三峡工程下游卵石夹沙河段床沙粗化计算

Riverbed Armoring Processes in a Sand-gravel Bed River After the Three Gorges Project Operation

三峡工程运用后,下泄清水持续冲刷河床,造成长江中游河段床沙逐年粗化,对长江中游河段水沙输移及河床演变规律产生重要影响。为研究三峡工程下游宜枝河段、枝江河段卵石夹沙河床年际床沙粗化及年内床沙交换过程,基于Markov三态转移概率矩阵,引入非均匀沙隐暴系数,得到卵石夹沙河床上床沙级配、推移质级配及悬沙级配的概率计算模型(沙量平衡方程),能够同时考虑前期水沙条件、床沙起悬及冲淤过程对床沙交换过程的影响。结果表明:2003-2017年,枝城站床沙发生明显粗化,泥沙颗粒级配分布曲线左移,2003年床沙粒径范围为0.002~41.000 mm,2017年变为0.031~86.000 mm;此外,该断面床沙中值粒径总体呈上升趋势,并在2010年以后有一定波动,但波动幅度较小;并以汛初、汛末为时间节点,将枝城站年内月均床沙变化过程分为3类。通过枝城站2003-2009年实测资料验证,本文建立的概率模型计算结果与实测资料符合较好,能够应用于三峡工程下游宜枝河段及枝江河段卵石夹沙河床年际床沙粗化及年内床沙交换过程的预报,为今后非均匀沙运动及非平衡输沙机理的研究提供理论基础。

Upstream damming can greatly alter the flow and sediment conditions entering downstream reaches in the middle reaches of the Yangtze River,with the bed material continuously coarsened,which has a significant influence on the sediment transport and bed evolution in the downstream of the Three Gorges Project(TGP).In order to study the riverbed armoring and sediment exchange processes among bed material,bed load,and suspended load layer in the Yizhi Reach and Zhijiang Reach,the model with three-state transition probability(Markov Chain)and hiding-exposure effects of non-uniform sediment are introduced.The equilibrium equation of sediment quality in active layer is proposed for calculating grain size distribution.In this model,the influences of flow and sediment regimes,riverbed erosion and deposition on SEP are discussed.The results show that,the composition of surface bed material at Zhicheng station was obviously coarsening,and the grain size distribution curves shifted to the left.The grain size range of surface bed material in 2003 was 0.002~41.000 mm,and changed to 0.031~86.000 mm in 2017.In addition,the median grain size of surface bed material at Zhicheng station showed an upward trend and fluctuated after 2010,but the fluctuation range was relatively small.The monthly change process of the median grain size can be divided into three categories according to the time node of flood periods.The probabilistic model developed in this study is verified against the field measures with both bed load and suspended load particles,and the modeling results show a reasonable agreement with the measured data at Zhicheng station from 2003~2009.Accordingly,the probabilistic model can be used to predict the riverbed armoring and annual process of sediment exchange in sand-gravel bed in the Yizhi Reach and Zhijiang Reach.The model can be used to reveal the mechanism of sediment motion and non-equilibrium sediment transport in sand-gravel bed rivers.