On the causes of “8·4” debris flow disaster at Zechawa Gully
GONG Xinglong1,2,3, CHEN Kunting4, CHEN Xiaoqing1,2,3, YOU Yong1, CHEN Jiangang1,2, ZHAO Wanyu1, LANG Jie5
1. Key Laboratory of Mountain Hazards and Earth Surface Processes, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences(CAS), 610041, Chengdu, China; 2. CAS Center for Excellence in Tibetan Plateau Earth Sciences, 100101, Beijing, China; 3. University of Chinese Academy Sciences, 100049, Beijing, China; 4. General Research Service Center, Department of Soil and Water Conservation, National Pingtung University of Science and Technology, 91201, Pingtung, Taiwan, China; 5. Jiuzhai Valley National Park Administration Bureau, 623402, Jiuzhaigou, Sichuan, China
Abstract：[Background] On 4 August 2016, a debris flow event occurred in Zechawa Gully, Jiuzhai Valley, Sichuan province. During this debris flow event, the pedestrian walkways were buried, and the only scenic road from Nuorilang Waterfall to Long Lake was blocked, causing serious impact on the scenic area. The rainfall data from precipitation stations showed that the maximum 1 h rainfall and 24 h cumulative rainfall on 4 August 2016 were 10.0 mm and 17.5 mm respectively, which were far below the rainfall intensity of the study area at once in 10-year, and the scale of the debris flow disaster was inconsistent with rainfall. [Methods] To clarify the cause of the debris flow disaster, field investigations were conducted, and the debris flow peak discharge was obtained by the cross-section survey method, rain-flood method and dam breach analysis. [Results] The debris flow peak discharges at the two typical mud mark cross sections estimated by the cross-section survey method were 33.29 m3/s and 36.69 m3/s respectively, which were similar to the debris flow peak discharge (32.73 m3/s) at once in 20-year period. The debris flow peak discharges calculated by dam breach analysis varied in 36.5 m3/s to 43.6 m3/s, which were similar with the peak discharge obtained by the cross-section survey method. The total volume of the debris flow material calculated by the equation recommended by Specification of Geological Investigation for Debris Flow Stabilization was 1.48×104 m3, which was consistent with the field measured value 1.39×104 m3, and further supported the calculated debris flow peak discharge obtained from the cross-section survey method and dam breach analysis. [Conculsions] Based on our research results, the debris flow on 4 August 2016 was caused by dam breach, and the scale of the debris flow was equivalent to that at once in 20-year. The destruction of check dam in the channel has produced an amplification effect on the peak discharge, aggravating the debris flow disaster. Affected by the Ms 7.0 Jiuzhaigou earthquake recently, more than 20×104 m3 of loose material are active for debris flow events in the future, and the frequency and scale of debris flow disasters in Zechawa Gully will increase in the future. The original design standard of the debris flow mitigation countermeasures for once in 20-year was low, and the engineering countermeasures with the design standard of 50-year period should be taken after the Ms 7.0 Jiuzhaigou earthquake, and concrete materials should be used to construct control projects (such as check dams) to reduce the risk of breach.
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2020, Vol. 18 
