Effects Of Rock Fragment Cover On Hydraulics By Overland Flow

Purple soil is a kind of primitive soil, It characterizes by loose structure, rock fracture development and easily weathered. Purple soil also has a very low degree of weathering and a high intensity of soil erosion. One of the most important characteristics of purple soil is to contain large numbers of rock fragments. Shallow overland flow is the main motive force factors of water erosion process. Study on hydraulic characteristics of overland flow is basic to further understand the mechanism of soil erosion. Rock fragments cover and size have significant effects on hydraulic parameters. The research was conducted to evaluate the changes of hydraulic parameters by different rock fragments cover and size. It is not only useful for deeper understanding the mechanism of rill erosion and rock fragment effectiveness, but also helping us lay a foundation for establishing the model of rill erosion, and provide scientific suggestions on soil and water loss control. It also plays an important role in soil and water resources conservation practices in sensitive areas of purple soils.The purple soil is derived from Wangjiaqiao watershed in Zigui county of Three Gorges Reservoir Region. A runoff scour simulator with a flume(2 m×0.5 m×0.15 m) was used. Smooth pebbles were selected and used. The treatments included rock fragment cover(0,10%,20%,30%,40%), rock fragment size(0.5-1cm,1-2cm,2-4 cm,4-6 cm),each with three levels of flow discharge(2L/min,4L/min,6L/min) that were tested at the 5 degrees slope gradient. Main research results on hydra ulic characteristics by overland flow and rill formation included:(1)Flow velocity and the Froude number decreased, but the runoff depth, flow shear stress, stream power, Darcy resistance coefficient and the Manning roughness coefficient increased with an increase in rock fragment cover, whereas the Reynolds number had no significant changes. The Reynolds number, average flow velocity, runoff depth and shear stress of rill flow are generally larger than those of inter-rill flow but the Darcy resistance coefficient and the Manning roughness coefficient of rill flow are smaller. The difference is more significant in high cover percentages. Moreover, the Reynolds number, runoff depth and flow shear stress were increased with increasing flow discharge.(2) Sediment yield increased with increasing flow discharge but decreased with rock fragment cover increased. The variation trend remained the same with different rock fragment cover. The sediment yield in high flow discharge(6 L/min) increased at first and then decreased, eventually remained stable; in lower flow discharge(2 L/min), the runoff sediment increased gradually. The rill with no rock fragment cover or large flow discharge develops more rapidly than that under the condition of rock fragment cover or little flow discharge. The variation was closely related to rill development and flow shear stress.(3) Darcy resistance coefficient and the Manning roughness coefficient decreased but flow velocity, runoff depth, flow shear stress and stream power increased with increasing rock fragment size, whereas the Reynolds number had no obvious changes. The Reynolds number, average flow velocity, runoff depth and shear stress of rill flow are generally larger than those of inter-rill flow but the Darcy resistance coefficient and the Manning roughness coefficient of rill flow are smaller. The Reynolds number, runoff depth and flow shear stress were increased with an increase in flow discharge. O n the same flow discharge, with the gravel particle size increases, the amount of erosion is increasing.