| Soil erosion is a process mechanism of interaction at soil-water interface under the composite action of multiple exogenic forces and a mechanism of coevolution of erosion landform and environmental factors,leading to prominent problems of erosion dynamics type and seasonal cycle.The seasonal freeze-thaw in winter and spring and frequent heavy rains in summer and autumn aggravate the occurrence of soil erosion.However,current research focuses on the erosion process under the action of a single exogenic force,while the seasonal erosion process and main controlling factors under the composite action of multiple exogenic forces are relatively weak.Therefore,this study comprehensively considers the soil erosion mechanism under the coupling of freeze-thaw conditions,hydrodynamic characteristics and soil texture through indoor flume scouring test,direct shear test and freeze-thaw simulation test.The dynamic evolution of different soil properties and the nonlinear physical process of soil detachment are deeply explained.The dynamic-resistance control mechanism controlling the separation process of different soil textures under runoff driving is revealed in order to clarify the soil erosion mechanism of different soil textures under the composite action of seasonal freeze-thaw erosion.1.To explore the mechanism of composite erosion of seasonal freeze-thaw erosion,quantify the contribution of each factor and its interaction to soil detachment capacity,clarify the main controlling factors and erosion resistance mechanism driving soil detached under freeze-thaw erosion,and quantify the effect of each influencing factor(such as freeze-thaw cycles,initial moisture content,flow intensity index,soil parameters,etc.)on soil detachment capacity.(1)Compared with the control group,the average soil detachment capacity of SM silty loam,WS sandy loam and KS clay loam increased by 37.60%,18.91%and 79.29%after one freeze-thaw cycle,and increased by 73.48%,44.39%and 150.21%after ten freeze-thaw cycles.In addition,the higher the energy of external flow,the higher the threshold required for soil detachment capacity to reach stability under freeze-thaw conditions.(2)The average soil detachment capacity of SM silty loam and KS clay loam both showed a decreasing and then increasing trend with the change of initial moisture content.The optimal moisture content was 8%and 25%respectively.It is worth noting that the average soil detachment capacity of WS sandy loam continued to increase with the increase of moisture content,regardless of whether it was destroyed by freeze-thaw cycles.(3)To further improve the prediction accuracy of the model,a model for predicting soil detachment capacity based on integrated water flow intensity index was established.The determination coefficient R~2,Nash coefficient NSE and relative root mean square error RRMSE of the model were 0.93,0.92 and0.21 respectively.(4)Based on path analysis,the contribution of each factor to soil particle detachment in the soil detachment process was quantified.The contribution rates of effective stream power,median particle size,freeze-thaw cycles,colloid content and initial moisture content were 45.05%,8.96%,13.72%,25.56%and 6.71%respectively.2.To explore the dynamic evolution of mechanical properties of different soil textures during freeze-thaw process,and clarify the change trend of shear strength and shear strength index with the number of freeze-thaw cycles.(1)In the initial stage of shear failure of three soils,the shear stress increased rapidly with the increase of shear displacement,showing an approximately linear trend.As the shear failure proceeded,it gradually remained stable;the normal stress was the main controlling factor affecting the shear strength,contributing87.00%,77.69%and 73.69%in SM silty loam,WS sandy loam and KS clay loam respectively,while the number of freeze-thaw cycles and soil moisture content played a secondary role.(2)After ten freeze-thaw cycles,the weighted average shear strength of SM silty loam,WS sandy loam and KS clay loam decreased by 20.64%,24.94%and 15.73%respectively.The number of freeze-thaw cycles contributed 81.82%,60.00%and 26.45%in SM silty loam,WS sandy loam and KS clay loam respectively,while the moisture content contributed 18.18%,40.00%and 73.55%respectively.(3)After five freeze-thaw cycles,the cohesion of SM silty loam and WS sandy loam decreased by 14.29%and 24.55%respectively and remained stable.The cohesion of KS clay loam did not decrease by 34.30%until after ten freeze-thaw cycles and continued to decrease.The initial moisture content in the soil had a greater effect on cohesion,contributing 56.90%,60.48%and 74.80%in SM silty loam,WS sandy loam and KS clay loam respectively.(4)The internal friction angle of SM silty loam showed a significant decreasing trend with the increase of freeze-thaw cycle times,while WS sandy loam showed no significant increase or decrease overall.KS clay loam showed an insignificant trend of decreasing first and then increasing.freeze-thaw cycles contributed 92.63%,78.31%and64.38%to the internal friction angle in SM silty loam,WS sandy loam and KS clay loam respectively.3.The driving mechanism of soil erosion resistance(soil erodibility parameters and critical shear stress)under freeze-thaw action was explained,and soil erodibility parameters and critical shear stress under different conditions were quantified using WEPP model.(1)The average determination coefficient of the three function forms of power function,exponential function and linear function fitting the relationship between soil erodibility and critical shear stress was relatively close,which were 0.69,0.69 and 0.71 respectively.The closeness of the relationship between soil erodibility and critical shear stress was also closely related to soil type.SM silty loam was the highest,KS clay loam was the second,and WS sandy loam was the worst.(2)Soil erodibility was significantly positively correlated with freeze-thaw cycle action,increasing with the increase of freeze-thaw cycles,tending to be stable after a certain number of cycles,while critical shear stress showed an opposite trend.With the increase of initial moisture content,the soil erodibility of SM silty loam showed a decreasing and then increasing trend,WS sandy loam increased monotonically,and KS clay loam increased significantly at high moisture content.The critical shear stress of SM silty loam and KS clay loam first increased and then decreased with the increase of initial moisture content,while WS sandy loam changed slightly.(3)The mechanical property index was used to predict and simulate soil erodibility.The fitting degree of soil erodibility parameters was better,with an average determination coefficient of 0.56,0.50 and 0.45 respectively.The fitting accuracy of critical shear stress was poor.(4)A good nonlinear regression model was established between weighted average shear strength,particle size content,number of freeze-thaw cycles,initial moisture content and soil erodibility parameters for prediction(R~2=0.85).The critical shear stress of soil can establish a good nonlinear relationship with weighted average shear strength,sand content,initial moisture content and number of freeze-thaw cycles(R~2=0.79). |
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