黄土坡面发育平稳的细沟流水动力学特性

由于天然降雨存在间歇性,常会出现前期降雨已经发育成型的细沟又经历了二次降雨的情况,但目前对这一条件下细沟流水动力学特性的研究还很少。本文采用室内人工模拟降雨的方法,通过间隔24 h两场不同雨强的降雨,研究黄土坡面经过第一场降雨形成比较稳定的细沟之后,在第二场较小雨强降雨过程中,细沟发育平稳时的流水动力学特性。研究结果表明:①细沟流速大小受坡长影响并不显著,相同坡度下不同坡长的细沟流平均流速差别不大;细沟流速受细沟形态影响较大,25°坡面比20°坡面细沟密度大,流速相对较小;②剪切力大小受流量和坡度共同作用;雷诺数与水流剪切力呈显著正相关关系,弗洛德数与水流剪切力则呈显著负相关关系;③随着距坡顶距离的增加,阻力系数呈增大趋势;第二次降雨强度相同,坡度大的坡面上阻力系数也较大,可见阻力系数与径流量和坡度的关系十分紧密;阻力系数与雷诺数之间呈显著正相关关系,雷诺数的增加意味着平均流速增大,水流强度增大导致细沟形态更为复杂,水流受到的阻力增大;虽然雷诺数增加同时也意味着水流深度增大,但从试验结果可见,陡坡条件下阻力系数受流速的影响较大。

Hydrodynamic characteristics of stable growth-rill flow on loess slopes

Rill erosion is the main way of slope erosion on farmland of the Loess Plateau. Rill erosion on slopes accounts for 70% of the total amount of erosion and plays an important role in soil erosion process on loess hillslopes. It is the beginning of qualitative change in the process of soil erosion. Studies of rill erosion can help control soil erosion on slopes, facilitate the development of agricultural production, and serve as the foundation of study of the development process of soil erosion. Natural rainfall may occur intermittently and rills may go through a second rainfall within a short time period. But studies on the dynamic characteristics of rill flow under these circumstances have been few. This research was carried out in the rainfall simulation lab of the State Key Laboratory of Soil Erosion and Dryland Farming on Loess from March to May 2010. Artificial rainfall was applied on a loess slope at an interval of 24 hours under two rainfall conditions, with the first rain that formed relatively stable rill followed by a light rainfall. The results show that: (1) rill flow velocity was impacted by slope length indistinctively. Average velocity of rill flow on slopes of different lengths was not very different. On the other hand, rill flow velocity was greatly influenced by rill morphology. Compared to 20° slope, rill density of 25° slope was higher, while its flow velocity was lower; (2) shear stress is jointly affected by flow discharge and slope. The Reynolds number and Froude number were significantly associated with flow shear stress-the Reynolds number is positively correlated with flow shear stress, but the Froude number had negative correlation with flow shear stress; (3) As the distance from the top of the hill increases, the Darcy-Weisbach resistance coefficient tends to increase as well. With the same rainfall intensity at the second time, the resistance coefficient of steep slope is higher. This indicates a close relationship between resistance coefficient and runoff and slope. There is a significant positive correlation between resistance coefficient and the Reynolds number. Higher Reynolds number means greater average flow rate and intensity. As a result of increased intensity of water flow, rill morphology is more complex and the flow resistance increases. Although the increased Reynolds number also means that the flow depth is larger, the test results show that the drag coefficient under the condition of steep slope is mainly affected by the velocity of flow.