模拟降雨下植被盖度对坡面流水动力学特性的影响

通过模拟降雨实验的方法,分析研究了坡度10°和20°,降雨强度30mm/h和60mm/h条件下不同盖度黑麦草对坡面产流产沙的调控过程,并从雷诺数、弗劳德数和阻力系数三个方面对水流运动过程和黑麦草调控坡面流的水力学特性进行了剖析。研究结果表明:雷诺数随坡度增加而相对增大,随降雨强度增大有明显增大趋势。黑麦草覆盖能够明显减小坡面径流雷诺数,在各降雨强度和坡度条件下,雷诺数随黑麦草盖度增加而减小,雷诺数大小一般呈现:裸坡20%40%60%80%。黑麦草盖度对坡面流弗劳德数有显著影响,随着黑麦草盖度增加弗劳德数减小,并且弗劳德数随盖度变化为:裸地20%40%60%80%,坡面阻力系数与坡面产沙率有良好的拟合关系,随坡面阻力系数的增大,坡面产沙率呈对数减小,并且阻力系数在0—1时减小速率很大,阻力系数大于1以后减小曲线较为平缓。

Impact of vegetation cover on surface runoff hydraulic characteristics with simulated rainfall

Soil and water loss has caused severe land resources depletion and eco-environment degradation in many areas of China, which leads to reduction of local agricultural and industrial productivity. Furthermore, as a severe environmental problem, soil and water loss will affect many respects, such as nature, economy, society and science. Many factors are influencing the degree of soil erosion, Plant vegetation coverage, rainfall intensity and duration, soil type and texture, etc. In this study we chose slope, rainfall intensity and vegetation coverage as impact factors to the degree of soil erosion. Plant ryegrass is one of the feasible measures to avoid soil and water loss. In order to understand the process of runoff movement under the complex surface conditions and the effect of flow and different surface conditions on hydraulics parameters, we utilized the artificial rainfall approach to study the process of runoff and sediment yield and its hydraulic characteristics to explore how ryegrass play a role in the work of soil and water conservation. The experiment was designed with 30 mm/h and 60 mm/h rainfall intensity and slopes of 10° and 20° with different coverage of ryegrass (0, 20%, 40%, 60%, 80% and 100%) in Jiufeng experimental forest farm of Beijing Forestry University. During the simulated rainfall process, the runoff samples were collected at fixed time: in the first ten minutes, the runoff was sampled every two minutes due to the fast increase of runoff amount, and then the runoff was sampled every five minutes in the last fifty minutes as the change of runoff amount went slow. At the end of the experiment, the volume of runoff was measured, and then standing the runoff samples were placed and dried for 24 hours, respectively, to obtain the amount of sediment. During the simulated rainfall process, the velocity of overland flow was measured every ten minutes. Then the surface runoff and hydraulic parameters were calculated with runoff, sediment and surface runoff velocity. By choosing Reynolds number, Froude number and drag coefficient as factors to characterize the water advance and water recession process, the results showed that: the Reynolds number increases with the slope degree and rainfall intensity. The increase of ryegrass could play important roles in both the decrease of the Reynolds number and the Froude number, and the Reynolds number and the Froude number under different coverage are: bare land > 20% > 40% > 60% > 80% and bare land > 20% > 40% > 60% > 80%, respectively. The drag coefficient and sediment yield rate were well-fitting, and the sediment yield rate decreased as the drag coefficient increased. When the drag coefficient was between zero and one, the growth rate of sediment yield was larger than the drag coefficient. By this study, we know that ryegrass has a great significance to soil and water conservation. Moreover, the increase of ryegrass coverage leads to the enhancement of the soil and water conservation function. The results of the ryegrass regulating runoff and preventing soil erosion in this study have theoretical significance to understanding the process of soil erosion.