干热河谷冲沟沟头后退的水力、重力协同作用机制

Dry-hot Valley Region of Jinsha River is now recognized as one of the most severe sediment production regions in the Upper Reach of Yangtze River, which is due to the fact that gullies are well developed in this region. It is of great importance to carry out research on gully erosion for protecting land resources and controlling river sediments. Long-term field investigations indicated that gully head was the most active part in mophology changing and sediment yielding, and its activeness would determine the development direction of whole gully. However, systematic studies on the dynamic processes and mechanisms of gully head retreat has been paid little attention...The applicants of this project found that gully head retreating process is caused by the synergistic effect of hydrodynamic and gravitational action. Hydraulic and gravitational action processes condition and accelerate each other, which drive the gully head developing backward...This project will seek to explore the increasement of effective freeing surface , the change of soil mechanical properties and their influencing mechanisms on soil stability under hydraulic effect using artificial rain, in-situ runoff scouring experiment, numerical simulation, lab experiment and analysis, and then determine the critical conditions of suspended soil collapse based on the Limit Equilibrium Theory. The results of the study will reveal the synergistic mechanism between hydraulic and gravitational effect and provide the theoretical basis for revealing the laws of gully initiation and development.

干热河谷是长江上游产沙最为强烈的地区之一，冲沟发育是其中的一个重要原因，开展冲沟侵蚀研究对于保护土地资源、控制江河泥沙意义深远。长期野外监测表明，沟头是冲沟形态变化、侵蚀产沙最为强烈的部位，其活跃的程度决定着整个冲沟的发展方向。但是，迄今为止，有关冲沟沟头后退的动力过程及机制仍不十分清楚，相关研究亦不够系统、深入。..申请者在科研实践中发现：冲沟沟头的后退是水力和重力协同作用的结果，水力作用过程和重力作用过程互为条件，相辅相成，共同推动沟头向后发育。..本研究拟通过人工降雨和原位放水冲刷试验，结合室内实验分析和数值模拟方法，基于极限平衡理论，探明水动力作用下沟壁有效临空面的增加与土体力学性质的变化对沟壁土体稳定性的影响机制，确定土体崩塌发生的临界条件，揭示沟头后退过程中水力、重力的协同作用机制，研究成果为进一步揭示整个冲沟发生发展规律奠定基础。

金沙江干热河谷冲沟侵蚀强烈，以沟头后退侵蚀过程为主。本研究通过构建不同跌坎高度沟头原位模拟试验平台，采用放水冲刷试验，研究了沟头后退侵蚀过程中的径流水动力特征、产沙特征及地形变化特征，查明了侵蚀过程中沟头径流能量转化、耗散特征及崩塌发生的临界条件，揭示了冲沟沟头后退侵蚀过程的动力学机制。这不但对于深刻揭示该区域冲沟侵蚀发生发展规律具有重要意义，而且对于采取针对性的冲沟治理措施具有重要的实践指导意义。主要研究结论及取得的创新性认识如下：（1）阐明了沟头径流能量变化特征及跌坎高度影响机理；坡面径流在沟头部位能耗大，且能耗随跌坎高度增大呈显著增加趋势；跌坎高度每增大25cm，120L/min和150L/min两种流量跌水势能转化量分别平均增加4.89J/s和6.10J/s，跌水剪切力分别平均增加24.1Pa和22.2Pa。（2）查明了沟头后退侵蚀过程中的产沙特征；沟头部位侵蚀量对总侵蚀量贡献大，验证了干热河谷冲沟侵蚀产沙主要集中在活跃沟头部位的野外判定；当沟壁跌坎高度大于75cm时，沟头侵蚀量贡献达到50%以上，认为75cm是引起沟头后退侵蚀特征突变的临界跌坎高度。（3）阐明了沟头后退侵蚀过程中的地形变化特征；沟头部位地形变化最为剧烈，包括沟壁底部跌穴垂直下切和沟壁后退；沟头平均侵蚀厚度是集水区平均下切深度的1.74~8.92倍，是沟床平均下切深度的1.08~3.20倍；随沟壁跌坎高度增大，沟头部位平均侵蚀厚度和跌穴平均下切深度分别呈指数和对数增长趋势。（4）阐明了沟头后退侵蚀过程的动力学机制，初步探索建立了沟头土体崩塌稳定性模型；基于极限平衡理论，综合考虑水力和地形条件，初步探索建立了沟头土体崩塌稳定性模型。沟头土体稳定性系数，随裂缝发育深度增大呈显著的对数函数减小趋势，随土壤含水量增大呈显著的指数函数降低趋势，随内凹洞发育高度增大呈显著线性降低趋势。野外验证试验表明，模型可靠度达到了80%左右。模型建立对干热河谷沟头崩塌发生预测具有重要理论参考意义。