小浪底水库调水调沙期水沙运动全过程模拟

Whole-processes modeling of flow movement and sediment transport during the period of water-sediment regulation in Xiaolangdi Reservoir

为模拟调水调沙期小浪底库区水沙运动过程,建立了考虑有干支流倒回灌的明流与异重流交替演算的一维水沙耦合模型.该模型采用了适用于复杂支流水系的零维水库法和考虑支流底坡的流量公式计算干支流倒回灌,采用潜入点判别条件衔接明流与异重流计算,并考虑了非静止水面假定下异重流与上层清水的耦合.应用该模型对选取的2002年及2012年小浪底水库调水调沙过程分别开展参数率定及不同工况下的过程反演.计算结果表明,库区自上而下可划分为零支流库区段、明流段和异重流段,考虑倒回灌过程的模型在3个区段均能较好还原实测水沙过程,不考虑的模型仅在零支流库区段和考虑的模型误差相近;由于其忽略了干支流倒回灌对水沙输移的影响,明流段的水位、流量及含沙量在回灌期偏小,在倒灌期偏大;异重流段的潜入点位置及时间滞后,异重流交界面高程、流速及含沙量偏大,计算排沙比过高.另外,模型相关参数的敏感性分析还表明,交界面阻力系数、倒灌阻力损失系数和水量掺混系数对计算结果的影响依次减小;其中交界面阻力系数的增大,一定程度上导致模型响应的滞后;倒灌阻力损失系数的影响,则需依据支流底坡的具体大小分情况讨论.

A one-dimensional coupled model has been proposed to consider the flow exchanges between main stream and tributaries during the alternative calculations between open-channel flow and turbidity currents,which is capable of simulating the whole process of flow and sediment transport covering these three different physical factors during the period of water-sediment regulation in the Xiaolangdi Reservoir.The modified storage-cell method was adopted for a complex tributary system to calculate the net flow exchanges caused by the water surface variation,and the discharge formula considering the effect of bottom slope of tributaries was applied to calculate the turbidity currents intrusion.Meanwhile,the plunge criterion of turbidity is used to connect the calculations of open-channel flow and turbidity currents,the static water surface assumption was no longer used so as to couple the interaction between the upper clear water and lower turbidity currents.The whole event of water-sediment regulation conducted in the Xiaolangdi Reservoir in 2002 was used for parameter calibration and the event of 2012 was simulated using both the proposed coupled model(Model I)and the model without considering the flow exchanges(Model II).The results show that the reservoir can be divided into three regions:the upper region with no tributary,the middle region with just open-channel flow and the lower region with turbidity currents,where both contain tributaries.Model I presented the results to closely agree with the measured processes in three regions.Among which the upper region is the only region the results calculated by Model II are as precise as Model I,However,with effects of the flow exchanges on the sediment transport and the conservation of water mass and flow momentum being omitted,Model II presented incorrect predictions in the lower two regions where there are tributaries:the calculated water level,discharge and sediment concentration in the middle region are rather high during the occasion of water intrusion and rather small during the occasion of water recharge;in the lower region,the location and occurrence of the plunge point were predicted to be delayed,and these turbidity factors were overestimated,covering interface level,discharge,flow velocity and sediment concentration,correspondingly,the sediment delivery ratio are rather large.Therefore,the effect caused by the tributary system is so enormous that it should not be neglected in the simulation.In addition,a detailed sensitivity analysis of coefficients in the turbidity module was also conducted in order to investigate the effects of these coefficients on the model predictions,with the proper variation range being determined for each coefficient.It indicates that the effects on the calculated factors caused by the drag coefficient at interface,the resistance coefficient of intrusion and the mixing coefficient between clear water and turbidity decreased in turn.Besides,with an increase in drag coefficient,the model response shows a time lag at a certain extent;with the bottom slope of tributaries not being constant,the effect of resistance coefficient requires a further discussion based on a specific slope.