【作者】 葛华；

【导师】 李义天；

【作者基本信息】 武汉大学 ， 水力学及河流动力学， 2010， 博士

【摘要】 水库作为人类开发利用河流功能的一种重要形式,在发挥防洪、发电、航运、灌溉等多方面工程效益的同时,由于改变了天然情况下河流的水沙条件,从而对水库下游的河道演变带来非常深远的影响。世界各国的河流开发历史和经验教训表明,对水库下游泥沙输移的认识及预计不足,将导致众多水利工程难以发挥其目的,甚至产生一些负面效益。因此,深入分析水库下游非均匀沙的输移特点,明确其内在机理,提高其模拟预测技术,对于做到防患于未然,以便及早采取措施,发挥水库的最大工程效益具有非常重要的意义。本文归纳和总结了水库下游非均匀沙输移以及河道冲淤调整的一般性规律,深化了对其内在机理的认识,并在当前研究成果的基础上,针对一维泥沙数学模型中的关键技术和参数进行了探讨,并根据实测资料对一些关键参数进行了率定。在此基础上,结合具体问题,建立了长江中下游宜昌-大通河段的一维河网非恒定水沙数学模型,在利用最新的实测数据对模型进行验证的基础上,针对三峡水库不同蓄水方案的下泄水沙条件对未来长江中下游的河床冲淤及水力特性变化等进行了预测计算,并对预测结果的合理性进行了分析。全文主要讨论了以下几个方面的问题：(1)水库下游非均匀沙恢复特性及机理：分析了不同水沙条件对构成河床冲淤的泥沙沉降与上扬两个方面的影响因素,在总结和归纳水库下游沙量恢复一般特性的基础上,根据泥沙沉降与上扬构成的泥沙交换特点,明确阐述了水库下游非均匀沙恢复特点的内在机理,并构造了不同冲刷历时沙量恢复略估的表达式,可较好的反映出已建水库下游的沙量恢复的一般性规律。同时,依据实测资料对泥沙输移方程中的恢复系数进行了分析,结果表明其数量级可达10-3-10-1,且一般随着粒径的增大而减小,随着冲刷历时的增加和床沙的粗化而呈递减的趋势。之后,对长江中下游非均匀沙冲淤量对来沙的响应及不同时期来沙减少后的沙量恢复差异进行了分析,指出就年统计值而言,长江中游非均匀沙年冲淤量与年输入沙量的关系比较密切,一般随着泥沙粒径的增大,两者相关性逐渐增强,单位年来沙量改变引起的下游河道年冲淤量逐渐增大,河道相应的输沙能力也逐渐减小,且沿程呈现递减趋势。分析表明,虽然90年代前后和三峡水库蓄水前后长江中游来沙量均有一定程度的减少,但由于后者各粒径组泥沙减少幅度更大、且水沙条件改变之前河床已基本处于冲刷状态,再加之水力条件的影响,其沙量恢复现象更加明显。(2)水库下游河流平衡趋向调整：依据水库下游实测资料以及水槽实验资料,按照河床组成性质的不同,对水库下游卵石夹沙河床和沙质河床在平衡趋向过程中的部分调整现象进行了归纳与总结,并深入分析了各调整方式在平衡趋向过程的作用及内在机理。分析指出,卵石夹沙河床经过冲刷以后,通常可通过形成卵石抗冲保护层而使河段达到平衡状态,而沙质河床在平衡趋向过程中的调整方式则更加多样化,其中,纵剖面的趋缓调整、床沙的粗化调整以及横断面的下切与展宽调整的终极方向是改变影响河床冲刷发展的两个方面,即增强河床自身的抗冲刷能力与削弱水流塑造河床的能力。在清水冲刷条件下,当两者相当时河床才能进入最终的绝对平衡状态,而此状态是由泥沙的起动条件决定的。(3)一维非均匀沙数值模拟关键技术探讨：在当前研究的基础上,针对泥沙数学模型中的关键技术进行了进一步的探讨：①基于泥沙运动统计理论的泥沙上扬通量与沉降通量推导了非均匀沙挟沙能力表达式,并与当前研究成果进行了对比,同时结合天然河道实际情况以及部分参数研究的不足,提出修正方法；②根据泥沙运动的扩散理论,通过引入调整系数,对非平衡输沙条件下的含沙量沿垂线分布公式进行了理论推导和参数拟合,其结果可用于基于泥沙运动统计理论进行的泥沙恢复饱和系数计算之中；③推导了基于沙波运动的混合层厚度计算方法,该方法可以体现非恒定泥沙数学模型中不同时间步长对其取值的影响；④对流速沿断面分布公式进行了简单的理论推导与参数拟合,以用于一维泥沙数学模型中基于水流不饱和程度构造的断面冲淤面积分配计算之中。(4)三峡水库蓄水后长江中下游一维水沙数值模拟研究：建立了适用于长江中下游宜昌-大通河段的一维河网非恒定水沙数学模型,并利用三峡水库蓄水前后2002-2006年的最新实测数据对模型进行了验证。之后,根据三峡水库基本蓄水方案以及不同提前抬高蓄水水位方案的下泄水沙条件,对沿程各河段的冲淤量、冲淤发展过程以及沿程的水位变化进行了模拟预测计算。计算结果符合水库下游泥沙输移的一般规律,并与相关极限冲刷量预测结果以及近年实测结果符合较好。更多还原

【Abstract】 As an important form of rivers function development for human, reservoir plays a tremendous benefit in the fields of flood control, power generation, navigation, irrigation, and so on. However, it also brings a far-reaching impact to the evolution of the downstream river reach of the reservoir, due to the changes in the water and sediment conditions after impoundment. History of the river development and lessons all over the world indicates that the inadequate knowledge of sediment transport downstream of the reservoir will result in a number of works hard to fulfill their purpose, or even negative benefits. Therefore, in-depth analysis of the sediment transport in non-uniform characteristics downstream reservoir, definitude of its internal mechanism and the elevation of its simulation technology will provide benefits of great importance to take early measures to against possible trouble and get the maximum effectiveness of projects.This paper first summarized the general laws of sediment transport and river adjustment in forms of erosion or deposition downstream the reservoir, deepened the understanding of their internal mechanism, and then explored the key simulation technologies and calibrated the model parameters by using the measured data combined with the current research results. On this basis, combined with specific issues, a one-dimensional river network mathematical model of unsteady water and sediment transport for the middle and lower reaches of the Yangtze River downstream form the Three Gorges Reservoir was established and tested by using the latest actual measured data, and then the water and sediment transportation, river erosion or deposition amount, and the changes of hydraulic properties were predicted under different water and sediment condition corresponding to different impoundment progress.Some key issues are shown as following:(1) Characteristics and mechanism of non-uniform sediment recovery downstream reservoir:the two aspects affecting the river bed erosion and deposition were summarized, and then based on the analysis of general characteristics of sediment recovery downstream reservoir, according to the characteristics of sediment exchange posed by deposition of the sediment in water and entrainment of the sediment on river bed, a clear elaboration of the internal mechanism for non-uniform sediment recovery downstream the reservoir was raised. And a journal expression for sediment recovery at different stage of erosion period was constructed. It can well reflect the journal law of sediment recovery downstream from reservoirs. At the same time, based on the measured data, the recovery coefficient in the sediment transport equations was analyzed. The results showed that its magnitude can reaches about form 10-3 to 10-1, generally increasing as the particle size decreasing, and it also showed a descending trend with the increasing of the duration of the erosion state. Meanwhile, the non-uniform sediment transport properties and recovering features after a reduction of input sediment amount in middle and lower Yangtze River reaches were also analyzed. It was pointed out that there is a closely relationship between annual non-uniform sediment amount of erosion and input in the middle reach of the Yangtze River. Generally with the sediment particle size increasing, the relevance is improved, and the erosion amount in the lower reach is raised due to the unit sediment input amount change, and the input amount, corresponding to the balance of erosion and deposition, is smaller. Analysis also showed that, although there are two reductions in the input sediment amount before and after 90 years and the Three Gorges Reservoir, but the sediment recovery phenomenon of the later period is more obvious, because the later sediment reduction is larger, and the reach has been in the state of erosion, combined with the impact of hydraulic conditions.(2) The river adjustment in the process tending to the balance of erosion and deposition downstream reservoirs:Based on the measured data downstream reservoirs under natural conditions and the tank experimental data, according to the difference of river bed composition, the phenomena of the adjustment in the process towards to the balance, both for sand-gravel and sand river beds, was summarized and summed up, and their roles in the process were also analyzed. It is pointed out that the ultimate result of the adjustment of various factors is to change two aspects which affect the river evolution:the river’s ability to anti erosion and the flow’s ability to shape the riverbed. When the two aspects come to an equal level, the river will enter into the state of equilibrium, which is ultimately determined by the sediment start-moving condition. In the process trending to the balance, the role of vertical gradient adjustment is inevitable, but not the only, and the role of the increasing resistance cannot be ignored.(3) Discussion and exploration on the key technologies in one-dimensional numerical simulation of non-uniform sediment:Based on the current study, further discussion on the key technologies in non-uniform sediment mathematical model was proposed:①Using entrainment and deposition fluxes based on statistical theory of sediment movement, expression of non-uniform sediment carrying capacity was derived, and contrasted with the earlier research, the inadequacy and improvement was also proposed;②According to the diffusion theory of sediment movement, through the introduction of the adjustment factor, formula of the vertical sediment concentration distribution under the non-equilibrium sediment transport condition was derived and its parameter was fitted, and it can be used to calculate the recovery coefficient based on the statistical theory of sediment movement;③It is pointed out that the impact of the time step on the values of the mixed layer thickness must be considered in unsteady flow mathematical model, and the method of calculating the thickness of the mixed layer based on the sand wave movement was proposed;④Velocity distribution formula along the cross-section direction was derived and fitted, which can be used in the method of allocating the erosion and deposition area along the cross-section direction based on the flow unsaturated level in one-dimensional sediment mathematical model.(4) Research on the one-dimensional numerical simulation and prediction of water and sediment in the middle and lower reaches of the Yangtze River after the impoundment of Three Gorges Reservoir:a one-dimensional unsteady flow and non-uniform sediment river network mathematical model for middle and lower reaches of the Yangtze River downstream the Three Gorges reservoir was established, and it was tested and verified by using the latest actual measured data. After that, the predictive calculation of the river erosion or deposition amount and its development process, and the water level changes at major sites, under the different impoundment program condition, was carried out. The results were better fitted with the general laws of the sediment transport downstream reservoir than the earlier research, and it was in good agreement with the relevant prediction of the limit erosion amount and the latest actual measured data.更多还原