差异不大，且明显高于轻度区域，说明当林火烈度达到中度时，坡面土壤便会受到较大程度的侵蚀。地形条件相似的沟道，林火烈度越严重，泥石流侵蚀能力越强，最终体现于沟道两岸崩滑体数量越多，沟道宽度和深度越大。
Abstract:On March 30, 2020, a forest fire broke out in Jingjiu Township, Xichang City, and the vegetation in Xiangshui gully watershed was burned in a large area by forest fire. In the rainy season of the same year, debris flow broke out in many gullies in the Xiangshui gully watershed, of which 1#, 2#, 3# gullies were adjacent to residential houses and cultivated land, and the impact of the debris flow was relatively serious. Through field investigation, remote sensing interpretation, and outdoor test, taking Xiangshui gully 1#, 2#, 3# gully as the research object, the differences of permeability characteristics, slope erosion, and gully erosion under different forest fire intensities are analyzed, so as to reveal the disaster mechanism of post-fire debris flow of Xiangshui gully. The results show that forest fire is an important inducement for the outbreak of debris flow, and the rainfall threshold of debris flow after the fire will be significantly reduced. Forest fire disturbance leads to the decrease of the permeability coefficient of slope soil in varying degrees. The more serious the forest fire intensity is, the smaller the permeability coefficient is, and the rainfall is transformed into slope runoff in a larger proportion to participate in slope erosion. With the increase of rainfall times, the depth of slope soil erosion in light, moderate and severe fire areas increased. The difference of erosion depth between moderate and severe forest fire intensity is small, and it is significantly higher than that in light areas, indicating that when the forest fire intensity reaches moderate, the slope soil will be eroded to a greater extent. For the gully with similar topographic conditions, the more serious the forest fire intensity is, the stronger the erosion capacity of debris flow is, which is finally reflected in the greater the number of landslide bodies on both sides of the gully and the greater the width and depth of the gully.
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表 1 研究区各沟道地形特征参数
Table 1. Topographic characteristic parameters of each channel in the study area
沟名 汇水
面积
/km2主沟
长度
/m流域相对
高差/m流域切
割密度
/（km·km−2）沟道纵坡降/‰ 1# 0.28 1059 402 10.50 305 2# 0.96 1 902 632 9.74 308 3# 0.25 969 305 10.51 323 
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表 2 研究区泥石流暴发情况统计表
Table 2. Statistics of debris flow outbreaks in the study area
沟
名是否发生泥石流 5月1日 6月17日 6月23日 7月18日 1# × × √ √ 2# × √ √ √ 3# √ √ √ √ 注：“√”代表暴发泥石流；“×”代表未暴发泥石流。
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表 3 研究区各沟道林火烈度分布情况
Table 3. Distribution of forest fire intensity in each gully in the study area
沟名 未火烧/% 轻度
火烧/%中度
火烧/%重度
火烧/%1# 68.24 8.10 20.35 3.31 2# 10.78 9.90 37.25 42.07 3# 25.25 32.86 32.73 9.16
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表 4 中度、重度火烧区与泥石流规模和次数的关系
Table 4. Relationship between forest fire intensity and debris flow scale and times
沟名 中度、重度林火烈度
占比/%泥石流累计规模
/（104 m3）泥石流
暴发次数1# 23.66 0.24 2 2# 79.32 2.24 3 3# 41.89 1.09 4
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表 5 中度、重度林火烈度占比与泥石流沟侵蚀的关系
Table 5. Relationship between the proportion of moderate and severe forest fire intensity and debris flow gully erosion
沟名 中度、重度林火
烈度占比/%崩滑物源
数量/个沟道宽度
/m沟道深度
/m1# 23.66 4 1.0～2.5 0.5～1.5 2# 79.32 22 3.0～6.0 2.0～5.0 3# 41.89 7 3.0～6.0 2.0～4.0
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表 6 研究区域不同降雨频率降雨强度值
Table 6. Rainfall intensity values of different rainfall frequencies in the study area
降雨时段 设计频率/% 20 10 5 2 1 10 min 17.57 20.57 23.38 26.92 29.51 1 h 44.87 53.71 62.14 72.88 80.80 6 h 71.81 87.92 103.50 123.57 138.50 24 h 91.39 111.90 131.72 157.28 176.27
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