均质黏性土坝漫顶溃决机理及溃坝过程模拟

Breach Mechanism and Breach Process Simulation of Homogeneous Cohesive Earthen Dam Due to Overtopping

水库大坝的溃决对下游人民生命及财产带来巨大威胁,而中国已溃大坝中有85%以上为均质黏性土坝,且50%以上为漫顶溃决,因此有必要深入研究均质黏性土坝的漫顶溃决机理,提高溃坝洪水流量过程的预测精度,为溃坝应急抢险提供理论与技术支撑。基于均质黏性土坝大尺度漫顶溃决模型试验,揭示了漫顶水流作用下溃口在3维空间的发展机理,在此基础上提出了一个模拟均质黏性土坝漫顶溃决过程的数学模型。该模型基于坝体形状和漫顶水流特征确定“陡坎”的形成位置,采用宽顶堰公式计算溃口流量;选择可考虑坝料物理力学特性的溯源冲刷公式模拟“陡坎”的移动,并通过力学分析判断“陡坎”上游坝体的坍塌;引入坝料冲蚀系数,通过分析水流剪应力与坝料临界剪应力建立坝料的冲蚀率方程,模拟坝顶与下游坡溃口的发展;采用极限平衡法模拟溃口边坡的失稳,并假设滑动面为平面。模型考虑了不完全溃坝与坝基冲蚀,以及坝体的单侧与两侧冲蚀。选择国内外3组具有实测资料的大尺度均质黏性土坝漫顶溃坝模型试验对模型进行验证,实测值与计算结果的比较表明,溃口峰值流量、溃口最终平均宽度及溃口峰值流量出现时间的相对误差均在±25%以内,并且计算获得的溃口流量过程线与实测结果基本吻合,验证了模型的合理性。

Breaching of the reservoir dams would pose great threats to the lives and property of people downstream. In China, more than 85% of failed dams are homogeneous cohesive earthen dams, meanwhile, over 50% of the failure caused by overtopping-induced breach. Therefore, it has great significance to deeply study the overtopping breach mechanism of the homogeneous cohesive earthen dam, so as to improve the prediction accuracy of the breach hydrograph, and to provide theoretical and technical support to the emergency rescue following dam breach. The mechanism of the breach evolution in the three-dimensional space for homogeneous cohesive dam due to overtopping failure was studied according to the large scale model tests, after which a numerical model for overtopping-induced dam breach is developed. The forming position of the headcut is determined by the dam configuration and characteristics of the overtopping flow, and the breach flow discharge is calculated using the broad-crested weir formula. Then a backward erosion formula which considers the physical and mechanical properties of the dam materials is adopted to simulate the migration of headcut, while the mechanical analysis is utilized to judge the collapse of the dam body upstream of the headcut. The erosion coefficient of dam material is introduced, and the erosion rate equation of dam material is established by analyzing the flow shear stress and the critical shear stress of dam material. The erosion rate equation is then used to simulate the breach development of dam crest and downstream slope. In addition, the limited equilibrium method is adopted to model the instability of the side slope of breach, and the sliding surface is assumed to be a plane. Moreover, the model can consider incomplete and base erosion, as well as one- and two- sided breach. Three large scale model tests for homogeneous cohesive dam due to overtopping failure with detailed measured data are chosen to validate the proposed model. The comparison of measured and calculated data indicated that the relative errors of peak breach flow, final breach average width, and time to peak are all within ±25%. In addition, the calculated breach hydrograph is in accordance with the measured one. In summary, the case studies verify the rationality of the proposed model.