Reconstruction of Debris Flow Disasters in Polong Gully Based on Dendrochronology
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摘要: 迫龙沟是帕隆藏布下游右岸一级支流，1983、1984、1985和2015年爆发过4次泥石流，重复堵江并形成长约1 km的堰塞湖.个别主沟泥石流事件和沟道内部滑坡、支沟泥石流没有冲出沟口，无文献记录，但是造成沟道中上游河床演变和树木扰动.基于树木年代学，通过分析树木年轮损伤组织、生长抑制和释放动态，重建了迫龙沟40年来的主沟泥石流和沟道内部滑坡、支沟泥石流灾害历史；从树木生长扰动强度等方面探讨了泥石流的流动范围；并通过Wit指数进一步分析了没有历史记录的主沟泥石流、沟道内部滑坡、支沟泥石流发生时间和流动范围.树木年代学所反映的泥石流爆发时间的准确性与泥石流规模有关，泥石流规模太大会导致受扰动树木的灾害历史记忆消灭，不利于大规模灾害发生之前的泥石流事件的定年.沟道内的众多支沟泥石流和主沟内的滑坡会对主沟泥石流事件的定年产生干扰，但是与主沟泥石流同一时间发生的支沟泥石流和主沟滑坡事件会增大Wit指数，利于主沟泥石流灾害事件的定年.Abstract: Polong gully is a primary tributary on the right bank of Palong Zangbu River. Four recorded debris flows in 1983, 1984, 1985 and 2015 repeatedly blocked the river and formed a 1 km-long barrier lake. But some individual debris flow was too small to flush out the gully and be recorded. Based on the method of dendrochronology and the analysis of damaged tissue, growth inhibition and release of tree rings, the temporal-spatial distribution of the main debris flow events, debris flows in the tributaries and landslides in the main gully in the past 40 years were reconstructed. From the intensity of tree growth disturbance, the inundate area of debris flow was discussed. The relationship between the occurrence time of debris flow and inundate area was further analyzed by the index of Wit. The results show that the accuracy of debris flow event dating reflected by dendrochronology was related to the debris flow scale. Extra-large-scale of debris flow would lead to the elimination of disaster historical memory of disturbed trees, which was not conducive to the debris flow dating. The debris flows in the tributaries and landslides in the main gully would disturb the debris flow dating in the main gully. Debris flows in the gully tributaries and landslides in the main gully occurring at the same time would increase the value of Wit, which was conducive to debris flows dating in the main gully.
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Key words:
- debris flow /
- dendrochronology /
- damaged tissue /
- growth inhibition /
- disaster reconstruction /
- engineering geology
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图 2 5~9月份总降雨量与平均气温
Fig. 2. Total rainfall and averaged temperature from May to September
图 3 树木年轮生长扰动类型
a.创伤与愈伤组织；b.倾斜树木造成的年轮压缩；c.生长抑制及测量结果；d.生长释放及测量结果
Fig. 3. Type of growth disturbances
图 6 支沟泥石流和滑坡扰动树木的活动指数分布
Fig. 6. Activity index of the tree disturbance in the tributary and landslides
图 7 基于加权指数Wit的泥石流时间重建
Fig. 7. Reconstruction of debris flow dating by weighted index Wit
表 1 各种测年方法优缺点（赖忠平等，2021）
Table 1. Advantages and disadvantages of various dating methods(Lai et al., 2021)
方法 材料 优势 不足 光释光测年法 矿物（石英或长石） 测年范围广、测年物质易于获取、对沉积物进行直接测年 受沉积物的晒退、含水量和周围环境等因素影响，单一样品的不确定性较大，需尽可能密集采样 14C测年法 沉积物中有机质、泥炭、贝壳、骨骼、植物残体等 技术成熟，应用广泛，应用加速器质谱技术后测量精度高、速度快 测定堰塞湖等沉积物时受“碳库效应”会高估年龄；测年物质难寻、测年范围较小 宇宙成因核素暴露年代法 岩石 测年物质分布广泛、测年范围较广、可以直接测定年龄 测试成本高昂、实验条件受限, 难以展开大规模应用 火山灰测年法 火山灰 主要应用于火山地质灾害的测年, 测年精确度较高 测年样本难寻且测年尺度相对较短 树木年代学法 树木年轮 定年准确、分辨率高、连续性强和地域分布广泛 受年轮响应滞后性影响，存在误差；样本采集难度大且测年尺度相对较短 地衣测年法 地衣 测年精确度较高 受环境其他因素影响大，通常用以辅助其他测年法；样本难寻且测年尺度相对较短 
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表 2 生长扰动强度的分类标准（Kogelnig-Mayer et al., 2011）
Table 2. Classification of growth disturbances (Kogelnig-Mayer et al., 2011)
扰动类型 参数 Ti Ts Tm Tw 创伤及愈伤组织 存在创伤及愈伤组织 生长抑制 年轮变化(%) ≥60% ≥60% < 60% 持续时间（a） ≥5 < 5且≥2 ≤2 生长释放 年轮变化(%) ≥60% ≥60% < 60% 持续时间（a） ≥5 < 5且≥2 ≤2
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表 3 生长扰动的数量与类型
Table 3. Number and type of growth disturbances
生长扰动区域 生长抑制 生长释放 损伤及愈伤组织 数量 % 数量 % 数量 % 上游 38 60.3 24 38.1 1 1.6 扇体 21 42.9 24 49.0 4 8.1 阶地1 45 50.0 41 45.6 4 4.4 阶地2 25 56.8 18 41.0 1 2.2 阶地3 11 68.8 5 31.3 阶地4 6 100 滑坡1 10 66.7 5 33.3 滑坡2 3 100 泥石流支沟1 23 48.0 24 50.0 1 2.0 泥石流支沟2 27 52.0 25 48.0 泥石流支沟3 1 12.5 7 87.5 泥石流支沟4 14 45.2 17 54.8 泥石流支沟5 9 40.9 13 59.1
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