大粗糙单元对坡面流水动力学特性的影响

Influence of Large-scale Roughness Element on Hydraulic Characteristics of Overland Flow

通过室内定床冲刷试验,选用砂布床面模拟土壤下垫面,选用砂布+塑料半球体(直径为40mm)床面模拟大粗糙单元下垫面,在较大流量(2.81~84.43L/min)、坡度(2°~10°)范围内,系统研究了大粗糙单元对坡面流水动力学特性的影响。结果表明:(1)大粗糙单元床面的平均流速、平均水深和弗劳德数与流量之间均呈幂函数正相关关系;平均流速和弗劳德数与坡度之间均呈幂函数正相关关系,而平均水深与坡度之间呈幂函数负相关关系;阻力系数随雷诺数的增大而逐渐减小。(2)大粗糙单元增加了坡面流阻力,延缓了坡面流流速,雍高了坡面流水深,并使得水流流型由急流趋向缓流发展。(3)坡面流绕流现象明显,大粗糙单元周围水深的大小为迎水深(h_1)>侧水深(h_2)>背水深(h_3),在较小坡度下(2°和4°),h_1-h_3随流量的增大先增大后趋于稳定,在较大坡度下(6°,8°和10°),h_1-h_3随流量的增大先增大后减小。(4)坡面流阻力由颗粒阻力和绕流阻力组成,绕流阻力占总阻力的29%~77%,绕流阻力随流量的增加而减小。研究结果可为坡面土壤侵蚀物理模型的构建提供一定理论依据。

Large-scale roughness element plays an important role in soil erosion control,but few studies have been performed to quantify the effects of large-scale roughness element on hydraulics of overland flow.Laboratory flume experiments were conducted to investigate the potential effects of large-scale roughness element on flow velocity,water depth,Froude number,and hydraulic resistance of overland flow.The plastic hemispherical were glued onto the flume bed to simulate the large-scale roughness element,and a sand cloth bed was used as control.The flow discharges varied from 2.81 L/min to 84.43 L/min and the slope gradients ranged from 2°to 10°.Results showed that：（1）The mean flow velocity,mean water depth and Froude number had power function relationships with the flow discharges and slope gradients.However,the resistance coefficient decreased as Reynolds number increased.（2）The variation trend of water depth around the large-scale roughness element was water depth in front of the large-scale roughness element（h_1）water depth on both sides of the large-scale roughness element（h_2）〉water depth behind the large-scale roughness element（h3）.At a smaller slope gradient,h_1-h_3 increased first and then became stable with the increasing flow discharges.At a larger slope gradient,h_1-h_3 increased first and then decreased with the increasing flow discharges.（3）Compared with the sand cloth bed surface,the large-scale roughness element had a function of reducing flow velocity,increasing resistance,and changing flow regime.（4）The total resistance on large-scale roughness element was partitioned into grain resistance and form resistance,and form resistance accounted for 29%~77% of the total resistance.In a word,the large-scale roughness element cloud retards the flow,reduce soil erosion and have obvious effect on soil and water conservation.