弯曲河道水沙与滩槽相互作用及反馈机制

Hyperconcentrated floods occurred in the Yellow River may experience the phenomena of downstream peak discharge increase, which cannot be explained by the lateral discharge input. Inducing great damages, its mechanism has not been fully revealed, though the contributions of bed roughness, sediment erosion have been partly appreciated. This project proposes a systematic investigation on the interactions between flows and sediments, between sediment-laden flows and the complicated channel-floodplain patterns. It is expected that the physical mechanism for the downstream peak discharge increase, and accordingly our understanding about flood propagation in the Yellow River can be greatly improved upon the completion of this project. A physically-based high-resolution two-dimensional modeling framework will be firstly established for the Yellow River characterized by meandering and complicated channel-floodplain patterns. Empirical relations will be derived for the dispersion terms arising from the meandering effects. Full 3D RANS modeling of hyperconcentrated flows through complicated channel-floodplain cross-sections will be conducted to obtain flow structures, which can be used to quantify empirical parameters of the dispersion terms. Systematic numerical case studies will be then conducted to investigate the interactions between flows and sediments, between sediment-laden flows and the complicated channel-floodplain patterns, based on which the relative contributions and the relations of the meandering planform, roughness, sediment erosion, channel-floodplain pattern can be revealed. Finally, the main factors giving rise to the downstream peak discharge increase will be identified, the conditions for the occurrence of this phenomenon will be delimited. These understanding will provide basis for comprehensive regulating and flood risk management of Yellow River floods.

黄河下游弯曲宽浅、滩槽发育，高含沙洪水与滩槽之间相互作用强烈，常表现出洪峰流量异常增值现象。本项目研究弯道高含沙水流与滩槽相互作用及反馈机制，旨在揭示洪峰流量增值的机理，增进对高含沙洪水传播过程的认识。项目将首先建立适应于弯曲河道、复杂滩槽断面的高含沙水流平面二维耦合数学模型。为准确将弯道效应考虑在平面二维模型中，项目将从理论上推导表征其影响的离散项计算式，利用雷诺平均三维数学模型模拟弯道内高含沙洪水漫滩后的水流结构，从而量化离散项的经验参数。最后，项目应用平面二维模型，针对高含沙水流与弯道滩槽的相互作用及反馈机制，开展数值算例对比研究，逐一揭示弯道滩槽平面和断面形态、糙率变化、泥沙冲淤等因素对洪峰流量增加的相对贡献程度、相互影响关系和作用机理。在此基础上，甄别影响洪峰增值的主要因素，界定洪峰增值发生条件，增进对漫滩高含沙洪水传播的理解，为黄河下游洪水防治和风险管理提供依据。

本项目以黄河天然（弯曲）滩槽河道为背景，研究了高含沙洪水与滩槽的相互作用及反馈机制。首先，基于全耦合水-沙-床数学物理方程，考虑漫滩含沙水流的紊动扩散作用和弯道二次环流效应，建立了适应于弯曲滩槽河道的高含沙水流平面二维耦合动力学数学模型，并通过黄河下游“04.8”、“92.8”高含沙洪水及河口三角洲河道2018年汛期洪水，验证了模型的准确性和可靠性。其次，通过数值算例对比研究，揭示了高含沙洪水与滩槽的相互作用机制。在黄河下游的滩槽河段，高含沙洪水对洪峰期间的滩淤槽冲、主槽摆动和洪峰后的主槽回淤起主导作用，低含沙洪水对地形的塑造作用较弱。洪水漫滩与归槽在顺直滩槽段主要表现为滩槽之间的水沙横向紊动扩散过程，在弯曲滩槽段以纵向滩槽之间的强对流作用为主，因而，弯曲滩槽的入口-弯顶段有明显的（滩地）淤积徒增现象。在河口三角洲，河道发育前期，小流量更易形成散乱游荡的滩槽格局，大流量促进主槽刷深和支汊浅沟发育；发育后期，水沙与滩槽冲淤相适应，始终呈现出单一归顺的河道形态，大流量对主槽的维持起主导作用。河口三角洲备用流路启用后，行河流路主槽冲刷，通过水下三角洲前缘的淤积维持流路向海延伸；原流路则发生淤积逐渐废弃。此外，河床阻力的变化和河床冲刷对非漫滩高含沙洪水的洪峰增值起主导作用，而强漫滩洪水受复杂滩槽冲淤影响，机理复杂。因而，进一步结合特征线理论，推导了洪峰流量在特征线上变化的相容方程，结合1973-2012年间的21场洪峰增值事件的实测洪水数据，对其增值影响因素进行了定量分析。结果表明，洪峰增值是多个因素相互作用的复杂过程。其中，由河宽梯度引起的压力项以及由重力和阻力组成的外力项是影响洪峰增值的主导因素，前者促进洪峰增值，后者抑制洪峰增值。以上研究成果为黄河滩槽河道的防洪治理与综合规划及河口流路的稳定维持提供了理论和技术支持。

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