坡面薄层水流与微地貌的耦合作用机制研究

This proposal is foucusing on the hydrodynamic characters of the sheet flow on the eroding hillslope in the rill initiation stage, and the coupling mechanism of microrelief change and the sheet flow will be studied. Through this study, the increase of knowledge on hydrodynamic process of the sheet flow on eroding slope, an improvement on flow resistance expression in rill development models, and a more univeral hydrodynamic critical condition of rill initiation are expected. To be specific, we are going to carry out a series of plot scale laboratory erosion experiments to study the evolution of microrelief on slopes, with especially attention on microrelief change in the initial stage of rills. The microrelief will be measured using our newly developed structure light microrelief scanner which could obtain up to 5 measurements result per second. Thus a set of very detailed microrelief evolution data, which is very rare in rill erosion studies as limited by experimental technology, would be obtained. The local geometrical characteristics and the statistical properties of the microrelief would be analyzed. And basing on the measured microrelief, a series of numerical studies of sheet flow characters would be conducted with Computational Fluid Dynamics (CFD) method based on 3D N-S equation. The Volume of Fluid (VOF) method will be used to describe the multiphase flow of air and water, and the free surface could be calculated. The boundary of the flow would be considered as rigid boundary which could simplify the simulation, and representative microrelief state in the evolution series will be used in flow simulations. In this way, the undiscovered detailed flow property could be studied and the connection between flow and microrelief gains its chance to show. The flow regime, flow patten and the resistance constitute, which could reflect the mechanism of the coupling between sheet flow and microrelief, will be discussed globally at the plot scale and locally at the microrelief scale. A comparison of measured microrelief change and calculated erosion capability of the sheet flow could also possibly provide important information on soil erodibility. When the mechanism of sheet flow has been further understood, the discussion on critical conditions of rill initiation could runs deeper. The effective range of the existent critial index will be studied, and we will attempt to set up a new indicator of rill development stage which would fully reflect the change of flow characters. In summary, this proposal is designed to discuss the coupling mechanism of the microrelief and the sheet flow on it, which could provide theorical support of hydrodynamic property and fundamental data for the prevalent rill development simulation studies. And pratical application of this study could very possibly set up a better critical indicator of rill initiation which could provide benefit for modern erosion models.

本项目拟针对坡面水力侵蚀过程中地表薄层水流与微地貌相互耦合这一关键问题，以微地貌如何影响地表薄层流动特征为切入点，探讨细沟发育初期微地貌与地表薄层水流的耦合作用机制。具体而言，本项目拟通过先进的地形测量手段与高精度薄层水流数值模拟技术的结合，对细沟发育初期的微地貌演化过程进行精细测量；并以测量的微地貌为基础建立数值模型，基于三维N-S方程和VOF两相流模型，求解相应微地貌条件下地表水流的流动状态；了解此时流动的分布、阻力和脉动等特性，进而讨论微地貌演化对流动特征的影响，建立合理的薄层流动表达；同时基于流动的侵蚀特性，结合实测微地貌改变来讨论薄层水流对微地貌变化的影响方式；最终提供众多细沟发育模型研究所需的坡面水动力特性，并为其提供完整的可验证实验数据和理论支持，促进这类研究的发展和应用；同时，依靠对微地貌与地表流动耦合作用的完整认识，探寻更有效的细沟发育表征参数及其临界值，为应用中的需求提

本项目以坡面水力侵蚀过程的发生和细沟发育为背景，以坡面水力侵蚀过程中地表薄层水流与微地貌相互耦合作为关键科学问题，从微地貌如何影响地表薄层流动特征的角度切入，探讨细沟发育初期微地貌与地表薄层水流的耦合作用机制。 主要研究内容有：1、通过先进的地形测量手段与高精度薄层水流数值模拟技术的结合，对细沟发育初期的微地貌演化过程进行精细测量；2、以复杂微地貌为基础建立数值模型，基于三维N-S方程和VOF两相流模型，求解相应微地貌条件下地表水流的流动状态；3、了解复杂为地貌条件下流动的分布、阻力和脉动等特性，进而讨论微地貌演化对流动特征的影响，建立合理的薄层流动表达；4、基于流动的侵蚀特性，结合微地貌改变、尤其是植被改变，来讨论薄层水流对微地貌变化的影响方式；5、将所建立的为地貌演化和细沟发展过程结果引入小流域尺度的产流和侵蚀模型，建立有工程应用价值的模型。. 得到的重要结果有：1、自然条件下实际微地貌的统计特性表现出一定的规律性，一般条件下，微地貌高程数据服从正态分布，而与坡面水流水深相同尺度的元地形空间上为均匀分布，个别较大的微地貌如植被凸起或明显凹坑分布具有随机性，但单位平方米内数量一般很少；通过上述分布规律重建的微地貌具有与实际微地貌相似的代表性；2、基于有限体积方法和VOF的两相流模型能够在准确地模拟薄层水流的流动特征的同时提供更高的求解速度，在表征复杂的自由表面时也更为准确；3、微地貌对坡面薄层水流流动结构和阻力的影响主要是通过产生流动的空间不均匀性造成的；宏观阻力的表达式中必须引入表征微地貌特征的参数，其将通过Re和Fr间接对阻力施加影响，Dunkley等人提出的淹没率等参数具有明显的局限性；微地貌特征尺度与水深的相对关系更加合理，是水流阻力大小和构成的关键控制因素。. 研究的主要科学意义有：1、探索并建立了一套基于N-S方程进行复杂微地貌坡面流动特征和阻力估计的方法，为进一步研究提供了新方法和新思路；2、认识到兴波阻力是薄层水流阻力复杂性的关键因素，大Fr数条件下兴波阻力的迅速增长是当前以Re数为基本要素的阻力公式失效的根本原因，而目前考虑淹没条件的众多阻力模型仅考虑了淹没条件造成的压差阻力特征，而未能考虑其对兴波阻力的影响，这是这些模型不够理想的关键；3、发展了可用于工程实际的反映微地貌影响的小流域尺度坡面流动模型，为坡面水力侵蚀的宏细观结合研究提供基础平台.

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