雨强和糙度对坡面薄层流水动力学特性的影响

基于流体力学和水力学基本理论,通过6个糙度、5个流量和5种雨强组合条件下的放水冲刷和模拟降雨试验,研究雨强和糙度对坡面流水动力学特性的影响。结果表明:在坡面薄层水流中,床面糙度对坡面流阻力有"增阻"效应,但在粗糙尺度为1.77 mm时产生拐点;降雨条件下,坡面流自由表面失稳的动态演化过程显著,其波动临界条件为黏深比0.382 4、0.599 3,在整个动态过程中阻力系数都随着降雨强度增加而减小;结合黏滞阻力、雨强阻力构建层流过渡区阻力计算公式,决定系数为0.92(P0.05),可以较好地为坡面流模型的建立提供参考依据。研究成果有助于从泥沙运动力学的角度揭示坡面流层流失稳的本质,为坡面流理论的发展奠定基础。

Influence of rainfall and roughness on hydrodynamic characteristics of overland flow

Abstract: Overland flow is the initial motivation of slope surface erosion and the key reason for soil degradation. In order to study the influence of rainfall intensity and roughness on slope surface water dynamic characteristic, the artificial simulated rainfall experiments were carried in the Soil and Water Conservation Engineering Laboratory in Northwest A&F University. Based on the basic theory of fluid mechanics and hydraulics, the influence of rainfall intensity and roughness on hydrodynamic characteristics of overland flow was meticulously investigated. The experiment included 6 roughness, 5 flow discharges and 5 rainfall intensity. The theory of friction velocity and viscous sublayer in sediment movement mechanics were also applied in this study. Results showed that under the stage of overland flow, the data points of flow resistance coefficient was far from the laminar flow line and turbulent flow line with the continuous increase of roughness when the roughness was smaller than or equaled to 1.77, indicating that the bed surface roughness of slope surface had an effect of resistance increase to flow resistance. However, the resistance coefficient fluctuated to being relative stable when the roughness was greater than 1.77 mm. Under the rainfall condition, the resistance coefficient decreased with the increase of rainfall intensity when roughness was closer to 0. In addition, the resistance coefficient was concentrated when the roughness increased. Under the stage of different rainfall intensity and roughness, the overland flow was accompanied by production and disappearance of roll wave. The phenomenon could be expressed by the ratio of viscous sublayer thickness and mean water depth. By observation, the ratio at roughness of 3.68 when the roll wave was obvious was calculated. When the ratio was between 0.382 4 and 0.599 3 (the difference was 0.24), the overland flow was in transition zone, otherwise, in roll wave zones. On the conditions of same roughness and rainfall intensity, Darcy-Weisbach resistance coefficient would decrease with the increase of turbulent flow Reynolds number. Under the condition of the same roughness, the resistance coefficient decreased with the increase of rainfall intensity. The resistance coefficient was 1.25-2.6, 1.3-2.7, 1.8-3.0, 1.8-3.15, 1.8-3.15, showing a trend of increase than being stable with increase of resistance coefficient. The turbulent flow Reynolds number was 0.35-36.1, belonging to transitional flow zone (0.35-900) according to turbulent flow Reynolds number standard but belonging to transitional and laminar flow zone according to traditional Reynolds number standard. It indicated that the standard for overland flow deserved further study. Based on the critical water depth, the overland flow was in rapid zone mostly. As the roughness increased, the flow transferred from rapid zone to slack zone. In addition, this study provided an overland flow resistance calculation formula by comprehensively considering viscous resistance, circle resistance and rainfall resistance. By validation, the formula had a high accuracy with R2 of 0.92 (P<0.05). The results are helpful to understand hydraulic flow erosion mechanism of slope surface, and provide data support for building soil erosion model.