金沙江白格堰塞湖溃决过程数值模拟

2018年10月10日和11月3日,在我国四川省与西藏自治区交界处的白格村同一位置连续发生两次滑坡,完全堵塞了金沙江形成堰塞湖。由于“10?10”滑坡形成的堰塞湖水位抬升迅速,堰塞湖于10月12日自然漫顶溃决。“11?03”滑坡堵塞了“10?10”堰塞体溃决形成的流道,形成了更大的堰塞湖,鉴于客观条件允许,采取了开挖泄流槽降低堰塞湖溃决水位的措施,至11月12日,堰塞湖发生漫顶溃决,溃口洪水峰值流量为31000m3/s。由于“11?03”白格堰塞湖溃决案例拥有较为完整的实测资料,为堰塞湖溃决过程的研究提供了宝贵的基础数据。基于堰塞体的溃决机理,建立了可考虑堰塞湖的水动力条件、堰塞体的形态和材料特征的堰塞湖溃决过程数学模型。模型采用宽顶堰公式模拟溃口洪水流量,并根据堰塞湖入湖和溃口流量以及堰塞湖的水位-湖面面积关系曲线确定堰塞湖水位的变化;采用基于水流剪应力和堰塞体材料临界剪应力,并可考虑宽级配堰塞体材料特性的冲蚀公式模拟材料的冲蚀过程;假设溃口在纵向下切和横向展宽过程中坡角保持不变,采用极限平衡法分析溃口在发展过程中发生的边坡失稳;采用按时间步长迭代的数值计算方法模拟堰塞湖溃决时的水土耦合过程。采用建立的模型对“11?03”白格堰塞湖溃决案例进行反演分析后发现,模型计算获得的溃口流量过程、堰塞湖水位变化过程、溃口发展过程与实测值基本吻合。参数敏感性分析表明,冲蚀系数对溃决过程具有重要的影响,残留坝高通过影响下泄库容也对溃决过程产生作用;另外,开挖泄流槽可降低堰塞湖溃决时的库容,从而对溃口流量过程产生影响,是降低灾害损失的有效手段。

Numerical Modeling of Breaching Process of Baige Dammed Lake on Jinsha River

On October 10th and November 3rd, 2018, two successive landslides occurred at the same place of Baige village, the border of Sichuan Province and Tibet Autonomous Region, China, which blocked Jinsha River twice. Owing to the rapid rising of water level of the "10·10" dammed lake, on October 12th, the landslide dam breached naturally. Then, the flow channel formed after the breaching of "10·10" dammed lake was blocked by the "11·3" landslide, resulting in an even larger dammed lake. The measures of excavation of drainage channel were taken to decrease the water level of the dammed lake. On November 12th, the dammed lake breached with the peak flow rate of 31 000 m³/s. For the "11·3" Baige dammed lake, the detailed hydrological data were documented, which provided valuable basic data for the study of the breaching process of dammed lake. Based on the breaching mechanism of landslide-formed dam, a numerical model which can consider the hydrodynamic conditions of the lake and the morphological characteristics of dam materials of the landslide dam, was developed. The broad-crested weir equation was adopted to simulate the breach flow discharge, and the variation of water level of the dammed lake was determined according to inflow and breaching flow, as well as the rating curve of water level and surface area of the dammed lake. Based on the shear stress of water flow and the critical shear stress of dam materials, the erosion process was simulated by using the erosion formula for wide-graded dam materials. Under the assumption that the breach slope angles remain unchanged during the longitudinal cutting and transverse broadening processes, the limit equilibrium method is used to analyze the slope instability during the breach evolution process. The coupling process of soil and water during dam breaching was simulated by the time step iterative numerical method. Then, the back analysis of the "11·3" Baige dammed lake was conducted, and the comparison showed that the breach hydrograph, water level variation of dammed lake, and breach morphology evolution calculated by the proposed model are consistent with the measured data. The sensitivity analysis showed that soil erodibility had significant influence on the breaching process, and residual dam height also played a role in the breaching process by affecting the discharged water storage. In addition, the excavation of spillway can reduce the reservoir capacity when the dammed lake breached, thus affecting the breach hydrograph, which is an effective measure to reduce disaster losses.