受控河道内洲滩冲积平衡机制及其对水沙变化的响应模式

With river basin management, more and more river channels are under control of human being. In such cases of rivers with extensive engineering of bank protection, the magnitude of planform adjustment has been constrained and the deformation of sand bars will be important channel response that is possible in relation to changes in discharge and sediment load. Effects of sand bar deformation had draw much concern in river management with altered flow and sediment discharge. Although it is found that the highly trained river often responds to changed flows by attempting to reform mid-channel bars rather than natural cutoffs of meanders, the geomorphological approach, accounting for bars morphology and dynamics, in providing useful guidance for stabilization schemes and trend prediction is not yet recognized. Insufficient consideration of unsteady hydrograph and boundary control factors are the main limitations in previous research, so that the balance of sediment transfer and energy dissipation can not be built. In this research project, with the middle Yangtze River as the study recah, the channel adjustment in highly regulated rivers will be viewed over smaller time and space scales and analysis of morphodynamics will be put forward using fixed reach boundaries. With intensive investigation of data gained both from prototype, numerical simulation and scale model, the following research questions will be addressed: (1) what is the relation between income flow an sediment process, boundary planform factors and morphodynamic features of alluvial sand bars; (2) how do the channel keep a balance of sediment load transfer with deformation of sand bars, and what is the law between channle morphology and flow energy dissipation with an unsteady hydrograph of flow and sediment input. With the knowledge of those two aspects, the mechanism of sediment transfer and energy dissipation will be investiagte in long period. The relation between sand bar morphological stability and balance of sediment transfer and energy dissipation will be built. Depend on those findings, the sand bar adjustments can be estimated when it is affected by changed flow and sediment discharge. This kind of research will help to understand the channel behavior to rebuild its balance after human disturbance, and some useful guidance can be provided for this kind of channel harnessment.

当冲积河流的岸线边界因人类守护工程而稳定之后，洲滩调整成为河床平面形态响应水沙过程变化的主要方式，对坝下游生产实践影响甚大。已有洲滩演变理论主要基于自由冲积河流或自由运移型洲滩，对受控河道内洲滩调整趋势难以准确判断，其关键原因在于非恒定水沙过程、边界形态因子对洲滩河段内水沙输移和能量耗散的影响机制未得到系统揭示。本项研究以长江中游典型洲滩为背景，在吸收、集成河流动力学和河流地貌学相关理论原理基础上，借助原型实测资料、数值模拟和概化模型试验，建立来水来沙、边界形态因子与洲滩特征之间的关系，研究洲滩形态变化对泥沙输移的调节机制，以及形态阻力变化对水流能量的耗散规律，在时段平均意义上构建洲滩稳定性与输沙平衡、能量平衡的相互作用机制，从而能够对水沙条件变化后的洲滩调整趋势实施预判。预期成果有助于增进对洲滩等微地貌单元演变特性的认识，丰富冲积河床自调整理论，也为工程作用下的河道治理提供科技支撑。

当冲积河流岸线被天然节点或人工护岸约束之后，洲滩调整成为河床形态响应水沙过程变化的重要方式，对生产实践影响甚大。本项目以边界受到控制，进口来水来沙受到水利工程调控的受控河道对象，研究洲滩变形与水沙过程变化之间的响应关系。研究工作以长江中下游为背景，在归纳若干典型洲滩河段演变规律基础上，重点分析受控河道内洲滩冲积平衡特征及其存在条件，以及受控河道内水沙输移和能量耗散的平衡机制，内容包括：采用下荆江、螺山-汉口等长河段多测次地形，通过数学模型开展了40组计算分析，研究了宽窄相间河段不同输沙模式；针对汉江下游、长江下游河段，开展20余组模型计算，研究了支流尾闾洲滩形态与水沙过程关系，感潮河段平滩水位、造床流量与上下游因素的关系；分别建立多个典型洲滩河段的数学模型或概化模型，分析了洲滩调整与水沙过程之间响应关系。研究的主要成果包括：针对长江中游复杂的江湖分汇关系，揭示了分汇流情况下基流和中洪水的流量概率密度函数差异；针对不恒定水沙过程，剖析了算术等间隔与对数等间隔下的地貌功差异，导出了不同情况下最有效输沙流量的解析表达式；解析了局部边界形态因子对主流摆动和洲滩发育的影响，提出了阻隔性河段判断指标；针对宽窄相间河道，提出了沿程输沙平衡和沿程能耗最小的沿程河相关系约束条件，揭示了存在洲滩的长河段内由枯水至洪水期水流能耗逐渐趋近于能耗最小状态的规律；揭示了宽窄相间河段内泥沙时停时走的暂态冲淤条件，并提出了冲淤临界流量；提出了类似长江中游这种存在较多分汇情况的冲积河道造床流量确定方法，以及受顶托的支流尾闾、感潮河段的造床流量和平滩水位联合确定方法；解析了三峡水库运行后坝下游洲滩河段冲淤调整机理。在当前三峡水库运行后坝下游河道不断发生调整的背景下，成果能够为河道形态调整趋势预测、河道和航道治理等提供直接参考。