干旱区荒漠植被对河岸侵蚀的影响机理及数值模拟

The erosion rate of riverbank can be reduced effectively by planting some desert riparian vegetation and other ecological measures in arid region. But the existed researches are limited to tropical vegetation and the influence results of vegetation on riverbank stability and erosion rate, which can not reflect the interaction between vegetation roots and soil, and the influence mechanism of desert riparian vegetation on riverbank erosion under the action of flow dynamic. Firstly, the erosion processes of main stream riverbank of Tarim River will be investigated in this research and several typical desert plants will be selected. Some relationships between the riparian vegetation roots and soil properties are researched in the process of riverbank erosion, including the physical properties, shear properties, incipient condition and scouring characteristics of soil. Secondly, combined with flume experiment of bank erosion process under the influence of typical desert vegetations, three influence actions of vegetation roots on bank erosion are analyzed, which are the network, retaining and pulling. The deposition pattern of the failed soil, the modes of disintegration and transport for the deposited soil mass under the fluvial erosion are also analyzed. The erosion and collapsed mode are proposed under different roots characteristics and different soil composition, and the influence mechanism of desert riparian vegetation on bank erosion can be discovered. Finally, a calculating method of riverbank stability will be proposed, and it is coupled with the longitudinal and lateral deformation model of riverbed. Thus a new integrated morphodynamic model is developed to predict the evolution of riverbank erosion considering the influence of desert riparian vegetation, which can provide the technical support and scientific basis for the future research on the erosion mechanism of desert vegetation in arid Region.

在干旱地区，利用荒漠植被等生态措施可以有效减少河岸侵蚀，但现有研究多针对热带植被以及植被对河岸稳定性和侵蚀速率影响结果，不能综合反映在水流动力作用下植被根系与土体相互作用以及荒漠植被对河岸侵蚀影响机理等。因此本项目以塔里木河干流河岸为研究对象，首先选取典型荒漠植被，研究河岸侵蚀过程中植被根系对河岸土体力学性质、剪切特性以及河岸土体起动和冲刷特性等的影响规律；同时结合典型荒漠植被影响下河岸侵蚀过程水槽试验，分析植被根系对河岸侵蚀的网络作用、护挡作用和牵拉作用以及崩塌后土体在坡脚堆积形式和进一步被水流分解、输移模式等，提出不同根系特征和不同土体组成下河岸侵蚀、崩塌模式，揭示荒漠植被对河岸侵蚀的影响机理；然后建立荒漠植被影响下河岸稳定性计算方法，并与河床纵向和横向变形模型进行耦合，构建新的河岸侵蚀数学模型，对河岸侵蚀过程进行数值模拟，为进一步研究干旱区荒漠植被固土护堤作用机理提供支持和理论保证。

本项目以塔里木河干流河岸为研究对象，采用实地查勘与现场取样、室内土工试验和理论分析等方法，开展了典型荒漠植被根系对河岸侵蚀影响机理定量分析及侵蚀过程模拟研究。首先，通过2次6个典型河段的实地查勘与现场取样，获取了6种不同植被（红柳、甘草、骆驼刺、芦苇、胡杨、白刺）的根-土复合体，分析了不同植被根系特征下根-土复合体的基本力学性能指标、抗剪强度指标的变化规律，得到了复合体抗剪强度与粘聚力和内摩擦角定量关系式；从根-土相互作用力学机制入手，分别建立柔性和刚性状态下根系力学模型，计算了根系对土体抗剪强度贡献值；其次，选用塔里木河原型岸坡土体进行了典型根系作用下土体侧向冲刷试验，包括3种典型根系（红柳、胡杨、芦苇）、3种放置方式（竖直、倾斜和相交）和5种根系体积密度（8%、13%、18%、23%、28%）的交叉试验，分别得到了不同放置方式下和不同根系特征下土体起动切应力和冲刷系数与根系体积密度和土体干密度之间的关系；同时，在无植被和有植被根系（红柳、胡杨、芦苇）条件下，分别进行了非粘性土体和粘性土体岸坡冲刷过程概化水槽试验，定量分析了不同根系特征对不同土体河岸侵蚀过程影响的网络作用、护挡作用和牵拉作用等机理，包括根系影响下河岸崩塌过程、崩塌类型及近岸流场分布等；最后，综合考虑了典型根系对河岸土体基本力学性质、剪切特性、冲刷特性等影响的定量关系，以及根系作用下崩塌土体对坡脚掩护作用等，建立了典型荒漠植被根系影响的河岸稳定性计算方法，分别选取塔里木河流域阿拉尔新大桥和沙雅大桥断面，运用模型对有植被覆盖与无植被覆盖河岸侵蚀过程分别进行了模拟，并对模拟结果进行了对比分析，定量评价了植被根系对河岸侵蚀过程的影响。本项目研究成果不仅为干旱区荒漠植被固土作用机理提供技术支持和理论保证，而且对于促进防治塔里木河水土流失和建设良好生态环境以及推动基础研究和交叉学科发展均具有重要的现实和理论意义。