| In order to reveal hydraulic characteristics of rill flow on loess slope,laboratory experiments were conducted using a rill flume under five slope gradients（5°,10°,15°,20°and 25°）,flow discharges(2,4,8 and 16 L·min-1)and three bulk densities(1.2,1.35 and1.5 g·cm-3).The objectives of this study were to analyze the hydraulic parameters of rill flow,evaluate the effects of sediment concentration on hydraulic parameters and determine the best predictors of sediment concentration.Main results showed that:（1）The measured flow velocities at different bulk densities increased from 0.246m·s-1 to 0.935 m·s-1 as the flow discharges and slope gradients increased.The bulk density was less important to the flow velocity.The variation of flow velocity was influenced by both sediment concentration and flow discharge.However,flow discharge was the best predictor for estimating flow velocity.The water flow was between supercritical laminar and supercritical turbulent in this study.The Reynolds numbers increased rapidly with increasing flow discharges.The Reynolds number was well simulated by flow discharge with a power function.The Reynolds numbers did not significantly increase with slope gradients.The Reynolds numbers changed little at different bulk densities,indicating that the Reynolds number was not particularly sensitive to slope gradient and bulk density in our study.The flow discharge seems to be the preferred predictor for estimating Reynolds number.The Froude numbers increased with increasing sediment concentrations and slope gradients.The Froude number was more sensitive to slope gradient than to bulk density and sediment concentration.The Darcy-Weisbach friction factor increased first and then decreased as the sediment concentrations,flow discharge,Froude numbers and slope gradient increased.The Darcy-Weisbach friction factor decreased as the Froude numbers increased.（2）The hydraulic characteristics between uniform rill and erosion rill were significantly different.Flow velocity increased as a power function with flow discharge and slope gradient under uniform rill and erosion rill.However,the effect of slope gradient on flow velovity in uniforms was significantly greater than that in erosion rills.Variation of Froude number with slope gradient under uniform rill was similar to that under erosion rill.However,there was a large difference with flow discharge.Resistance coefficients increased as a power function with flow discharge and slope gradient under uniform rill.However,the resistance coefficients increased first and then decreased with increasing slope gradient and flow discharge under erosion rill.（3）The electrolyte tracer pulse method was employed to measure the flow velocities at locations of 1,2,3,4,6,8,10 and 12 m from the inlet of the flowing water.The flow velocities measured by the dye tracer method served as a control.The results showed that the flow velocities measured by the electrolyte tracer pulse method initially accelerated and then reached a steady value.An equation for estimating flow velocity on the basis of the measurement distance was established.The flow velocities calculated by the equation agreed well with those measured by the electrolyte tracer pulse method.In addition,the equation established in this study was shown to successfully predict flow velocities measured in other studies.The steady flow velocities and the distances when water flow reached stability under different slope gradients and flow discharges were determined by this equation.By comparing the steady flow velocities with the flow velocities measured by the dye tracer method,the results indicated that the steady flow velocities were 0.702 to0.735 times that of the flow velocities measured by the dye tracer method at various slope gradients and flow discharges.A linear function existed between the two,and the regressed parameter k（i.e.,0.718）could be used as the correction factor between the two.（4）The leading edge velocities varied from 0.237 to 1.290 m·s-1.The leading edge velocities increased with increasing flow discharges and slope gradients.The leading edge velocity was more sensitive to flow discharge than to slope gradient.The leading edge velocity could be predicted using a power function of slope gradient and flow discharge.By comparing the leading edge velocities with the flow velocities measured by the dye tracer method and the electrolyte tracer pulse method,the results indicated that the leading edge velocities showed good agreement with the flow velocities measured by the dye tracer method and the electrolyte tracer pulse method.The velocities measured by the dye tracer method were larger than the leading edge velocities.The relative errors between the two varied from-15.018 to-40.825 percent.The regressed parameter k（i.e.,0.758）could be used as the correction factor between the two.The leading edge velocities were relatively close to the velocities measured by the electrolyte tracer pulse method.The relative errors between the two decreased with increasing flow discharges and slop gradients.The regressed parameter k（i.e.,0.946）could be used as the correction factor between the two.The correction factor was more sensitive to Reynolds number than to flow discharge and slope gradient.The equations established in this study could be used to predict the correction factors.（5）The measured sediment concentration varied from 87.08 to 620.80 kg m-3 with a mean value of 343.13 kg·m-3.The flow discharge,slope gradient and unit stream power were good predictors of sediment concentration.The flow velocity and stream power were not good predictors of sediment concentration.The relationships of measured sediment concentration to slope gradient,flow discharge and unit stream power could be described by power functions.In addition,using the equations to simulate the measured sediment concentration of other studies,it was found that a flow and slope model and a shear model were more applicable than the other three models.In general,the equations established in this study can facilitate the prediction of sediment concentration. |
