斑块状植被覆盖下坡面流水动力学特性

Hydrodynamic characteristics of overland flow under patchy vegetation cover

为阐明斑块状植被随机覆盖下坡面流水动力学特性,通过5个覆盖度、6个坡度和7个流量组合条件下的室内放水冲刷试验,系统研究了斑块状植被覆盖下坡面流的流型流态及阻力机制。研究结果表明:1)该试验工况下,坡面水流位于虚拟层流区和过渡流区,水流流态发育受覆盖度和坡度相互制约。2)在虚拟层流区,综合阻力系数与雷诺数呈负相关,而在过渡流区,二者关系在临界覆盖度处发生转捩,随覆盖度的增加,二者关系逐步由负相关变为正相关。3)综合阻力系数与覆盖度呈幂指函数相关,而随淹没度变化趋势则受制于覆盖度。低于临界覆盖度时,二者整体上为负相关;高于临界覆盖度时,综合阻力系数随着淹没度增大先减后增。此外,基于水流阻力等效原则,综合考虑水力坡度、覆盖度、雷诺数、淹没度的影响,建立了坡面流阻力计算模型。

The flow of surface water under vegetation cover has a significant impact on the hydraulic erosion of slope. Most previous studies focused on vegetation type, coverage, stiffness, diameter, submergence and height, particularly on the relationships between some botanical attributes and flow resistance. In recent years, the spatial location and distribution pattern of vegetation, such as sloping land, have become drawn much attention. However, few researches have been conducted on the sloping land under natural conditions, where less disturbed by human activities and covered by fragmented patchy vegetation. To clarify hydrodynamic characteristics of overland flow covered by patch vegetation, a fixed-bed resistance simulation scouring test was conducted at the State Key Laboratory of Soil Erosion and Dry-land Farming on the Loess Plateau, China. The test was set up with 6 slopes with a range of 0.034 9 to 0.207 9. According to the critical intensity of erosion rain on the Loess Plateau, seven single-wide flows were designed, ranging from 0.278 × 10-3 to 2.500 × 10-3 m2/s. To simulate the vegetation distribution in the natural state, the grass was randomly arranged in patches, where5 coverages were set up to explore the critical coverage of vegetation on the slope. A systematic analysis has been made to investigate the resistance mechanism of slope flow zone under patch-like vegetation. The results showed that: 1) Under the experimental conditions, the overland flow over vegetation coverage could be considered as the virtual laminar flow and transitional flow,and this evolution process of the water flow pattern was restricted by vegetation coverage and slopes. As the slope gradient increased, the flow pattern extended from the subcritical flow to supercritical flow, while it showed an adverse trend when the coverage degree increased. 2) The formation mechanism of slope resistance was closely related to the coverage condition,flow discharge and water depth. The proportion of particle resistance decreased when these three factors increased, but it had nothing to do with the slope gradient. 3) In the transitional flow zone, the comprehensive resistance coefficient was negatively correlated with Reynolds number, while the aforementioned relationship was mainly influenced by the varying vegetation coverage degree in the transitional zone. With the increase of coverage degree, it gradually changedfrom the negative correlation to positive correlation, and the influence of slopes gradually became dominate. 4) The comprehensive flow resistance coefficient showed a power function relationship with the coverage degree. When the coverage degree did not reach a critical value, the comprehensive flow resistance coefficient rose significantly as the coverage degree increased, but this trend tended to be stable as the coverage degree was relatively larger than the critical value. This relationship was alsorestricted by the flow discharge and slope gradient, that is why the f-Cr relationship curve would cross under the different flow discharges when the coverage degree increased. 5) The relationship between the comprehensive flow resistance coefficient and submergence degree was also related to the coverage degree. When it did not reach to the critical coverage degree, they were negatively related. But as it increased continuously, the comprehensive flow resistance coefficient first decreased, and then increased as the submergence degree increased. 6) In addition, usingthe stress analysis of water body covered by patch vegetation, a calculation model of overland flow resistance considering slope gradient, vegetation coverage,Reynolds number and submergence degree was established based on the equivalent principle. The determination coefficient was 0.831 and 0.806, respectively in the virtual laminar flow and transitional flow zone. It can be seen that the Reynolds number played a leading role in the virtual laminar flow zone, while the coverage degree was more important in the transitional zone.As the Nash coefficient was 0.836 and 0.784, it indicates that this model can simulate the flow resistance on slopes covered by patch vegetation. This finding provides a theoretical basis for adopting vegetation measures for soil and water conservation.