植被作用下河槽中水流对沙床冲刷机理及水沙两相运动规律研究

This project takes the channel flow and the non-cohesive uniform sediment-bed in water-sediment-vegetation circumstance as research objects. Through combination of field observation, model experiment, numerical simulation and theoretical analysis, the mechanism of sediment-bed erosion by channel flow and the motion characteristics of water and sediment phases under effect of vegetation patch are deeply studied. The main research contents include: (1) The vegetated flow in typical reach of Taihu lake is observed, which is used to provide basis for laboratory experiments.（2）Considering the effect of sediment erosion by channel flow at the root of vegetation, a new zoning model subdividing vegetation patch in water-sediment-vegetation circumstance is developed based on the previous zoning model in water-vegetation circumstance. (3) Turbulent coherent structures in different zones of vegetation patch are analyzed after the profiles of sediment-bed changing, then the correlation between turbulent coherent structures and sediment erosion at the root of vegetation is studied. (4) A numerical model simulating sediment erosion by channel flow under the effect of vegetation patch is established, by which the motion mode of sediment particle is explored. The motion characteristics of pore water and sediment particle are analyzed to reveal mechanism of sediment-bed erosion by channel flow under the effect of vegetation. The project involves aquatic phytoecology, experimental technique, theory of sediment transport, numerical simulation, multiphase flow theory and turbulent flow theory, which has important practical values to engineering practice and scientific research.

本课题选择水-沙层-植被环境中的水流和非粘性均匀沙床为研究对象，结合野外观测、室内试验、数值模拟及理论分析方法，深入研究植被作用下水流对沙床的冲刷机理及水沙两相流动规律。研究内容有：（1）选取太湖典型河道边滩植被水流进行野外观测，为室内试验提供依据；（2）基于前人对水-植被环境中植被区域分区模式的研究，考虑植株根部沙层受冲刷发生形态变化，提出水-沙层-植被环境中植被区分区模式；（3）植被区分区后，分析植被底部沙床形态发生变化后的各区水流紊动相干结构，揭示其与植株根部床面冲刷的关系；（4）建立刚性植被边界条件下的水沙两相流冲刷模型，模拟植被环境中水流对沙层的冲刷过程，利用该模型确定植被底部沙层内沙粒运动方式，分析沙层沙粒和孔隙水的运动规律，揭示植被作用下水流对沙床冲刷机理。内容涉及水生植物生态、试验技术、泥沙输移、两相流、数值模拟及紊流理论等学科领域，对工程实践和科学研究有实用价值。

天然河道中，植被的存在会增加河床阻力，阻沙淤槽，抬高水位，降低河道行洪能力，同时在植被附近形成局部强剪切流，对植被根部沙层产生冲刷侵蚀，植被稳定性受到影响甚至植被遭到破坏，而要准确描述这种冲刷侵蚀作用，须对这种植被环境下的特殊水沙两相流动、河道水流结构（包括植被区和非植被区）及沙床床面形态变化进行精细化分析研究。本文以植被为研究对象，对刚性植被环境中的特殊水流流动过程及床面泥沙运动形态进行了试验、数值模拟研究。所得到的植被环境中水流流动特性等研究成果对河流生态恢复、防洪工程、临水景观美化和环境保护等方面的研究和工程实践具有一定理论价值，同时也丰富了环境水力学方面的理论研究内容。所建立的描述泥沙起动的半经验函数表达式，为环境评价和生态植被河槽建设提供参考依据。通过理论分析、实验室试验及数值模拟相结合的方法分别对动床条件下植被区水流流动特性、植被环境中的泥沙起动问题、稀疏植被及植株带条件下的水流流动特性进行了精细化分析研究。