坡面含沙水流波流耦合特性及泥沙输移机理研究

Soil erosion is one of the important environmental issues faced by people, slope surface is the basic elements of the loess plateau, The study of the hydraulic characteristics of overland flow and erosion and sediment yield rules is the key scientific problem of watershed sediment yield mechanism and ecological environment restoration and reconstruction in soil and water losses regions. Thus, this study will focus on the temporal and spatial variations of hydraulic properties of slope sediment flow through the combination of indoor experiment, theoretic analysis, and numerical simulation. It intends to investigate the driving mechanism of rolling wave evolution by inertial force, gravity, and shearing force and the critical condition for instability; to provide a critical judging criterion for the transition of slope flow regimes; to illuminate the “resistance enhancement” mechanism of flow based on the principles of turbulence bursting and boundary layer; to give the calculation formula for bed shear stress under the condition of wave and flow coupling; and then on this basis to establish a new function of sediment transport capacity that is suitable for slope flow in the Loess Plateau based on the balance between sediment suspension power and turbulent kinetic energy. The results of this study will help to reveal the internal mechanism of suspended sediment discharge of slope rolling wave from the perspective of hydraulics and lay a theoretic foundation for the establishment of an erosion forecast model with physical causation

土壤侵蚀是人类面临的重要环境问题之一，坡面是黄土高原流域组成的基本单元，坡面水流动力学特性及侵蚀产沙规律的研究是认识流域产沙机制和水土流失区生态环境恢复重建的关键科学问题。为此，本项目融紊流力学、泥沙运动力学、土壤侵蚀学、细尺度流体力学等理论为一体，通过室内试验观测、理论分析与数值模拟相结合的方法，研究坡面含沙水流水力要素的时空变化规律；系统分析径流滚波演化、失稳及猝灭临界条件的基础上，探讨惯性力、重力分力、床面剪切力对滚波演化驱动机制,提出坡面水流流态转捩临界判数；并根据紊流猝发理论和边界层理论阐明水流“增阻”机理；基于波动理论，推导出波流耦合条件下床面切应力计算式；在此基础上，以泥沙悬浮功与水流紊动能相平衡建立适应于黄土高原地区坡面流输沙能力的新公式，该研究成果将有助于从动力学角度揭示坡面滚波流悬移输沙的内在机制，进而为构建具有物理成因的侵蚀预报模型提供理论。

坡面水流动力学特性及侵蚀产沙规律的研究是认识流域产沙机制和水土流失区生态环境恢复重建的关键科学问题。本项目主要开展了定床阻力试验、动床降雨和人工加沙试验，探究了雷诺数大区间内坡面流滚波产生的力学机理及动态演化规律。发现：坡面水流失稳产生滚波，其流区的界定区间随糙度的增大而减小。随单宽流量增加，滚波波高及相对波高先增大后减小，波频及波速不断增加，相对波速呈幂函数的规律减少。其中滚波波速可由单宽流量、能坡、粗糙度三者以幂函数的形式表示，单宽流量和能坡对滚波波速贡献显著，粗糙度对滚波波速影响较小。关于流态界定，坡面流流态指数均值0.543，坡面流属于过渡流区为主，层流区为辅的混合流区，这与传统雷诺数判法结论一致。如据试验现象采用滚波判数δ/h判别，水流处于滚波流区与紊流区的混合流区，且随糙度增大有向紊流区转捩的趋势。相同雨强条件下，阻力系数随着糙度的增大而增大。相同糙度条件下，阻力系数随着雨强的增大而减小。糙度的“增阻”的机理主要与边壁摩阻力、黏性底层厚度、压差阻力以及滚波四方面有关，而雨强的“减阻”机理可利用光面和人工加糙试验的对比得出，并根据阻力分割的思想和阻力叠加的方法推得降雨条件下坡面流阻力模型。含沙坡面流雷诺数变化范围是296.6~1249.4，弗劳德数在2.032~2.874范围变化，即含沙水流为急流，处于层流区和过渡流区。阻力系数初期随含沙量的增大相应减小，后期不断增大。含沙水流的平均流速与流量和含沙量均存在一定关系，但流量对平均流速的影响更为显著。各水流强度指标与输沙能力关系中，有效水流功率相关性最强。选取无量纲有效水流功率的水流强度指标和体积含沙量作为影响因子，可率定得坡面水流输沙能力计算公式，精准预测不同粒径土壤输沙能力。目前课题组基于该项目研究实际发表学术论文24篇，其中SCI 6篇，EI 8篇，培养10名硕士生毕业。该成果将有助于从动力学角度揭示坡面滚波流悬移输沙的内在机制，进而为构建具有物理成因的侵蚀预报模型提供理论。