| More than75%of the total land areas suffer greatly from serious soil erosion in Loess Plateau. Rill erosion is one of the major erosion ways on hillslopes, which accounts for-50%of the total amount of soil erosion in Loess Plateau. Hydraulic characteristics and soil properties may influence rill erosion profoundly. Knowledge of hydraulic characteristics of rill flows provides a basis for better understanding and modeling the rill erosion processes and mechanisms, which is also essential for planning the appropriate conversation measures in Loess Plateau. We conducted rainfall simulation experiments to investigate the hydraulic characteristics of rill flows during the development of rills on loess slopes. The soil used in the experiments was a clay loam soil collected from Yangling in Shanxi Province, China. Soil samples were packed in soil trays with two sizes:one is5m (length)X1m (width) and the other one is10m (length) X1.5m (width). Four slope gradients (10°,15°,20°and25°) and three rainfall intensities (1.5mm/min,2mm/min and1mm/min) were selected. The main results are presented as follows:(1) Reynolds number, Froude number and mean flow velocity of rill flows was larger than that of inter-rill flows. Resistance coefficient of rill flows was smaller than that of inter-rill flows, and moreover, the difference was more significant on larger slope gradients. Resistance coefficient of Darcy-weisbach and resistance coefficient of Manning increased with increasing slope gradient. Stream power and shear stress increased first and then decreased with increasing slope gradient, the threshold slope was between20°and25°(2) Sediment concentration was correlated significantly with slope gradient and rainfall intensity, but was not correlated significantly with slope length. Moreover, sediment concentration was correlated significantly with shear stress, stream power and unit stream power. Unit stream power has the highest correlations with sediment concentration.(3) The development of rill formation was in accordance with the variation of sediment concentration. In the early development of rill, the distance between rill certain effect on rill distribution, among the earliest rill backward-step will not occur, the distance between the range is between12.5and17.5cm; Rill erosion process is mainly the following cutting erosion and headward erosion, the effects of bank landslip are relatively small; the flow velocity decreased with rill width increased, and there is a significant negative correlation between flow velocity and width of rill.(4) Rainfall simulations under smaller rainfall intensity were conducted following larger rainfall intensity for investigating hydraulic characteristics of rills when the development of rills was stable. The results showed that flow velocity in rills wasn’t affected significantly by slope length, but was affected significantly by rill forms. Shear stress was influenced by runoff and slope. Resistance coefficient was correlated with runoff and slope. Flow velocity affected resistance coefficient profoundly under steep slopes. |
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