Study On The Relationship Between Rill Morphology And Hydraulics On Saturated Purple Soil Slope

Rill erosion is one of the most important forms of soil erosion on purple soil slope farmland,and it is also the cause of erosion and structural degradation of soil.Due to its special binary geotechnical structure,purple soil is prone to saturation in arable soil with shallow soil layers,high porosity,and strong water permeability under continuous rainfall conditions.After the soil is saturated,the slope will suffer more severe erosion than the unsaturated soil slope,which is intuitively manifested as the difference of rill morphology on the slope and the significant increase of rill erosion,and is internally manifested as the change of rill hydrodynamic characteristics.In the whole rill erosion process,rill morphology and hydraulics characteristics interact,but the correlation between rill morphology and hydraulics characteristics is still unclear,which greatly restricts the understanding of rill erosion process and mechanism on saturated purple soil slope.In view of this,this study calculates morphological parameters such as rill width,depth,and width to depth ratio under different rainfall intensities(30,60,90 mm/h)and slope gradients(2°,5°,10°,15°,20°),rill erosion amount,and hydraulic parameters such as rill flow velocity,Reynolds number,Froude number,and DarcyWeisbach resistance coefficient through indoor simulated rainfall experiments with saturated water supply devices.The spatial variation characteristics of rill morphology and hydraulic parameters on saturated purple soil slopes under different operating conditions were quantified,the effects of rainfall intensity and slope gradient on rill morphology and hydraulic characteristics on saturated soil slopes were analyzed,and on this basis,the correlation between rill morphology and hydraulic characteristics on slope surfaces was clarified.The main conclusions are as follows:(1)The morphological changes of rills on saturated purple soil slope surface have spatial heterogeneity.The variation range of rill width,depth,and the ratio of rill width to depth along the slope is between 2.94～10.02 cm,0.68～9.92 cm,and 1.11～5.42,respectively.The rill width and depth show an overall trend of increasing and then decreasing from top to bottom along the slope surface.The cross-sectional morphology of rills shows a change pattern from"wide-shallow"to"narrow-deep"from top to bottom along the slope;There is a significant correlation between the morphological parameters of rills on saturated purple soil slopes and rainfall intensity and slope.Under different intensities and slope gradients,the average width,depth,and the ratio of rill width to depth on saturated purple soil slope range from 3.44 to 9.64 cm,1.01 to 8.14 cm,and 1.18 to 3.87,respectively.The average width and depth of rills are significantly positively correlated with rainfall intensity and slope,while the average ratio of rill width to depth is significantly negatively correlated with rainfall intensity and slope.Slope gradient has a greater impact on rill morphology parameters than rainfall intensity.The amount of rill erosion on saturated purple soil slopes increases first and then decreases along the slope surface,reaching its maximum value at the middle slope position.Therefore,when implementing soil and water conservation measures on the slope surface,it is necessary to strengthen the deployment of measures at the middle slope position.The amount of rill erosion significantly increases with the increase of rainfall intensity and slope,and the impact of slope on rill erosion is more significant than that of rainfall intensity.(2)The variation of hydraulics parameters of saturated purple soil slope surface has spatial heterogeneity.The average velocity,Reynolds number,Froude number and Darcy-Weisbach resistance coefficient of saturated purple soil slope surface vary from 0.102 to 0.441 m/s,248.399 to1915.108,0.380 to 2.658 and 0.384 to 2.070,respectively.The average velocity increases first and then decreases along the slope surface;Reynolds number increases gradually along the slope;The Froude number increases first and then decreases along the slope surface;The Darcy-Weisbach resistance coefficient shows a trend of decreasing first and then increasing along the slope surface.There is a significant correlation between hydraulics parameters of saturated purple soil slope surface and rainfall intensity and slope.Under different rainfall intensities and slope gradients,the average velocity,Reynolds number,Froude number and Darcy-Weisbach resistance coefficient are0.175～0.350 m/s,393.011～1313.626,0.711～2.184 and 0.558～0.982,respectively.The average velocity and Reynolds number are more significantly affected by rainfall intensity,and Froude number is more significantly affected by slope gradient.(3)There is a significant correlation between rill morphology on the slope surface of saturated purple soil and concentrated flow hydraulic parameters.The main source of rill erosion on saturated purple soil slope is rill wall collapse erosion.Under the condition of p<0.01,there was a significant positive correlation between rill erosion and average velocity and Froude number;Under the condition of p<0.05,it has a significant positive correlation with Reynolds number and a significant negative correlation with Darcy-Weisbach resistance coefficient.The rill width of saturated purple soil has a significant positive correlation with average velocity,Reynolds number and Froude number,but has no significant correlation with Darcy-Weisbach resistance coefficient(p<0.01);There was a significant positive correlation between rill depth and mean velocity and Froude number,but no significant correlation between rill depth and Reynolds number and Darcy-Weisbach coefficient(p<0.01);The ratio of rill width to depth has a significant negative correlation with average velocity,Froude number and Darcy-Weisbach resistance coefficient,and a significant positive correlation with Reynolds number(p<0.01).