小浪底水库浑水异重流出库理论判别
Theoretical Discrimination of Turbidity Current Venting in Xiaolangdi Reservoir
浑水异重流是多沙河流水库中泥沙常见的运动形式,在小浪底水库拦沙运用初期对减少库区泥沙淤积起到重要作用。异重流持续运移至坝前实现排沙出库所满足的条件对多沙河流水库的运行管理至关重要。本文引入河流功的概念研究小浪底水库异重流出库的动力因素。在忽略水沙交换的情况下,异重流运动受控于河床纵比降和自身水沙动力。用潜入点至坝前的平均河床纵比降J0与异重流临界比降Jc的比值J0/Jc表示相对河床纵比降,代表异重流运动的地形条件;用异重流平衡速度u和持续时间T的乘积与潜入点至坝前的径向距离L比值uT/L表示异重流持续运动的相对距离,代表异重流水沙动力因子;进而确定相对河床纵比降J0/Jc和自身水沙动力因素uT/L双参数构成的异重流出库判别图。分析小浪底水库1999年建库至2023年的逐日水沙资料,确定了92场次异重流过程,从整体上看,水量和沙量越大的洪水过程能够产生较为明显的异重流排沙过程。采用该判别图能成功将大部分能否到达坝前的场次异重流进行区分,同时为多沙河流水库异重流高效排沙提供了可靠依据。
Objective The Xiaolangdi Reservoir, as a representative sediment-loaded river reservoir, shows that turbidity current venting plays a crucial role in mitigating reservoir sedimentation during initial sediment retention operations. Understanding the mechanical conditions under which the turbidity current reaches the dam for effective sediment discharge or venting presents a critical operational challenge in managing sediment-loaded river reservoirs. The theoretical identification of turbidity current venting provides direct technical support for water and sediment regulation strategies, which form the objective of this study. Methods Turbidity current venting and the turbidity current reaching the dam were considered equivalent situations. The turbidity current events were selected based on the conditions that, for the inflow, the daily sediment concentration exceeded 20 kg/m³ and the daily sediment transport rate exceeded 10 000 kg/s, which were used as the thresholds for turbidity current event occurrence. For practical significance, a daily sediment concentration greater than 1 kg/m³ for the outflow was employed as the standard to determine whether sediment discharge occurred. The concept of river work was introduced, as the movement of the turbidity current needs to overcome resistance to perform work, referred to as turbidity work. It was assumed that the energy dissipation of turbidity current movement from plunging to cessation was equal to turbidity work, and the turbidity work when it reached the dam was defined as critical work. Therefore, the theoretical discrimination of turbidity current venting was that the turbidity current work needed to exceed the critical work before reaching the dam. The turbidity current movement was controlled by its sediment content and the longitudinal gradient of the riverbed, by neglecting the water and sediment exchange and mixing processes. The ratio J0/Jc of the average longitudinal gradient J0 of the riverbed from the plunge point to the dam and the critical gradient Jc of the turbidity current is utilized to represent the relative longitudinal gradient of the riverbed, indicating the topographic conditions of turbidity current movement. The ratio uT/L, where u is the turbidity current equilibrium velocity, T is the duration, and L is the radial distance from the plunge point to the dam, is utilized to represent the relative distance of motion, indicating the dynamic factor of the turbidity current. The theoretical discrimination is expressed as J0/Jc · uT/L > K, where K is a parameter related to a specific reservoir. Therefore, the discriminant diagram of turbidity current venting, composed of the relative longitudinal gradient of the riverbed (J0/Jc) and its hydrodynamic factor (uT/L), was determined. Results and Discussions A total of 92 turbidity current events were identified, of which 55 resulted in sediment discharge. Statistically, flood events with higher sediment concentrations and larger water volumes produced turbidity currents with more significant sediment discharge. There was no clear correlation between the event-averaged flow discharge and sediment concentration, and these parameters cannot be directly employed to distinguish between turbidity currents that reached the dam and those that did not, i.e., for discrimination purposes. A clear positive correlation existed between the event water and sediment amounts; however, these two parameters failed to serve as effective discriminants. The sediment amount of turbidity current events that did not reach the dam was primarily concentrated below 80 million tons, whereas that of the events that reached the dam was mainly distributed in the range of 0.2~320.0 million tons, exhibiting a bell-shaped distribution. The water volume of turbidity current events that did not reach the dam was mainly concentrated in the range of 0.1~1.6 billion m3, while that of the reached events was mainly concentrated in the range of 0.4~1.6 billion m3, both showing bell-shaped distributions. The larger the water and sediment amounts in a turbidity current event, the higher the probability of its reaching the dam. The entire coordinate area of the discriminant diagram was divided into two regions, namely the dynamic region at the top left and the inertial region at the bottom right. As the parameters J0/Jc and uT/L increased, the density current was more likely to advance toward the dam and occupy the dynamic region. In the dynamic zone, the turbidity current quickly reached the dam by its potential energy. In the inertia zone, a significant deceleration of the turbidity current occurred due to insufficient potential energy. For most turbidity current events in the inertia zone, the head of the current was still able to reach the dam due to the energy supplied by the trunk of the turbidity current. The flood events in which the turbidity current did not reach the dam were mainly located in the lower left corner of this coordinate system. The critical condition for the sediment discharge of turbidity currents was represented by a hyperbolic curve with K=12 for the Xiaolangdi Reservoir. This discrimination chart effectively distinguished the outflow of density currents. Thirty-three of the 37 non-vented events were located at the lower left of the curve, and 45 of the 55 vented events were located at the upper right of the curve. The discrimination diagram effectively distinguished sediment discharge by turbidity currents. Conclusion Most of the turbidity current events since the establishment of the Xiaolangdi Reservoir were successfully identified using the discriminant diagram. The proposed theoretical sediment discharge conditions effectively predict the sediment discharge of multiple floods that have already occurred. The discriminant diagram also provides a dependable basis for achieving efficient sediment discharge of turbidity flow in the reservoir of the sediment-laden river.
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国家重点研发计划项目(2021YFC3200400)
国家自然科学基金项目(52179066)
国家自然科学基金项目(52309091)
水利部重大科技项目(SKR‒2022088)
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