Study On Mutual Feedback Mechanism Of Soil Detachment And Sediment Transport On Loess Slope

The sediment transport in hillslope erosion process is restricted by both soil detachment capacity and sediment transport capacity and depend on the comprehensive action of soil properties and hydraulic characteristics of overland flow.The quantitative characterization relationship and mutual feedback mechanism between soil detachment and sediment transport are the key to establish soil erosion prediction models but there is a lack of unified form relationship.The Loess Plateau is one of the most serious areas of soil and water loss in the world.The loess hillslope has a wide range of slope distribution diverse soil types and complex erosion environment.Meanwhile it is easy to form high concentration sediment flow on the Loess Plateau.The investigation of soil detachment sediment transport derived by overland flow and their mutual feedback mechanism of typical soil types in on the loess hillslope is conducive to understanding physical process and mechanism of sediment transport in soil erosion on loess hillslope as well as providing theoretical support for soil erosion control and food security improvement on the Loess Plateau.Therefore on the basis of summarizing domestic and foreign studies five typical soil types of the Loess Plateau were selected in this study including Lou soil（YL）Yellow shan soil（CH）Yellow Mian soil（AS）Aeolian sand soil（YY）and alluvial soil（WR）.Soil detachment and sediment transport processes were simulated to a hydraulic flume with variable slope.The soil detachment capacity and sediment transport capacity by overland flow for different soil types were measured.The physical process of soil detachment and sediment transport by overland flow was analyzed and the mutual feedback mechanism of soil detachment and sediment transport by overland flow was also investigated.The main results are as follows:1.The relationship between soil detachment capacity by overland flow and hydraulic characteristics and soil properties was clarified and the soil detachment capacity prediction equation on loess hillslope was established.（1）There were significant differences in soil detachment capacity among different soil types under slope gradient of5.24%～44.52%and flow discharge of 0.33-1.25×10^-3 m² s^-1.The soil detachment capacity of YL CH AS YY and WR ranged from 7.57×10^-3 to 3.54 kg m^-2 s^-1 2.03×10^-2 to 2.87 kg m^-2 s^-11.06×10^-2 to1.36 kg m^-2 s^-1 7.30×10^-3 to2.70 kg m^-2 s^-1 and 1.02×10^-2 to 4.23 kg m^-2 s^-1respectively.（2）The soil detachment capacity by overland flow showed an obvious increasing trend with the increase of slope gradient and flow discharge.The variation in soil detachment capacity of different soil types with slope gradient and flow discharge can be fitted as a binary power function of slope gradient and flow discharge.The contribution of slope gradient flow discharge soil type and their interaction to the variation in soil detachment capacity were 42.85%23.24%8.00%and 23.96%respectively.（3）The power function of hydraulic parameters can better express the variation trend of soil detachment capacity with hydraulic parameters.The soil detachment capacity could be expressed as a power function of slope gradient flow discharge and soil median particle size(D₅₀)（R²=0.91;NSE=0.91）or a power function of stream power and D₅₀（R²=0.91;NSE=0.91）.2.The effects of soil properties and hydraulic characteristics on sediment transport capacity（T_c）under non-erodible beds were revealed and the sediment transport capacity prediction equation coupled with soil properties and hydraulic characteristics was constructed.（1）There were significant differences in sediment transport capacity in non-erodible beds among different soil types at slope gradient of 5.24%-44.52%and flow discharge of 0.33-1.25×10^-3 m² s^-1.The sediment transport capacity by overland flow of WR was the largest which were 2.15 1.38 1.32 and 1.16 times greater than those for YL CH AS and YY respectively.The sediment transport capacity of different soil types was significantly negative correlated with clay content aggregate mean weight diameter（MWD）and soil organic matter content（SOM）.（2）The sediment transport capacity in non-erodible beds of different soil types increased as a power function with the increase of slope gradient and flow discharge.The contribution of slope gradient flow discharge soil properties and their interaction to the variation in sediment transport capacity in non-erodible beds were 45.59%22.04%6.38%and23.96%respectively.Stream power（w）was the most appropriate hydraulic parameter to express sediment transport capacity in non-erodible beds for different soils.（3）The sediment transport capacity in non-erodible beds of different soil types can be expressed as the quaternary power function of slope gradient flow discharge clay content and MWD（R²=0.94;NSE=0.94）or the ternary power function of stream power clay content and MWD（R²=0.94;NSE=0.94）.The newly constructed prediction equation of sediment transport capacity in non-erodible beds can reflect the influence of soil properties and hydraulic characteristics on it.3.The dynamic mechanism of sediment transport on loess hillslope was clarified and the unified characterization relationship of sediment transport capacity in non-erodible beds and erodible beds is realized.（1）The sediment transport capacity in erodible beds of different soil types was significantly different at the slope gradient of 5.24%-36.40%and flow discharge of 0.33-1.25×10^-3 m² s^-1.The sediment transport capacity in non-erodible beds of YL CH AS YY and WR was 0.03-1.02 kg m^-1s^-1 0.07-1.08 kg m^-1s^-1 0.06-1.21 kg m^-1s^-1 0.06-1.41 kg m^-1s^-1 and 0.06-1.51 kg m^-1s^-1respectively.The sediment transport capacity in non-erodible beds was significantly negative correlated with clay content MWD and SOM.（2）The variation in sediment transport capacity in erodible beds with slope gradient and flow discharge can be fitted as a binary power function of slope gradient and flow discharge.The contribution of slope gradient flow discharge soil properties and their interaction to the variation in sediment transport capacity in in erodible beds were 38.93%44.91%7.18%and8.58%respectively.（3）The power function of stream power can better express the variation trend of soil detachment capacity in erodible beds with hydraulic parameters.（4）The sediment transport capacity in erodible beds of different soil types can be expressed as the quaternary power function of slope gradient flow discharge clay content and MWD（R²=0.96;NSE=0.96）or the ternary power function of stream power clay content and MWD（R²=0.87;NSE=0.87）.（5）The mean values of sediment transport capacity in erodible beds of YL CH AS YY and WR were 1.74 1.21 1.45 1.54 and 1.58 times than that of sediment transport capacity in non-erodible beds respectively.Sediment transport capacity in erodible beds and non-erodible beds can be uniformly expressed as the power function of slope gradient flow discharge clay content and MWD（R²=0.88 NSE=0.88）.4.The reducing effect of sediment load on soil detachment rate on loess hillslope was analyzed and the mutual feedback mechanism of soil detachment and sediment transport on loess hillslope was clarified.（1）The soil detachment rate in non-erodible with sediment load of 0,10%T_c 25%T_c 50%T_c 75%T_cranged 0.008 to 3.54 kg m^-2 s^-1 0.007 to 2.92 kg m^-2 s^-1 0.006 to 2.26 kg m^-2 s^-1 0.002 to 1.97 kg m^-2 s^-1 and 0.0004 to 1.03 kg m^-2 s^-1 respectively at slope gradient of 5.24%-44.52%and flow discharge of 0.33-1.25×10^-3 m² s^-1.The soil detachment rate by overland flow in non-erodible beds showed an obvious decreasing trend with the increase of sediment load.The soil detachment rate in non-erodible beds with the sediment load of 10%T_c 25%T_c,50%T_c and 75%T_c decreased by 15.26%30.53%49.19%and 71.94%respectively compared with that at clear water.（2）The soil detachment rate in erodible beds along the hillslope decreased as a power function with the increase of slope length.The sediment concentrations and sediment load in erodible beds increased as a logarithmic function with the increase of slope length.The soil detachment rate in erodible beds along the hillslope decreased as a linear function with the increase of sediment concentrations and sediment load.（3）The soil detachment rate in erodible beds decreased with the increase of sediment load and the soil detachment rate in erodible beds with sediment load of 20%Sd（sediment discharge at clear water）40%Sd 60%Sd and 80%Sd decreased by 16.43%31.76%42.35%and 51.19%respectively compared with that at clear water.（4）There is a significant linear negative feedback relationship between soil detachment rate and sediment load of overland flow in erodible beds and in non-erodible beds.The linear negative feedback relationship can be used to satisfactorily predict the variation in soil detachment rate of overland flow with sediment load.