红壤缓坡水流动力学特性及其对侵蚀影响的试验研究

摘要

水动力学特性对阐明红壤坡面侵蚀过程和产沙机理有重要意义.采用动床模拟径流冲刷方法,研究不同坡度(5°、8°、10°、12°)和不同放水流量(5、7 5、10 L/min)下,第四纪黏土发育红壤坡面侵蚀规律和水动力学特性.结果表明:1)放水冲刷初期坡面流为短时间薄层水流,随后发育为细沟流.薄层水流流态大部分处于“过渡流-缓流区”,细沟流大部分处于“过渡流-急流区”.薄层水流单宽流量、水深、流速、雷诺数以及弗劳德数均小于细沟流,而曼宁糙率和达西-韦斯巴赫阻力系数则为薄层水流大于细沟流.2)曼宁糙率和达西-韦斯巴赫阻力系数均与弗劳德数呈幂函数关系(R2分别为0.49和0.53),过渡流达西-韦斯巴赫阻力系数与雷诺数呈现显著幂函数关系(R2为0.29).3)从水动力学角度解析了红壤缓坡面水蚀过程——水力坡度和雷诺数2种因子共同组成的复合水动力参数可作为表征坡面侵蚀产沙的特征参数,建立了薄层水流和细沟流条件下基于水力坡度与雷诺数乘积的土壤侵蚀速率指数函数方程(R2分别为0.94和0.52).研究初步表明红壤缓坡的侵蚀规律和水动力学特性,为进一步研究该区域坡面侵蚀动力学机制奠定一定基础.%[Background] Soil erosion by water in red soil hilly region in tropical and subtropical zones of central China cause severe soil quality degradation and environmental issues.Soil erosion is primarily controlled by large volumes of surface runoff in this region due to abundant rainfall.Previous studies show that hydraulic characteristics are important in elucidating the mechanisms of erosion and sediment yield on slope.So far several laboratory and field experiments had conducted to understand the correlations between hydraulic parameters and soil erosion.However,most researches showed diversified results on those correlations due to various types of soils and different conditions in different experiments.The objectives of this study are to assess the difference in hydraulic parameters and sediment yield under red soil hilly-slope.[Methods] In order to simulate the surface runoff erosion process,an indoor experiment,involving four slopes (5°,8°,10°,and 12°),three flow rates (5,7.5,and 10 L/min)was carried out in a red soil flume (3-meter length and 0.5-meter wide).The bulk density and initial moisture for the soil sample was 1.35 g/cm3 and 30％,respectively.Experimental indices were measured by different approaches,the staining method was utilized to measure the flow velocity;the flow width was measured using a ruler at 3 positions;the sediment yield rate and discharge rate were deduced and calculated from the water bottle collected the runoff samples at the bottom of the flume during a set time interval.A single trial lasted 15 min.The hydraulic characteristics mainly used were Reynolds number (Re),Froude number (Fr),and other two different expressions of flow resistance Manning roughness coefficient (n) and Darcy-Weisbach friction (f).[Results] 1) Water erosion of red soil slope presented obvious periodic development process.There was short time sheet flow at the beginning,and soon transformed to rill flow.The sheet flow regimes generally indicated in transitional-sub-critical zone,and rill flow regimes indicated in transitional-super-critical zone.The unit discharge,mean flow velocity,mean flow depth,Re and Fr of sheet flow were larger than those of rill flow,while the n and f exhibited inversely in a same period.2) The n and f of the flow varies between 0.038-0.092 and 0.73-4.41,respectively.Both n and f showed a power function with Fr (R2 =0.49;R2 =0.52),and besides this,f also showed a power function with Re of transitional flow (R2 =0.29).4) A composition hydraulic parameter JRe,which was define as the product of Re and J (J was water surface slope),was an appropriate indicator for soil erosion.Sediment yield rate in the red soil slope could be estimated by JRe with an exponential function in both sheet flow phase and rill flow phase (R2 =0.93;R2 =0.51).[Conclusions] The results are conducive to the comprehensive understanding flow hydraulic characteristics and provide a reference for further studies on rill erosion in red soil hilly region.