Spatial Development Characteristics of Post-Fire Debris Flow in Bajiaolou Town
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摘要: 作为一种与林火密切相关的特殊泥石流,火后泥石流的形成具有明显的空间差异性,对其空间发育特征及影响因素的分析有利于此类泥石流的防灾减灾. 运用现场调查、室内外试验和遥感解译等手段,对雅江县八角楼乡火烧迹地15条沟道的火后泥石流发育特征及地形、林火、植被特征、降雨条件进行了多元分析. 结果表明:火烈度越强,土壤的物理力学性质劣化越严重,斥水性越强,渗透性越低,泥石流越易形成;流域过火面积越大,植被覆盖率越低,泥石流暴发频率越高;相对于大流域,小而陡的流域反而更易暴发火后泥石流. 此外,研究区内暴发的5次火后泥石流中有4次泥石流其激发雨强重复周期均小于5 a,且随着时间的增长激发雨强也逐渐升高.Abstract: As a special type of debris flows closely relevant to forest fire, the generation of post-fire debris flow often shows a spatial variation. It is of great significance to analyze the spatial development characteristics and its influencing factors for hazard mitigation and reduction. By means of field investigation, in-situ and laboratory tests, and remote sensing interpretation, multi-factor analysis was conducted on the 15 channels in the burned area in Bajiaolou town of Yajiang county in terms of their spatial development characteristics, topography, forest fire, vegetation and rainfall conditions. The results show that the higher the fire intensity, the stronger the soil water repellency and the lower the permeability; moreover, the physical and mechanical properties of soil deteriorate more seriously, which contribute to the generation of post-fire debris flow. The occurrence frequency of post-fire debris flow rises when the burned area become larger and the vegetation cover coefficient lower. In contrast to the large basins, the smaller basins show more vulnerability to post-fire debris flow for their short channel lengths and high longitudinal slopes. Besides, the recurrence time of the triggering rainfall intensity that corresponds to four of five debris-flow events in the study areas is less than 5 years, and the triggering rainfall intensity also increases with time.
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表 1 2018年火烧迹地泥石流暴发情况统计
Table 1. Statistics of post-fire debris flow occurred in burnt area in 2018
流域 是否发生泥石流和洪水 是否过火 暴发频次/次 易发性 6月13日 6月24日 6月30日 7月5日 7月12日 D1 × × × √ √ 是 2 L D2 √ × √ √ √ 是 4 H D3 √ √ √ √ √ 是 5 H D4 × × √ √ √ 是 3 L D5 √ √ √ √ √ 是 5 H D6 × × √ √ √ 是 3 L D7 √ √ √ √ √ 是 5 H D8 × × √ √ √ 是 3 L D9 √ √ √ √ √ 是 5 H D10 √ √ √ √ √ 是 5 H F1 × × × × × 是 0 N F2 × × × × × 是 0 N F3 × × × × × 是 0 N F4 × × × × × 否 0 N F5 × × × × × 否 0 N 注:“√”表示发生了泥石流,“×”表示只发生了洪水;“L”表示易发,“H”表示高易发,“N”表示不易发. 表 2 火烧迹地坡表土壤物理性质试验结果
Table 2. Tested physical properties of soil on slope in study area
火烈度 密度/(g•cm−3) 含水率/% 比重 干密度/(g•cm−3) 孔隙度/% 未火烧(对照组) 1.51 ± 0.08 (a) 4.77 ± 0.30 (a) 2.62 ± 0.01 (a,b) 1.44 ± 0.08 (a) 45.14 ± 3.08 (b) 轻度 1.30 ± 0.03 (a,b) 3.89 ± 0.44 (a,b) 2.62 ± 0.01 (a,b) 1.25 ± 0.03 (a,b) 52.28 ± 1.28 (a,b) 中度 1.20 ± 0.08 (b) 2.69 ± 0.34 (b) 2.63 ± 0.01(a) 1.17 ± 0.08 (b) 55.66 ± 3.17 (a) 重度 1.14 ± 0.04 (b) 1.14 ± 0.26 (c) 2.59 ± 0.01 (b) 1.12 ± 0.04 (b) 56.66 ± 1.40 (a) 注:表中数据为均值 ± 标准误差;a、b、c反映单因素方差分析结果,同列数据不同字母表示均值差异性显著(P值小于0.05). 表 3 各火烈度区坡表不同深度处不同斥水土壤样本含量占比
Table 3. Proportion of hydrophobic soil samples at different depths in areas with different fire intensities
深度/cm 火烈度 样本含量占比/% 斥水性土壤
占比合计/%N L M H 0 W 72.48 23.91 2.45 1.16 27.52 Q 42.22 26.93 7.81 23.04 57.78 Z 29.79 28.61 14.62 26.98 70.21 Y 25.42 28.59 12.05 33.94 74.58 1 W 93.04 4.16 1.53 1.27 6.96 Q 93.53 2.57 0.86 3.04 6.47 Z 51.07 32.01 7.64 9.28 48.93 Y 52.24 33.81 5.97 7.98 47.76 2 W 97.05 1.23 0.44 1.28 2.95 Q 97.12 1.54 0.53 0.81 2.88 Z 88.21 6.53 2.64 2.62 11.79 Y 87.53 7.62 1.56 3.29 12.47 3 W 97.13 0.82 1.21 0.84 2.87 Q 96.43 1.68 0.87 1.02 3.57 Z 96.49 1.07 0.99 1.45 3.51 Y 94.35 1.86 1.98 1.81 5.65 注:“W”为未火烧,“Q”为轻度火烧,“Z”为中度火烧,“Y”为重度火烧;“N”为亲水,“L”为轻度斥水,“M”为中度斥水,“H”为严重斥水. 表 4 各流域基本地形参数
Table 4. Basic topographic parameters of each basin
流域编号 面积/km2 流域高
差/m主沟长/km 主沟纵比降/‰ 沟道密度/
(km•km−2)D1 3.19 1430 3.88 336.04 1.87 D2 0.85 825 2.00 370.61 3.58 D3 0.25 650 1.28 501.70 8.80 D4 2.20 1445 3.35 395.28 2.67 D5 0.20 624 0.95 717.63 9.93 D6 0.31 761 1.17 722.32 6.42 D7 0.64 870 1.48 555.21 4.95 D8 2.82 1077 2.72 335.14 3.13 D9 0.31 592 0.97 590.58 6.29 D10 0.24 492 0.83 549.63 8.21 F1 4.13 1449 5.06 286.82 1.80 F2 6.98 1501 6.45 206.52 2.54 F3 5.15 1443 4.34 289.46 1.48 F4 0.44 697 1.13 581.34 6.18 F5 1.38 1062 2.81 348.70 3.01 -
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