坡面薄层水流水动力学特性试验

坡面流水动学特性对阐明土壤侵蚀和坡面产沙机理均有重要意义,采用坡面定床阻力试验,定量研究了6种不同粗糙度床面、5种不同坡度下坡面薄层水流水力要素关系及阻力的变化特征,以期揭示坡面薄层水流阻力的内在规律性。结果表明,坡面薄层水流流态指数随坡度呈现出的先减小后增加的变化趋势,当试验坡度小于0.15 rad时,流态指数随坡度的增加而逐渐减小,当坡度大于0.15 rad时,出现相反的变化趋势。流态指数随床面粗糙度呈抛物线变化趋势,其均值为0.376,总体上坡面薄层水流属于滚波流区和过渡流区的范畴;水流弗劳德数与单宽流量和试验坡度均成幂函数关系,临界流对应的单宽流量随粗糙度的增加而增大,随坡度的增加而减小,水流流型处于临界流和急流型态;阻力系数与单宽流量呈幂函数关系,而与雷诺数成反比关系,关于增阻的原因主要与绕流产生压差阻力和坡面滚波流引起的局部阻力有关,并根据薄层水流阻力特征,提出了滚波流区阻力计算公式。研究成果可为坡面土壤侵蚀预报模型构建提供理论依据,从而促进明渠水流理论向坡面水流方面扩展。

Experiment on hydraulic characteristics of shallow open channel flow on slope

Abstract: Hydraulic characteristics of overland flow are of importance in elucidating the mechanisms of soil erosion and sediment yield on slope. By a fixed-bed resistance experiment, the relationship of hydraulic parameters and the resistance characteristics for the shallow flow are quantitatively studied under conditions of six roughness beds and five gradients. Results showed that the index of the shallow flow regime exhibited a saddle-shape changing trend with increased gradient. The index of the shallow flow regime would gradually decrease with the increase of gradient when the gradient was less than 0.15 rad, but the trend was opposite when the gradient was higher than 0.15 rad. The index of the shallow flow regime presented a parabolic changing trend with increasing roughness, and the average index was 0.367. For the bed roughness less than 0.38, the flow range per unit width in unstable flow region became narrower with the increase of roughness, and the flow extended to the transitional zone; For the bed roughness greater than 0.38, the flow range per unit width in the unstable flow region gradually became wider with the increase of roughness. Generally, the shallow flow may be summarized by the category of rolling slope flow zone and transition flow zone. The Froude numbers of the shallow flow were both a power function with the flow of per unit width and the gradient, and their exponents are 0.436 and 0.278, respectively. The flow per unit width of the critical flow increased with the increase of roughness but decreased with the increase of gradient. It had a positive relationship with power 0.573 of the roughness and power 0.636 of slope, and the patterns of the shallow flow were in the critical flow and the supercritical flow. The resistance coefficient was in an opposite relationship with power 0.872 of per unit width flow, a positive relationship with power 0.445 of slope, and also an inverse relationship with the Reynolds number. However, the resistance coefficient had large difference with open channel flow, and increased with increasing roughness and slope. The increased resistance was likely to be mainly associated with the pressure drag by flow around a body and the local resistance by rolling slope flow. A formula of resistance in rolling slope flow zone was proposed based on the resistance characteristics of the shallow flow. Results from the study may provide information on the prediction of soil erosion on slope, and promote the application of open channel flow theory in overland flow.