Rill Erosion Development Process And Its Morphology Characteristics On Sand-Covered Slope

The wind erosion and water erosion interlaced area on the Loess Plateau is an area with a fragile ecological environment.Under the influence of both hydraulic and wind effects,the resulting soil erosion has severely damaged the local agricultural land and natural ecology.Rill erosion is an important way for soil erosion on the Loess Plateau.Rill formed on slopes can not only cause serious soil erosion,but changes in the rill morphology during development also have an important impact in erosion process.Therefore,it is of great significance to study the rill development process and morphological characteristics in the wind-water erosion interlaced area for revealing the law of soil erosion.In this paper,based on the indoor simulated rainfall test,the sand-covered slopes in the erosion area of wind and water were taken as the test objects,the infiltration and runoff processes of the sand-covered slopes were studied,the spatial distribution characteristics of erosion and sand production were revealed,and the erosion of rills were quantified.The rill development process and its response to erosion were explored.The main research conclusions as follows:(1)The initial runoff time of sand-covered slopes under different rainfall intensities are 1～12min longer than loess slopes.The infiltration rates of sand-covered slopes and loess slopes decrease with the increase of rainfall;When the stable infiltration rate were reached before,the initial infiltration rate of the soil on the sand-covered slopes were about 1.23 times that of loess slope,and the infiltration amount of the sand-covered slopes were more in the same time.The water content curve of sand-covered slopes under different rain intensities rises earlier than the loess hillslopes,and the soil moisture can reach the deep soil earlier and the amount of infiltration were more.With the increase of rainfall time,the runoff on the hillslopes surface increases first and then tends to be stable,and the time for the sand-covered hillslopes to reach stable runoff is longer than that on the loess hillslopes.Under the rain intensity of 1.0,1.5 and 2.0 mm/min,the total runoff of loess hillslopes were always greater than that of sand-covered hillslopes,and as the rain intensity increases,the difference between the two tends to decrease.(2)Runoff flow rate under both slopes conditions increased with increasing rain intensity,and the runoff flow rate on the sand-covered slopes had greater volatility with changes in rainfall duration.The runoff shear force on the slopes surface generally shows that the loess slopes surface were larger than the sand-covered slopes surface,and with the increase of rain intensity large linear increase trend.The runoff shear force of the sand-covered slopes increases faster.When the rain intensity increases by 0.5 mm/min,the increase value of the runoff shear force of the sand-covered slopes were about 1.5 times of the loess hillslopes.The change law of runoff power was basically consistent with the change of runoff shear force.There was a good power function relationship between hydrodynamic parameters and sediment transport rate.(3)Analysis of the spatial distribution of soil erosion revealed that on the sand-covered slopes,a lot of sediment from inter-rill area can be clearly seen,while the loess slopes were not obvious;Amount of erosion on sand-covered slopes during rainfall was about 2.18 times that of loess slopes.(4)In terms of the morphology characteristics of rill,the development duration and development rate of the maximum trench length on loess slopes were larger than sand-covered slopes.The trenches were mostly "narrow and deep";the deepening rate of the bottom of rill on the sand-covered slopes increases with the increase of rain intensity.There was a good quadratic function relationship between the rill surface area and the rainfall duration,and R2 was all above 0.94;with the increase of the rain intensity,the rill density and its splitting degree were increase.With the increase of the rain intensity,the large increase rate is also increasing,and the increase rate of sand-covered slopes were greater than loess slopes;the overall complexity of fine trenches also increases with the increase of rainfall intensity.The average rill complexities of sand-covered slopes were greater than loess slopes.When the rain intensity was small,the rill complexities of two type’s slopes were smaller,and the differences were greater when the rain intensity change larger.The overall performance of fractal dimension under different rain intensity on loess hillslopes was:2.5>2.0>1.5>1.0 mm/min,and the performance of sand-covered hillslopes was:2.5>1.5>2.0>1.0 mm/min.(5)By analyzed the erosion contribution rate of different rill development patterns,it was found that the average erosion contribution rate of inter-rill area on the loess hillslopes under different rain intensities was 27%,the average erosion contribution rate of rill was 73%.And the average erosion contribution rate of traceability erosion was 18.2%.The average contribution rate of trench erosion was 14.5%.The average contribution rate of undercutting erosion at the bottom of the rill was 40.9%;The average erosion contribution rate of inter-rill area on sand-covered hillslopes was 33%.The average erosion contribution rate of rill was 67%,the average erosion contribution rate of traceability erosion was 12%,the average contribution rate of trench erosion was 21%,and the average contribution rate of undercutting erosion at the bottom of the rill was 34.4%.Moreover,each rill morphological parameter and its cumulative sediment yield have a good exponential function relationship.Under the condition of the loess hillslopes,the fitting effect of rill split degree μand rill surface area S is better,the R2 was 0.97;The fitting effect of rill split degree μ under the condition on sand-covered hillslopes were better than that using the two morphological parameters μand S,R2 was 0.78.