多沙河流水库溯源冲刷与异重流排沙耦合过程的数值模拟研究

Retrogressive erosion and turbidity current venting are proven techniques for reservoir sediment management, however it is still a challenge to make accurate predictions while giving full consideration to the coupling effects between these two physical processes with the present level of understanding to their mechanisms and numerical modelling technique. Studying approaches of analysis to observed data, application of mechanical theory and numerical modeling are used in this project. Firstly, the mechanism of bed deformation in the process of retrogressive erosion is investigated from the aspects of the stability of deposit slope and the dilatancy effect. The sediment entrainment flux formula for retrogressive erosion is proposed, which can be used to achieve simulations to different deformation modes of the longitudinal profiles. Then, the data from laboratory experiments and field measurements are used to investigate the unsteady characteristics of turbidity current intrusion. The approximation equation is obtained for the interface profile between the turbid and clear water layers, and a calculation method is proposed for turbidity current intrusion. Finally, the proposed calculation methods for retrogressive erosion and turbidity current intrusion are integrated with the coupled model for open-channel flow and turbidity current in reservoirs. The coupled processes of retrogressive erosion and turbidity current venting are simulated to evaluate the reservoir operation schemes for sediment management. Study results from this project will not only help to enhance the understanding of characteristics of flow and sediment movement and bed evolution in heavily sediment-laden reservoirs, but also improve the management ability to reservoir sedimentation.

水库溯源冲刷与异重流排沙是在国内外水库泥沙的治理实践中探索出的有效管理措施，但是现有机理研究与模拟技术难以准确预测两者的演化过程并充分考虑耦合作用。本项目将采用实测资料分析、力学理论分析与数值模拟相结合的研究方法。首先从淤积体坡面稳定性和床面层剪切膨胀作用出发分析溯源冲刷过程中的床面变形机理，提出溯源冲刷过程的床沙上扬通量计算公式，从而实现对不同纵剖面变形模式的模拟；然后采用水槽试验与原型观测资料研究异重流倒灌过程的非恒定特性，得到稳定状态下交界面曲线形态的近似方法，提出异重流倒灌流量的计算方法；最后将所提出的溯源冲刷与异重流倒灌计算方法与水库明流及异重流控制方程的交替求解模式相结合，建立水库溯源冲刷与异重流排沙的耦合过程模型，用于评估不同调度方式的排沙效果。研究成果将有助于深入认识多沙河流水库水沙运动与河床冲淤规律，提升水库泥沙的管理水平。

水库溯源冲刷与异重流排沙是在国内外水库泥沙的治理实践中探索出的有效管理措施，但是现有机理研究与模拟技术难以准确预测两者的演化过程。本项目从溯源冲刷过程中的泥沙侵蚀强度和有限长度支流内的异重流倒灌着手，在以下三个方面开展研究并取得了相应成果：（1）将考虑了大底坡影响的修正后的浅水方程与基于高速冲刷条件下悬浮通量理论的泥沙输移与河床变形方程耦合，建立了溯源冲刷模型；（2）基于立面二维水沙输移模型的自相似解，得到了异重流倒灌的最大长度表达式，建立了对末端封闭以及无限长河渠均适用的异重流倒灌流量和输沙率计算方法；（3）采用准二维算法实现了河网计算中河段连接单元与一维控制单元间的数值通量计算，使用OpenMP技术完成了河网算法的并行化，建立了能够实现库区、下游河道和滩区联合模拟的水沙模拟系统。数值模拟研究证明了忽略大底坡影响会高估水流的重力加速作用导致计算冲刷速度比实际速度快，采用传统的挟沙力公式计算悬浮通量会导致计算冲刷速度在溯源冲刷的早期远小于实测值。对于三门峡水库溯源冲刷过程，冲刷范围和各断面的冲刷深度均能做出较准确的预测。对于异重流倒灌的研究成果填补了过去倒灌长度公式缺乏理论依据而单纯依靠回归分析的空白，弥补了过去异重流倒灌强度预测方法不能考虑支流末端的反射影响的缺点，提高了预测精度。应用建立的黄河中下游水库-河道-滩区水沙模拟系统，评估了“1958”型洪水重现时，小浪底水库的防洪作用和黄河下游的洪水风险。本项目成果揭示了非粘性沙溯源冲刷机理以及地形条件对异重流倒灌均衡状态的影响机制，有助于提高水库减淤调度的效果。