| A series of step-pools formed on the loess slope have a vital effect on soil erosion.However,the studies on rill erosion processes under the action of step-pools remain inconclusive.This study aimed to investigate the influence of step-pools on rill erosion processes on a loess slope using three slopes(15°,20°,25°),five flow discharges(2.4,3.6,4.8,6.0,7.2 L min-1),and seven step-pool characteristics(0,10,15,20,25,30,and 40).These conditions were combined in an indoor water-scouring test,with hydrodynamic characteristics of rill flow,sediment content,and rill section development studied.The results showed that:(1)The response of rill flow hydrodynamics to slope,flow rate and characteristic length of step-pool under preset step-pool condition was studied.Under the action of the characteristics of the step-pool,the variation range of the flow velocity in the rill is 0.15m/s to 0.63 m/s.Velocity increased with the increase of step-pool characteristics,slope and flow rate,and showed significant correlations with step-pool characteristics,slope and flow rate(p≤0.01).Under the action of step-pool characteristics,the variation range of Reynolds number of rill flow is 275.60~2789.39,and the variation range of Froude number is0.53~2.82;Reynolds number and Froude number both increase with the increase of step-pool characteristics,slope and flow rate.The variation range of the rill flow resistance coefficient is 0.35 to 2.60,and the Darcy resistance coefficient decreases with the increase of step-pool characteristics,slope and flow rate.There was a significant correlation between Darcy resistance coefficient and step-pool characteristics,slope and flow rate(p≤0.01).The Darcy resistance coefficient has a good power function relationship with the Reynolds number.(2)The evolution law of rill section characteristics under preset step-pool conditions was studied.Slope,flow rate and step-pool characteristics can significantly affect the rill depth,width and width-depth ratio(p≤0.01).Both rill depth and rill width increased with slope,flow rate and step-pool characteristics.When the slope,flow rate and step-pool characteristics are large,the rill is fully developed and reaches a stable state,and the stable width-depth ratio is about 0.8 to 0.9.Slope,flow rate and step-pool characteristics all significantly affected the rill development coefficient(p≤0.01).The rill development coefficient increases with slope,flow rate and step-pool characteristics.The resistance coefficient and the rill development coefficient satisfy an inverse proportional function relationship,which is mainly related to the energy dissipation of the rill flow.(3)The response of rill erosion sediment yield to slope,flow rate and characteristic length of step-pool under preset step-pool condition was studied,and the prediction model of rill erosion sediment yield was established.After 14 minutes of the test,the sediment content of different slopes and different flow rates is basically maintained at a lower and the same level.Sediment content increases with flow rate,slope,and step-pool characteristics.Sediment production was significantly correlated with slope,flow rate and step-pool characteristics.The prediction formula of sediment content is:G=0.004S1.454Q0.848η0.825.The variation range of sediment carrying coefficient of rill flow is13.00~398.90,with an average value of 205.90,and the corresponding non-deposition guarantee rate is about 50%,that is,the rill flow is in the state of non-erosion and non-deposition.The sand-carrying coefficient was significantly correlated with the characteristics of the step-pool and flow rate(p≤0.01),and the sand-carrying coefficient was significantly correlated with the slope(p≤0.05).The contributions of flow rate,slope and step-pool characteristics to the sediment carrying coefficient are 38.506%,28.637%and 20.725%,respectively.(4)The coupling relationship between the sediment yield of rill erosion and the hydrodynamic characteristics of rill flow and the characteristics of rill section under the preset step-pool condition was studied.There is a significant positive correlation between the sediment yield and the flow velocity of the rill,which satisfies the linear function relationship,and the critical flow velocity is 0.44 m/s.Sediment production has a significant positive correlation with flow power,unit flow power and shear force,which satisfies the linear function relationship;flow power has the best fitting effect on sediment production,followed by unit flow power,velocity and shear force.The critical water flow power is 0.44 N/(m·s),the critical unit flow power is 0.04 m/s,and the critical shear force is 5.25 Pa.There is a power function relationship between the dimensionless soil erosion rate and the dimensionless flow velocity,and a linear relationship with the dimensionless unit flow power.The relationship between the dimensionless soil erosion rate and the dimensionless flow velocity(determination coefficient R~2=0.73)is better than the dimensionless unit flow power(determination coefficient R~2=0.65).The relationship between the dimensionless flow velocity and the dimensionless unit flow power and the dimensionless soil erosion rate has no obvious partition phenomenon under different working conditions.The relationship between dimensionless shear stress and dimensionless soil denudation rate has obvious partition phenomenon under different working conditions.The rill development coefficient,rill depth and rill width were all significantly positively correlated with sediment yield,while the width-depth ratio was significantly negatively correlated with sediment yield.The width-depth ratio,rill depth and rill width all meet the power function relationship with sediment yield,while the rill development coefficient and sediment yield meet the linear function relationship.The prediction effect of rill width on sediment yield is the best,followed by rill depth,width-depth ratio of section and rill development coefficient. |
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