Soil Erosion Process On The Steep Slopes

This paper oriented to study soil erosion process and mechanism of steep loess slopes（25°⁵0°）,in an attempt to lay down a foundation for elucidating in-depth relationships of the runoff time,characteristics of particle size composition and hydrodynamic characteristics of shallow water flow on slopes with soil erosion.An indoor experiment was carried out simulating rainfalls on steep loess slopes.The experiment was designed to have only one soil type（loessal soil）,three rainfall intensity(1.0mm min^-1,1.5 mm min^-1 and 2.0 mm min^-1)and six slope gradient（25°,30°,35°,40°,45°and 50°）.And the main results was as followed:（1）The sediment process of steep loess slopes are analyzed,results show that the runoff rate increased rapidly in the first few minutes of each experiment,and the rainfall intensity played a leading role in the runoff production rate.The runoff rate increased with the increase of rainfall intensity,and the larger the rainfall intensity was,the greater the rainfall intensity was.When the slope is the same,the total runoff increases with the increase of rainfall intensity.When the rainfall intensity is the same,the influence of slope on the total runoff is more complex,and the soil erosion rate of each rainfall generally shows that the velocity increases with the increase of rainfall intensity at the same slope.Soil erosion rate varies greatly under the condition of the same rainfall intensity and different slope.There exists threshold effect on soil denudation rate change trend by slope,and rain intensity plays a leading role in soil erosion rate change,when the slope is the same,the total erosion amount increased exponentially with the increase of rainfall intensity.When the rainfall intensity was 1.0 mm min^-1 and 1.5 mm min^-1,the total erosion amount increased first and then decreased with the increase of slope,and there was a peak value when the slope was 40⁴5°,and when the rainfall intensity was 2.0mm min^-1,the total erosion amount increased firstly and then decreased with the increase of rainfall intensity.The effect of slope on total erosion is complex.（2）The results show that flow velocity increased with rising rainfall intensity on slopes the same in gradient,and under the same rainfall intensity,the effect of slope gradient on flow velocity exhibited a critical point in gradient,that is,between 40°and 45°under the same rainfall intensity.On slopes with gradient lower than the critical point,flow velocity increased with rising slope gradient,whereas on slopes with gradient higher than the point,flow velocity decreased with rising slope gradient.Mean runoff depth increased steadily with rising rainfall intensity on all slopes regardless of gradient;but decreased with rising slope gradient under rainfalls the same in intensity.Reynolds numbers of steep loess slopes were on the whole relatively small,often far below 580.In this study shallow flows on slopes were of laminar flow in destabilized layers,while shallow flows with Froude number beyond 0.8 were of rush.And Darcy-Weisbach resistance coefficient（f）was positively related to rainfall intensity and negatively to slope gradient.（3）The Results show that the relationship between erosion rate and hydrodynamic parameters on steep loess slopes:when the gradient is the same,flow shear stress,stream power and unit energy of water-carrying section can increase with the increase of rainfall intensity,and the best regularity is runoff power.Under the same rainfall intensity,flow shear stress,stream power and unit energy of water-carrying section single.The potential energy increases with the increase of slope.Soil erosion rate increases power function with flow shear stress,unite stream power and unit energy of water-carrying section with the determining coefficient of R²（ω）>R²（τ）>R²（E）.So stream power was the best one of all the hydrodynamic parameters tested to describe the soil erosion process on the steep loess slopes.（4）The Results show that the content of clay particles is the least,the content of fine particles is the second,the content of coarse silt and sand is more,and the influence of slope on sediment is more complex,the rule is not obvious.The sediment content of each grain is mainly controlled by rainfall intensity.When the slope is small（≤35°）,the clay content and fine silt content increase with the increase of rainfall intensity,while the fine silt content and sand content decrease with the increase of rainfall intensity.When the slope is large（>35°）,the content of clay and fine powder increases with the increase of rainfall intensity,while the content of fine silt and sand decreases with the increase of rainfall intensity.In each experiment,the change of clay content with runoff duration showed two stages:the first stage was the rapid increase stage of clay content,the second stage was stable or fluctuating phase of clay content;As a whole,the sediment particles with small diameter were transported preferentially during the rainfall,and the clay and fine silt were enriched,but the coarse silt and sand did not change obviously during the erosion,which was consistent with the original soil composition.