不同土壤前期含水率和坡度下黄壤分离临界水动力特性

土壤分离临界水动力学参数是侵蚀预报研究的基础内容,但关于不同近地表土壤水分条件下坡面侵蚀发生临界起动关键影响因子及其内在机制尚不清楚。本研究选取长江中上游地区典型黄壤为研究对象,试验设计5个土壤前期含水率(5%~23%)和5个坡度(1.0°~10.0°),利用冲刷槽实测土壤分离临界水动力学参数,探讨土壤分离临界水动力学参数对不同土壤前期含水率和坡度耦合作用的响应。结果表明:土壤分离临界流速、临界水深和流态均与坡度和土壤前期含水率呈幂函数减小关系;在坡度小于5.0°时,土壤分离临界水动力参数受坡度和土壤前期含水率耦合作用的影响;而在坡度大于5.0°时则主要受坡度的影响。因此,当坡度大于5.0°时,可直接采用简化幂函数方程对土壤分离临界水动力参数进行估算。在本研究试验条件下,坡面土壤分离临界水流流态基本属于层流、缓流,发生紊流、急流的概率很小。坡度和土壤前期含水率对坡面流阻力有重要的影响,阻力系数随临界单宽流量和雷诺数的增加呈幂函数下降趋势。

Critical Hydraulic Characteristics of Yellow Soil Detachment under Different Antecedent Soil Moisture Contents and Slope Gradients

Critical hydraulic characteristics of soil detachability are essential factors for soil erosion predicting. There is still insufficient knowledge on key factors and governing mechanism of soil detachability, especially for various near-surface soil water conditions. The typical eroded yellow soil area in the upper and middle Yangtze River was taken as the research area. In order to illustrate the response of critical condition of soil detachment under different subsurface soil water conditions, scour flume was used to measure hydrodynamics parameters. Five antecedent soil moisture contents (5%~23%) and five slope gradients (1.0°~10.0°) were designed. Results showed that critical flow velocity, flow depth and flow pattern of soil detachment all showed power function decreasing trends with the increase of slope gradient and antecedent soil moisture content. When the slope gradient was smaller than 5.0°, critical hydrodynamics parameters of soil detachment were influenced by coupling effects of slope gradient and antecedent soil moisture content;otherwise, it would be mainly influenced by slope gradient. It was also proposed that the simplified power function equation could be used accurately when slope gradient was larger than 5.0°. Most of the flow patterns belonged to laminar flow and subcritical flow. The slope gradient and antecedent soil moisture content played an important role in the overland flow Darcy-Weisbach friction factor. When Reynolds number and critical discharge per unit width were increased, the Darcy-Weisbach friction factor was decreased as power function.