草地坡面径流侵蚀动力特征模拟

Hydrodynamic characteristics of overland flow under grass cover

坡面是土壤发生侵蚀的基本单元,坡面流是泥沙运动的载体,其水动力学特征直接影响坡面侵蚀的强度和时空分布。利用坡面放水冲刷试验,研究了不同植被覆盖下坡面流水力学参数和侵蚀产沙特征,结合灰色关联分析,确定影响坡面侵蚀产沙的关键水力学参数。结果表明:冲刷流量和植被覆盖对坡面流水力学参数和侵蚀产沙过程有显著影响,冲刷流量越大,径流流速越大,阻力系数、糙率系数和弗汝德数越小,径流剪切力和径流动能越大,侵蚀产沙量越大;随着植被覆盖度的增大,坡面径流平均流速、弗汝德数和单位水流功率呈指数减小,径流剪切力、平均阻力系数和糙率系数呈指数增大,径流动能呈直线减小。每增加1%的植被,径流流速平均降低1%~3%,不同覆盖下侵蚀减沙效益达到15%~89%。灰色关联分析结果表明,由于径流紊动性较小,在3.2 L·min-1冲刷流量下的各参数与侵蚀产沙量的相关系数均小于5.2 L·min-1,在不同冲刷流量下,弗汝德数和单位水流功率与侵蚀产沙量相关关系最为密切,其次是水流动能。侵蚀产沙量随着单位水流功率的增加呈现显著指数减小,可以预测不同覆盖下的侵蚀产沙量。

Overland flow is the main carrier for sediment. Understanding hydraulic characteristics of overland flow is of great importance to clarify soil erosion and sediment yield mechanism. The study used a bed to perform indoor scouring test under the condition of different flow rate(3.2 L·min-1,5.2 L·min-1)with different coverages(0,30%,50%,70% and 90%)for grass surface. The parameters of hydraulic characteristics were measured and the correlations between those parameters with sediment yield amount were analyzed by gray relational analysis.The results showed that the discharge and grass coverage both exert great impacts on erosion process.(1)A larger scouring discharge will cause a faster velocity,smaller resistance and roughness coefficient and bigger shear stress and flow kinetic energy and thus larger sediment yield amount.(2)With the increase of grass coverage,the mean flow velocity,Fr and unit stream power decreased exponentially,and shear stress,resistance coefficient and roughness coefficient increased exponentially. However,the runoff kinetic energy showed a linear decrease with the increase of grass coverage. For 1% increase in grass coverage,runoff velocity decreases by an average 1%~3% and the sediment reduction benefit reaches from 15% to 89% at different coverage.(3)The gray correlation analysis found that the correlation coefficients between the above parameters and sediment yield amounts are smaller under the discharge of 3.2 L·min-1 because of the smaller turbulence. Fr number,unite stream power are the most correlated two parameters with sediment yield,and then is the runoff kinetic energy. Sediment yield amounts decreased exponentially with the increase of unit stream power,which indicated that unit stream power can be a good hydraulic parameter for predicting sediment yield. The stream power is a suitable factor to express the influence of underlying surface on flow dynamic characteristic and erosion process.