Study on the development characteristics of post-fire debris flow and its early warning risk aversion in Zhongba Village, Xide County
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摘要:
2020年5月7日四川省凉山彝族自治州喜德县鲁基乡中坝村发生森林火灾，过火面积约6.7 km2。林火后火烧迹地坡面灰烬厚约2～6 cm，同时表层土壤结构被扰动，在6月4日傍晚20 min左右的强降雨下，中坝村后山4条沟道同时暴发了火后泥石流灾害，对沟口的41户164名居民构成严重威胁。通过遥感解译、野外调查，查明了火烧区4条沟道的火烈度及物源分布特征，分析了火后泥石流的发育特征，并进行了危险度评估。基于火后泥石流暴发降雨阈值、实时雨量监测数据与群测群防于一体，提出了一套适用于山区火后泥石流的预警避险方案，在该沟得到成功应用，并确保了沟口居民安全。
Abstract:On May 7, 2020, a forest fire broke out in Zhongba Village, Luji Township, Xide County, Liangshan Yi Autonomous Prefecture, Sichuan Province, covering an area of about 6.7 km2. After the forest fire, the ash on the slope was about 2～6 cm thick, and the surface soil structure was disturbed. Under the heavy rainfall of about 20 min in the evening of June 4, the post-fire debris flow disaster broke out in the four gullies at the back mountain of Zhongba Village, which is a great threat to the 164 residents of 41 households in the gully mouth. Through remote sensing interpretation and field investigation, the fire intensity and deposit distribution characteristics of four gullies in the burned area are identified, the development characteristics of post-fire debris flow in Zhongba Village, Xide County are analyzed, and the risk assessment is carried out. Based on the real-time rainfall data obtained from group survey and group prevention, a set of early-warning and risk avoidance scheme for debris flow in mountain areas after fire is proposed, which has been verified by this example.
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表 1 中坝村后山火烧迹地不同强度火烈度判别特征
Table 1. Distinguishing characteristics of fire intensity of different intensity in Zhongba Village burned area
火强度 特点 未火烧 火烧前后地表覆盖物无变化 轻度火烧 超过50%的枯枝落叶未完全燃烧 中度火烧 大部分枯枝落叶被烧毁，但是大部分粗可燃物未完全燃烧 严重火烧 枯枝落叶和粗可燃物均被完全烧毁，地表为灰烬覆盖 
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表 2 中坝村森林火烧区不同火烈度面积占比数据统计
Table 2. Statistics on the proportion of different fire intensity areas in the forest fire area of Zhongba Village
/% 火烧区 轻度火烈度区面积占比 中度火烈度区面积占比 重度火烈度区面积占比 中度及重度火烈度区面积占比 整个火烧迹地 14.31 19.76 65.93 85.69 1#沟 2.11 26.56 69.41 95.97 2#沟 3.69 16.94 78.37 95.32 3#沟 3.18 18.51 74.78 93.28 4#沟 7.36 26.52 63.82 90.33
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表 3 火烧区4条泥石流沟形态特征统计
Table 3. Morphological characteristics statistics of 4 debris flow ditches in burned area
沟道编号 流域面积/km2 主沟长/km 主沟纵坡比降/‰ 相对高差/m 岸坡坡度/（°） 沟谷形状 1# 0.13 0.652 450 244 40～50 “V”形 2# 0.23 0.893 429 317 40～50 “V”形 3# 0.36 1.040 360 326 35～45 “V”形 4# 0.73 2.090 311 395 35～45 “V”形
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表 4 火烧区各泥石流沟动、静储量统计
Table 4. Statistics of dynamic and static reserves of debris flow gully in burned area
/(×104 m3) 沟号 物源储量 崩滑物源 沟道物源 坡面侵蚀物源 总计 1#沟 静储量 1.10 0.20 3.20 4.50 动储量 0.33 0.06 0.32 0.71 2#沟 静储量 1.60 0.23 8.40 10.23 动储量 0.48 0.07 1.10 1.65 3#沟 静储量 1.32 0.25 14.00 15.57 动储量 0.39 0.08 2.00 2.47 4#沟 静储量 0.75 0.20 35.00 35.95 动储量 0.22 0.06 4.80 5.08
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表 5 中坝村不同频率降雨强度值
Table 5. Rainfall intensity values of different design frequencies in Zhongba Village
降雨时段/h 设计频率/% 20 10 5 3.33 2 1 H1/6P 19.23 23.02 26.63 28.69 31.23 34.63 H1P 45.70 55.95 65.86 71.55 78.64 88.14 H6P 86.73 107.16 127.01 138.45 152.72 171.89 H24P 90.43 109.23 127.25 137.54 150.32 167.40
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表 6 中坝村2020年“6·4”泥石流暴发实时降雨量监测数据
Table 6. Real-time rainfall monitoring data of 6·4 debris flow outbreak in Zhongba Village in 2020
雨量站位置 距中坝村距离/
km10 min雨量/
mm1 h雨量/
mm漫水湾镇松林村1-4组 4.5 11.7 15.4
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表 7 中坝村火后泥石流各沟道沟口平均流速计算结果
Table 7. Calculation results of the average flow velocity of each channel gully of post-fire debris flow in Zhongba Village
沟道编号 1# 2# 3# 4# 泥深/m 1.3 1.4 1.6 1.0 主沟纵坡降/‰ 450 429 360 311 平均流速/(m·s−1) 7.85 7.28 8.04 7.03
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表 8 中坝村2020年6·4火后泥石流峰值流量计算结果
Table 8. Calculation results of peak flow of 6·4 post-fire debris flow in Zhongba Village in 2020
沟道编号 1# 2# 3# 4# 平均流速/(m·s−1) 7.85 7.28 8.04 7.03 计算断面面积/m2 5.88 11.56 11.31 2.75 峰值流量/(m3·s−1) 46.17 74.19 90.92 19.33
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表 9 中坝村1#~4#沟道沟域基础数据
Table 9. Basic data of 1#~4# trench trench area in Zhongba Village
沟道编号 泥石流规模m/
(103 m3)暴发频率f/
(次·100年−1)流域面积s1/
km2主沟长度s2/
km流域相对高差s3/
km流域切割密度s6/
km不稳定沟床比例s9/
%1# 6.30 32.43 0.130 0.652 0.244 9.33 76 2# 11.40 28.73 0.230 0.893 0.317 9.04 85 3# 12.30 27.98 0.360 1.040 0.326 9.00 80 4# 2.60 56.15 0.730 2.090 0.395 11.40 65
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表 10 中坝村泥石流1#~4#沟危险度评价
Table 10. Evaluation of dangerous degree of ditch 1#~4# in Zhongba Village
沟道编号 1# 2# 3# 4# 泥石流规模M/103 m3 0.266 0.352 0.363 0.138 暴发频率F/(次·100年−1) 0.756 0.729 0.723 0.875 流域面积S1/km2 0.085 0.130 0.160 0.225 主沟长度S2/km 0.175 0.285 0.296 0.416 流域相对高差S3/km 0.163 0.211 0.217 0.379 流域切割密度S6/km 0.466 0.452 0.450 0.570 不稳定沟床比例S9/% 1 1 1 1 评价结果 0.42 0.45 0.46 0.48
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表 11 单沟泥石流危险度分级标准
Table 11. Classification standard of danger degree of single ditch debris flow
单沟泥石流危险度 0.0～0.2 0.2～0.4 0.4～0.6 0.6～0.8 0.8～1.0 危险度分级 极低危险 低危险度 中危险度 高危险度 极高危险
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