FORMATION CONDITIONS OF LOW FREQUENCY DEBRIS FLOW IN WESTERN SICHUAN MOUNTAINOUS AREA: A Case Study of Xiangzhagou Debris Flow in Jiuzhaigou County
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摘要:
近年来川西山区屡次暴发低频泥石流灾害，造成大量生命财产损失，危害性较大.通过对九寨沟县香扎沟泥石流进行深入调查、访问，分析了沟域地形地貌特征以及泥石流物质组成和暴发特征，揭示了泥石流的物质大多源于沟床启动，且与早年林木采伐、木材腐朽密切相关.另外，集中强降雨及陡峻的地形也是泥石流形成的有利条件.通过对比中、高频泥石流，提出低频泥石流具有隐蔽性、突发性、危害大的特点.川西山区有过采伐史且具备相应地形条件的沟域，应作为低频泥石流灾害的重点调查研究和防治对象.
Abstract:In recent years, low-frequency debris flow disasters often occurred in western Sichuan mountainous area, causing massive loss of life and property with great harm. Through the in-depth investigation and visit to Xiangzhagou debris in Jiuzhaigou County, the authors analyze the topographic and geomorphic features of gully area, as well as material compositions and outbreak characteristics of debris flow. It is concluded that the materials of debris flow are mostly initiated from gully bed and closely related to forest cutting and wood decay in early years. Besides, concentrated heavy rainfall and steep terrain also contribute to the formation of debris flow. Comparing with the medium- and high-frequency debris flows, the low-frequency debris flow has the characteristics of being hidden, sudden and dangerous. The gully areas with a logging history and corresponding topographic conditions in western Sichuan mountainous area should be the focus of investigation and prevention of low-frequency debris flow disasters.
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Key words:
- low-frequency debris flow /
- geological disaster /
- wood decay /
- rainfall /
- Jiuzhaigou Valley /
- Sichuan Province
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表 1 香扎沟流域各支沟地形地貌特征
Table 1. Landform characteristics of branch gullies of Xiangzhagou valley
编号 地形地貌特征 N1 位于主沟左岸，距主沟沟口约450 m，沟长约2.44 km，纵比降352‰；左岸坡度45~60°，局部近直立，基岩出露，为变质砂岩、板岩；右岸坡度35~50°，崩坡堆积层由含粉质黏土碎块石组成；沟内常年有水，流量0.025 m3/s，在降雨时也为清水，沟床及岸坡植被茂密，植被覆盖率 > 80% N2 位于主沟下游鸟饶附近，主沟左岸，沟长约2.0 km，纵比降515‰；左岸坡度30~45°，崩坡堆积层由含粉质黏土碎块石组成，未见明显的裂缝及滑坡、崩塌，整体稳定性较好；右岸坡度35~50°，为崩坡堆积层，主要由含粉质黏土碎块石组成，整体稳定性较好；沟内常年有水，流量0.008 m3/s，洪水时颜色略深，沟床及岸坡植被茂密，植被覆盖率 > 90% N3 位于主沟下游香扎村2组居民房附近，源头位于曹结卡，主沟左岸，沟长约3.64 km，纵比降385‰，两岸坡度40~55°；崩坡堆积层由含粉质黏土碎块石组成，未见明显的裂缝及滑坡、崩塌，整体稳定性较好；沟内常年有水，流量0.3 m3/s，洪水时颜色略深，沟床及岸坡植被茂密，植被覆盖率 > 80%；近年来沟床下切较深，约0.5~1 m，切深处土体裸露，沟床见伐木残留的枯树枝等 N4 位于主沟下游香扎村2组居民房附近，主沟右岸，沟长约1.58 km，纵比降323‰，两岸坡度40~55°；崩坡堆积层由含粉质黏土碎块石组成，未见明显的裂缝及滑坡、崩塌，整体稳定性较好；沟内常年有水，流量0.2 m3/s，洪水时颜色略深，沟床及岸坡植被茂密，植被覆盖率 > 90%，沟床见枯树枝 N5 位于香扎村3组居民房上游，主沟右岸，沟长约2.42 km，纵比降236‰，沟床宽缓（5~10 m，局部大于15 m），呈“U”型，两岸坡度30~50°；崩坡堆积层由含粉质黏土碎块石组成，未见明显的裂缝及滑坡、崩塌，整体稳定性较好；沟内常年有水，流量0.12 m3/s，洪水时颜色略深；曾进行过伐木，并于1971年暴发过小型泥石流，但未进入主沟，目前沟床仍见伐木残留的枯树枝及树桩；岸坡植被茂密，植被覆盖率 > 80% N6 位于香扎村3组居民房上游，距离N5沟口约500 m，主沟左岸，沟长约4.75 km，纵比降291‰，沟床较狭窄（2~5 m），两岸坡度30~50°；崩坡堆积由含粉质黏土碎块石组成；曾暴发“8·6”泥石流，下切原始沟床3~6 m，在两岸形成较陡的土质边坡（50~60°），在水流、降雨等因素作用下，局部出现垮塌；沟床内见伐木残留的枯树枝及树桩，岸坡植被茂密，植被覆盖率 > 80%；沟口有大量泥石流固体物质堆积，主要为碎块石；沟内水流量0.5~1 m3/s N7 位于主沟左岸，沟长约2.89 km，纵比降329‰；下游沟道宽缓（5~10 m，局部大于10 m），两岸岸坡基岩出露坡度 > 50°，局部近直立，为变质砂岩、板岩，整体稳定性较好；中上游主要由崩坡积层含粉质黏土碎块石组成，未见明显的裂缝及滑坡、崩塌，整体稳定性较好，两岸坡度35~50°；沟内常年有水，流量0.3 m3/s，洪水时颜色较深；沟床见大量伐木后形成的原木、枯枝、腐根堆积，岸坡植被茂密，植被覆盖率 > 90% N8 位于主沟右岸，沟长约1.55 km，纵比降548‰，两岸坡度30~45°；崩坡堆积由含粉质黏土碎块石组成，未见明显的裂缝及滑坡、崩塌，整体稳定性较好；沟内常年有水，流量0.05 m3/s，洪水时颜色浅；沟床分布有枯枝、残根，岸坡植被茂密，植被覆盖率 > 90% 
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表 2 沟床堆积物颗粒分析统计表
Table 2. Grain size analysis statistics of gully bed deposits
取样位置 样品号 粒度组成比例/% > 200 mm 200~20 mm 20~2.0 mm 2.0~0.5 mm 0.5~0.25 mm 0.25~0.075 mm 0.075~0.002 mm N3（QJ14） RT-2 15.2 48.3 16.5 5.6 3.3 8.5 2.6 N4（海拔2710 m） RT-3 12.3 45.5 16.1 8.7 5.3 6.3 5.8 N5（海拔2770 m） RT-4 17.8 38.8 20.1 11.4 6.4 4.3 1.2 N6下游（海拔2710 m） RT-5 26 49.1 6.3 5.2 4.1 4.6 4.7 N6中游（QJ9） RT-6 23.2 47.1 7.1 6.3 3.2 5.1 8.0 N6上游（QJ13） RT-7 19.3 53.3 6.8 3.8 4.8 3.7 8.3 N7（海拔2930m） RT-8 12 39.6 18.3 10.4 6.7 5.8 7.2 主沟上游（海拔3140 m） RT-9 13.1 37.1 10.3 9.4 7.1 13.8 9.2 主沟中游（海拔2600 m） RT-10 13.5 52.5 9.5 7.9 3.5 5 8.1 主沟中游（海拔2690 m） RT-11 14.9 54.1 9.1 7.9 3.2 4.5 6.3
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