Comparative Study Of Rill Erosion Characteristics Between Saturated And Unsaturated Loess Soil

Complex and diverse topography,loose soil structure,and extremely uneven precipitation had led to the extensive rill erosion in the Loess Plateau.Eroding rills can result in the loss of nutrients and soil clay of the cultivation layer,which triggers serious soil degradation and acute erosion activities.It has become a major factor limiting the sustainable development of agriculture in the Loess Plateau.Dense plant roots lead to widespread distribution of soil channels of preferential flow,which enhanced soil permeability.Besides,continuous rainy weather can make the depressions being fully saturated.Soil saturation can cause significant changes in soil cohesiveness,pore-water pressure,surface roughness,etc.,thereby enhancing the erosional sensitivity and aggravating soil erosion.Up to now,there are few studies on slope erosion of saturated soil.Using a unique sectional water supply device to make the test soil fully saturated,and combined with the well-defined rill simulation scouring experiment,the rill erosion behavior and process of saturated loess soil were studied.The erosion parameters of saturated loess are estimated by experimental and analytical methods respectively,and these methods and results are verified by the previous test data of unsaturated loess.The main conclusions are as follows:1.The rill flow velocity of saturated loess soil has a good power function relationship with flow rate and slope.And slope is the mainly limiting factor for rill flow velocity.The rill flow velocity of saturated loess is obviously higher than that of unsaturated loess under the same test conditions.The rill flow velocity of saturated loess is about 1.17 times of that of unsaturated soil.The Darcy resistance coefficient of saturated loess slope is only 70%of that of unsaturated soil.When the soil is saturated,the voids between the soil particles are filled by water,which results in the decrease of the roughness of soil surface.2.The sediment yield of saturated loess has a good power function relationship with slope and flow rate.Besides,the effect of slope on the sediment yield is much better.The average sediment yield of saturated loess is about 1.86 times that of unsaturated loess,which reveals that the development degree of rill erosion about saturated loess soil is obviously greater than that of unsaturated soil under the same test condition.The maximum sediment yield of saturated loess is about 890 kg/m~3 and that of unsaturated loess is about 520 kg/m~3.The sediment yield and rill flow erosive energy of saturated and unsaturated soils can be expressed by logarithmic function.3.The experimental results showed that the detachment rates of saturated and unsaturated soils both increased with the rising of slope and flow rate.The detachment rate of saturated loess soil increased from 0.036 to 0.686 kg/(m~2·s),and the detachment rate of unsaturated loess soil spanned from 0.015 to 0.357 kg/(m~2·s).As for analytical results,the detachment rate of saturated loess soil increased from 0.040 to 0.839kg/(m~2·s)with the increase of the slope and the flow rate.Correspondingly,the detachment rate of unsaturated soil ranged from 0.017 to 0.390 kg/(m~2·s).Both analytical and experimental results showed that the soil detachment rate of unsaturated loess soil is about half of that of saturated soil,which is far more less than these saturated values.Results showed that the process of saturation evidently aggravated the soil erosion and promoted the formation of eroding rills.Generally speaking,the experimental results are in good agreement with the analytical values.4.The sediment transport capacity of these two kinds of soil has a good power function relationship with slope and flow rate,and the effect of flow rate on sediment transport capacity is more significant.Both analytical and numerical results confirm that soil saturation can trigger sudden change of sediment transport capacity in eroding rills,and the average sediment transport capacity of saturated soil is 1.77 times that of unsaturated soil.The power function regression model can accurately describe the relationship between sediment transport capacity and stream power.Besides,the analytical value of the sediment transport capacity is much closer to the results estimated by the Nearing formula,whose fitting effect is better than that of experimental values.Overall,both analytical method and experimental approach can be well applied to estimate the sediment transport capacity,and the stream power is an ideal predictor of that.The critical rill length of saturated soil ranged from 4.97 to 11.61 m,and that of the unsaturated soil spanned from 5.87 to 13.05 m.The attenuation of the critical rill length with the flow rate is obviously greater than that of the slope,which also reveals that the effect of flow rate on the sediment transport capacity is much greater than that of slope.Under the same test conditions,the critical rill length corresponding to saturated soil is obviously smaller than that of unsaturated soil,and the critical rill length of saturated soil is about 84.1%of that of unsaturated soil.5.The analytical values of rill erodibility parameters of saturated and unsaturated loess soil are 0.095 and 0.064 s/m respectively,and the critical shear stress is 0.77 and1.61 Pa,respectively.According to the experimental results,the rill erodibility parameter of saturated and unsaturated loess soil is 0.088 and 0.057 s/m,while the critical shear stress is 0.72 and 1.56 Pa.In general,both the analytical and experimental methods can be applied to the solution of rill erodibility parameters.The rill erodibility of the saturated loess soil is about 1.6 times that of unsaturated soil,and the critical shear stress is less than half of that of unsaturated soil.Given these realities,the erosional sensitivity of loess soil is obviously increased after saturation,and shear resistance to external forces of soil is also greatly reduced.Therefore,saturated loess soil is more easily dispersed and eroded than unsaturated soil.