坝下冲积河道调整驱动水位变化的时空分异规律及演变机制

In the downstream river reach of dams, unexpected changes of water stages has significant impacts on flood control, navigation and ecological conditions. However, common law of water stage adjustment is difficult to generalize because of the temporal and spatial fragmentation of the stream-gage information. Furthermore, due to simplification of factors such as flow process, geomorphology, and bed composition, the spatial and temporal differentiation in the process of water stage changes cannot be reflected in theory analysis and simulation. The key problem in predictions is clarification of the main controlling factors and causes of changes in the change process of water stage in different periods and different river sections. In this proposed research project, the response relationship between different factors to the channel adjustment is to be studied with the middle reach of the Yangtze River taken as the background study reach. The study will be based on data of prototype observation and numerical simulation. Conceptual flume experiment will be conducted to analyze the effects of main factors. The theory of riverbed self-adjustment will be used to explore the mechanisms. The influences of key characteristics of large alluvial rivers, such as inhomogeneous riverbed boundaries, uneven distribution of scouring and deposition along stream, and unequal water stages changes at different discharges, will be intensive investigated. The following research objectives will be addressed: reveal the feedback mechanism between water slope increasing and knickpoint backward transfer process in sand-gravel transitional reach; clarify the mechanism between hydraulic and morphologic changes in wide and narrow sand bed reaches from the perspective of channel self-adjustment. The result of this research will help to provide theoretical foundation for prediction of water stage with the combined impacts of flow regulation and sediment reduction.

坝下水位变化易引发意想不到的洪灾风险增大、航道和生态栖息地恶化等不利局面。但原观资料的时空片段性使得共性规律难以归纳。而由于对流量过程、地质地貌、河床组成等因素过分概化，理论和模拟研究中难以充分反映水位变化的空间不均匀性和时间非线性特征。解决预测与防控的关键，在于系统解析水位变化在各时期、各河段呈不同规律的主控因素及成因。本课题拟以长江中游河段为背景，以数值模拟延展观测资料实现过程复演，补充概化试验实施因素解析，通过河床自调整理论开展机制探索，辨析河道边界不均一、冲淤分布不均一、水位变幅不均一等现象的关联，研究水沙调控—河床调整—水位变化的全过程驱动响应关系，阐明沙卵石至沙质过渡河段内水位落差增大与深泓剖面转折点溯源转移的互馈机制，揭示宽窄相间冲积河段内不同的形态、阻力调整模式对河段整体纵向水沙输移平衡倾向性的促进机制。预期成果可为流量调节和沙量减少双重作用下洪枯水位变化趋势判断提供支撑。

河流上修建大坝之后，坝下河道形态和阻力调整会导致洪、枯水位变化，易引发意想不到的洪灾风险增大、航道和生态栖息地恶化等不利局面。预测与防控这种不利变化的关键，在于系统解析水位变化在各时期、各河段呈不同规律的主控因素及成因。本项目以地处三峡大坝下游的长江中游河段为背景，以近20余年来大量水文资料为基础，采用数理统计、理论分析与模拟计算等多种方法相结合，开展的研究内容包括：对同流量下水位变化规律开展了分析，对比了沙卵石河段、荆江沙质河段、城陵矶以下分汊河段洪、中、枯各级流量下的水位变化差异；研究了长江中游沙卵过渡带自1970年代至三峡水库运行后2015年的迁移特征及其主导因素；量化了河床冲刷和床面粗化过程中的河道阻力变化，比较了洪枯水阻力变化异同，分析了河道阻力增大的主要成因；探讨了水文条件变化后的洲滩植被变化规律，探究了河道形态调整、阻力调整两种因素对长江中游洪、枯水位的影响。研究所取得的主要成果包括：提出了考虑回水影响的多值型水位~流量关系确定方法，方法较传统单值型方法在比降平缓、支流众多的河道中更具适用性，为量化河床调整导致的水位变幅提供了有效工具；揭示了河道宽度对长江沙卵过渡带位置的控制作用，阐明了细沙来量减少后长江沙卵过渡带的河道纵剖面变化特征，以及河道调整引起的水面比降调整趋势性；归纳了沙质河段冲刷分布特点，揭示了河道冲刷主要由平滩以下流量导致，而平滩以上流量可能导致滩地淤积的规律；确认了沙质河段河道阻力主要来源于沙波阻力，三峡建库后阻力增大主要由水动力条件减弱引起沙波尺度增大导致；揭示了不同流量级下河道形态和阻力对水位的迭加影响，发现了三峡建库后洪水位不下降的成因是由于平滩流量附近河床下切与阻力增大效应相互抵消所致。研究成果可为坝下沙卵过渡带的河道治理，以及沙质冲积河道行洪能力预测和调控提供参考。