则查洼沟“8·4”泥石流灾害成因分析

On the causes of“8·4”debris flow disaster at Zechawa Gully

位于四川省九寨沟景区内的则查洼沟在2016年8月4日暴发泥石流,掩埋景区栈道,堵断从诺日朗至长海的唯一景区公路。降雨资料显示,2016年8月4日的最大1 h降雨量和24 h累计降雨量只有10.0和17.5 mm,远远低于研究区10年一遇的降雨强度,泥石流灾害规模与降雨量不相符。为了厘清泥石流灾害发生原因,笔者进行了野外调查,通过形态调查法、雨洪法和溃坝分析获取泥石流峰值流量。结果表明:溃坝分析法与形态调查法计算的峰值流量大致吻合,该值与20年一遇的泥石流峰值流量(32.73 m3/s)相当。说明:2016年8月4日暴发的泥石流属于溃决型泥石流,拦砂坝的溃决产生放大效应,加剧泥石流灾害,其灾害规模相当于20年一遇,未来泥石流灾害的频率和规模将增大;原有20年一遇的泥石流防治工程设计标准偏低,地震后应按照50年一遇的设计标准对则查洼沟进行工程治理,且应当使用混凝土材质修建拦砂坝等控制性工程,从而降低溃决风险。

[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.