Characteristics of burned area and susceptibility assessment of post-fire debris flow of Chongqing wildfire in August, 2022
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摘要:
2022年8月重庆市江津区、巴南区、北碚区相继爆发森林火灾，总过火面积约35 km2。火灾后火烧迹地植被及土壤结构遭到破坏，坡表堆积了厚度1～5 cm不等的灰烬层，受强降雨影响一旦爆发火后泥石流，将对附近村镇基础设施及群众生命财产构成严重威胁。通过遥感解译、现场调查及试验，查明了本次森林火灾火烧区空间分布特征，实测了火烧迹地灰烬层厚度、土壤饱和导水率等参数。结果表明研究区重度、中度和轻度火烧区分别占9.6%、34.0%和56.4%，其对应的坡面灰烬-结构扰动层平均厚度分别为 5.10，3.28 ，1.15 cm。基于专家经验预测模型对火烧区共95条潜在泥石流沟道进行火后泥石流易发性评价，其中1条为高等易发，43条为中等易发，44条为低等易发，7条为不易发，分别占总数的1.05%、45.26%、46.32%、7.37%。考虑到现状条件下各泥石流沟域内中度和重度火烧区仍残留有大量灰烬、泥沙等松散固体物源，且土壤表现出较强的斥水性，中等及高等易发性沟道在雨季一旦受集中降雨影响，爆发火后泥石流的可能性较大。研究成果可为火烧迹地火后泥石流灾害预防与风险管理提供科学依据。
Abstract:In August, 2022, forest fires occurred in Jiangjin District, Banan District, and Beibei District in Chongqing, respectively, with a total burned area of approximately 35 km2. After the wildfire, an ash layer with a thickness of 1～5 cm was deposited on the slope, and the vegetation and soil structure of the burned area were devastated. Affected by concentrated rainfall, it will pose a serious threat to the infrastructure of nearby villages and towns and residents’ lives and properties once the post-fire debris flow occurs. Based on the remote sensing, field investigation, and in-situ tests, the distribution characteristics of burned areas after the fire were identified. The measured data including the thickness of the ash layer, and the saturated hydraulic conductivity of the soil were derived. The severe, moderate, and low burned areas accounted for 9.6%, 34.0%, and 56.4% of the entire area, respectively, with the corresponding average thickness of ash and soil disturbance layer of 15.10 cm, 3.28 cm, and 1.15 cm. According to the prediction model based on expert experience, a total of 95 potential debris flow gullies in burned areas were evaluated for the susceptibility of post-fire debris flow, of which 1 was of high susceptibility, 43 were of medium susceptibility, 44 were of low susceptibility, and 7 were not prone to occur, contributing to 1.05%, 45.26%, 46.32%, and 7.37% of the total, respectively. Considering that a large number of loose solid sources besides ash and sediment still exist in the moderate and severe burned areas within watersheds under the current conditions, with strong water repellency of burned soil, once the medium and high susceptibility gullies are affected by concentrated rainfall in the rainy season, there is a relatively high probability for the post-fire debris flow occurrence. This study provides basic information for the prevention and risk management of post-fire debris flows.
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Key words:
- Chongqing wildfire /
- fire severity /
- burned area /
- post-fire debris flow /
- susceptibility
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表 1 研究区火烧迹地火烈度判别标准
Table 1. Fire severity criterion of burned areas in the study area
火烈度 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 重度火烧 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 研究区火烧迹地火烈度面积统计
Table 2. Fire severity area statistics of burned area in the study area
火烧迹地所在区域 火烈度 轻度火烧 中度火烧 重度火烧 过火面积/km2 面积/km2 占比/% 面积/km2 占比/% 面积/km2 占比/% 江津区 3.81 56.5 2.14 31.7 0.80 11.8 6.75 巴南区 8.24 54.4 5.06 33.5 1.83 12.1 15.14 北碚区 7.61 58.7 4.64 35.8 0.70 5.4 12.95 总计 19.66 56.4 11.84 34.0 3.33 9.6 34.84
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表 3 研究区中—重度火烧区面积不同占比沟道数量统计
Table 3. Number of gullies with different proportions in moderate and severe burned area in the study area
火烧迹地所在区域 中—重度火烧区面积占比 <15% 15%～30% 30%～60% >60 沟道数量/条 占比/% 沟道数量/条 占比/% 沟道数量/条 占比/% 沟道数量/条 占比/% 江津区 1 5.3 5 26.3 11 57.9 2 10.5 巴南区 19 34.6 13 23.6 11 20.0 12 21.8 北碚区 5 23.8 9 42.9 7 33.3 0 0 总计 25 26.3 27 28.4 29 30.5 14 14.7
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表 4 火后泥石流易发性评价因子等级划分及评分标准
Table 4. Grade division and scoring criteria of assessment factors of susceptibility
评价因子 不易发 低等易发 中等易发 高等易发 中—重度火烧区面积占比/% ＜15 15～30 30～60 >60 坡面灰烬-结构扰动层平均厚度/cm <1 1～2 2～3 >3 产沙区坡度/（°） <15 15～25 25～32 >32 主沟平均纵坡降/‰ <52 52～105 105～213 >213 流域面积/km² ≥100 <0.2或10～100 5～10 0.2～5 评分区间 [0,25） [25,50） [50,75） [75,100]
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表 5 火后泥石流易发性总评分等级划分标准
Table 5. Grading criteria for the total score of post-fire debris flow susceptibility
易发性 不易发 低等易发 中等易发 高等易发 打分 [0,25） [25,50） [50,75） [75,100]
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表 6 重庆森林火灾不同火烧区火后泥石流沟道易发性数量统计
Table 6. Statistics on the susceptibility of debris flows in different burned areas of Chongqing wildfire
/条 火烧区 沟道数量 高等易发 中等易发 低等易发 不易发 江津区 19 0 12 7 0 巴南区 55 1 23 25 6 北碚区 21 0 8 12 1 总计 95 1 43 44 7
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