坡面含沙水流非稳态特性及泥沙驱动机理研究

Soil erosion is one of the global environmental problems that restrict the survival and development of human beings. The slope is the basic unit of hilly and gully landform in Loess Plateau. The study on the unsteady characteristics of overland flow and the driving mechanism of sediment is the key scientific issue to understand the mechanism of watershed erosion and sediment production, guide the ecological restoration in the soil erosion area, and guide the prevention and control of debris flow disasters. Many different subjects are combined in this project such as hydraulics and river dynamics, sediment transport mechanics, fluid mechanics, soil erosion, data mining. Moreover, modern image processing technology is adopted such as high-definition photography, correction and automatic classification. In this paper, a new method of measuring hydraulic elements for the overland flow is proposed. Based on above research, the temporal and spatial variation of the hydraulic parameters on overland flow will be studied systematically. The critical threshold of free surface instability will be proved, and a new idea of dividing flow pattern division is revealed. The driving mechanism of viscous force, surface tension and shear stress near the wall layer on the evolution of unsteady flow will be discussed. The essence of "increasing resistance" will be clarified. Based on energy dissipated in turbulence is equal to that needed for sand suspension, a dimensionless new model to calculate the capacity of sediment transport is proposed for different soils. This study will be helpful to reveal the internal mechanism of overland sediment laden flow “"roll wave lifts sediment and flow transfers sediment” from the point of view of dynamics. Moreover, it can provide a theoretical basis for establishing erosion prediction model based on physical causes.

土壤侵蚀是制约区域发展的全球性环境问题之一。坡面是黄土高原丘陵沟壑地貌的基本组成单元，坡面水流非稳态特征及泥沙驱动机理的研究是认识流域侵蚀产沙机制和指导水土流失区生态修复，泥石流灾害防控的关键科学问题。本项目融水力学及河流动力学、泥沙运动学、流体力学、土壤侵蚀学、数据挖掘等于一体，采用高清摄影技术、正射纠正及自动分类等影像处理技术，提出坡面水流水力要素测量新方法；在此基础上，系统研究坡面薄层水流各水力参数的时空变化规律；探明自由表面失稳临界阈值，提出薄层水流流态划分新思路；探讨粘滞力、表面张力、近壁流层切应力对坡面非恒定流演化驱动机理，阐明水流"增阻"现象的本质；基于水流紊动耗能与泥沙悬浮功相平衡理论建立适应不同土壤质地的无量纲输沙能力计算新模型，该研究成果将有助于从动力学角度揭示坡面含沙水流“滚波掀沙、水流输沙”内在机制，进而为构建基于物理成因的侵蚀预报模型提供理论依据。

坡面薄层水流非稳态特征、阻力机制和泥沙驱动机理的研究是构建土壤侵蚀模型的基础，也是新形势下坡面侵蚀机理研究的难点问题之一。本项目通过坡面径流试验、水槽加沙试验和人工模拟降雨试验，探究坡面含沙水流滚波演化机理、阻力特性、泥沙驱动机理及非平衡输沙能力预测模型，主要取得以下成果：开发了高效科学的薄层水流水力参数测定新方法，即摄影测量法，相较传统测量方法时间精度提高了0.01s，雨强、时间对该技术提取流速的结果影响很小，可适用于不同雨强、不同时间间隔的实际流速测量过程。揭示了坡面薄层水流流态归属及阻力机制，对于80%甘油与20%水的混合流体，流区指数m=0.30，水流为层流；清水时流区指数m=0.48，流态介于层流和紊流之间；降雨条件下，水流主要处于层流和过渡流区。清水和混合流工况下的阻力系数皆随淹没度的增加而减小，在淹没度相等的条件下，混合流工况下的阻力系数要大于清水工况下的阻力系数。对于所有试验条件下，淹没度可以解释阻力系数15%的变异性。借助量纲分析法构建了清水和混合流体下的阻力系数计算模型。滚波沿传播路径存在显著聚合现象，粗糙度和雨强是影响滚波产生及演化的主导因子，滚波波速、波长及波高随粗糙度的增加而减小，随流量的增加而递增，但波频随粗糙度的增大而增大，表明雨强促进了滚波的产生，而粗糙度则起相反作用；另外层流区滚波产生的阻力系数介于0.64~4.85之间，雷诺数范围为142~416，水流失稳的弗劳德数临界值约为0.57。根据平均流速、剪切应力、径流功率和单位水流功率间的内在联系提出了统一水流强度参数，建立了基于统一水流强度指标的输沙能力预测模型（决定系数R2=0.92，模型效率值ME=0.88，相对均方根误差RRMSE=6.6%）。目前课题组基于该项目发表学术论文19篇，其中SCI收录15篇，EI收录3篇，北大核心收录1篇，培养16名硕士生、1名博士生毕业。该成果既为滚波的理论研究、坡面径流侵蚀产沙机理及侵蚀演变规律研究奠定理论基础，也对水力学和河流动力学理论向细观尺度的延伸提供有益参考。