| Soil erosion is one of the serious problems that affects the global ecological environment,and the sediment transport process directly affects the intensity of soil erosion.As the core of sediment transport processes,sediment transport capacity determines whether the sediment-laden flow is in the state of soil separation or sedimentation,it is an important part in the construction of soil erosion process modelling as well.However,the kinetic mechanism of sediment transport of overland flow is still not clear,and the influence of soil aggregate and sediment settling velocity on sediment transport capacity is less studied and further explorations are necessary.In this study,flume experiment,sediment settling velocity experiment,soil mechanical composition and soil aggregate composition experiment were adopted.Seven textured soils（black soil from Heilongjiang,cinnamon soil from Beijing,sand soil from Inner Mongolia,sandy loessial soil and loessial soil from Shaanxi,purple soil from Sichuan and red soil from Jiangxi）in China were used as experiment materials.Variation of sediment transport capacity and its influencing mechanism of soil textures were studied under five slope gradients（5.23%,10.45%,15.64%,20.79%and 25.88%）and six unit flow discharges(0.68×10-3,1.35×10-3,2.03×10-3,2.70×10-3,4.05×10-3 and 5.41×10-3 m2·s-1),effects of slope gradient,unit flow discharge,hydraulic parameters and sediment characteristics on the sediment transport capacity of overland flow,sediment characteristics on sediment settling velocity,and the differences of sediment transport capacities amongy seven soils were investigated and analyzed.The accurate calculation of sediment transport capacity of overland flow is not only the basis for revealing the mechanism of soil erosion processes,but also is the key to construct soil erosion mechanism models.The results of this study show that:（1）The differences in the sediment transport capacity of different soils were compared.Measured sediment transport capacities of sandy loessial soil are significantly different from that of black soil,cinnamon soil,sand soil,purple soil and red soil.Measured sediment transport capacities of loessial soil are significantly different from that of black soil,cinnamon soil,sand soil,purple soil and red soil.However,there was no significant difference among measured transport capacities of black soil,cinnamon soil,sand soil,purple soil and red soil;either between sandy loessial soil and loessial soil.In the case of slow slope,the increase of sediment transport capacity caused by the increase of slope gradient is larger;in the case of small flow discharge,the increase of sediment transport capacity caused by the increase of flow is larger.Sediment transport capacities are more sensitive to unit discharge than slope gradient.（2）The relationship between the sediment transport capacity and slope gradient,unit flow discharge and hydrodynamic parameters was elucidated.Sediment transport capacities of seven soils rise with the increase of slope gradient and unit flow discharge in power functions（R2 between 0.96 and 0.99,NSE between 0.96 and 0.99）,where the power indexes of slope gradient and unit flow discharge ranges from 0.213 to 0.909 and 0.788 to1.083,respectively.Under the experimental conditions of this study,the mean flow velocity increases with the increase of slope gradient and unit flow discharge（p<0.01）.Similarly,sediment transport capacities of the seven soils increase linearly with the mean flow velocity（R2 ranged from 0.83 to 0.94,and NSE ranged from 0.83 to 0.99）.The threshold flow velocities required for the sediment transport capacities of the seven soils range from 0.108to 0.431 m·s-1,and the threshold flow velocity decreases with the increase of median sediment size.Relationships between the sediment transport capacity of overland flow and shear stress（R2 between 0.67 and 0.93,NSE between 0.68 and 0.93）,stream power（R2between 0.75 and 0.97,NSE between 0.76 and 0.97）,unit stream power（R2 between 0.32and 0.80,NSE between 0.32 and 0.80）,and effective stream power（R2 between 0.59 and0.93,NSE between 0.59 and 0.93）can be expressed as power functions for seven soil slopes.),while the relationship to the unit energy of flow cross section can be expressed as linear functions or power functions（R2 between 0.83～0.98,NSE between 0.84～0.98）;Effects of selected hydrodynamic parameters on the simulation of sediment transport capacities of overland flow are rank as:unit energy of flow cross section>stream power>mean flow velocity>effective stream power>shear stress>unit stream power.（3）The mechanism of the influence of sediment characteristics on sediment transport capacity was revealed.All seven soils showed a significant negative correlation between sediment transport capacity and the selected aggregate characteristics(WSA0.25,WMD,GMD,AI),with the best correlation with WSA0.25;The sediment settling velocity of undispersed soil particles containing agglomerates in seven soils showed a highly significant negative correlation（p<0.01）with the aggregate characteristics index,the sediment settling velocity of undispersed soil particles was significantly greater than that under dispersed conditions,the presence of agglomerates had a significant effect on the sediment settling velocity,and the rate of increase in sediment settling velocity was significantly and positively correlated with the absolute clay content,effective clay content and MWD.No significant correlations were found between the sediment transport capacity of the seven soils and the three characteristic sediment settling velocity values(ω25,ω50,ω75)obtained under the dispersed condition,while highly significant negative correlations were found with the three characteristic sediment settling velocity values(udω25,udω50,udω75)obtained under the undispersed condition,with the best correlation being found with udω25.Compared with the empirical equation for the sediment transport capacity based on slope gradient,unit flow discharge and hydrodynamic parameters only,adding sediment characteristics indicators can effectively improve the simulation and prediction accuracy of the equation,and the sediment settling velocity improves the accuracy of the equation better than the aggregate characteristics,in which the empirical equation for the sediment transport capacity based on slope gradient,unit flow discharge and sediment characteristics indicators has the best performance,followed by energy-based indicators such as stream power and unit energy of flow cross section.（4）An equation for calculating sediment transport capacity was constructed.For pre-selected equations,Zhang’s equation（S,q）(P.O.0.5-2=58.57%),Zhang’s equation（τ）(P.O.0.5-2=51.43%),Luan’s equation(P.O.0.5-2=74.29%)and Govers’equation(P.O.0.5-2=50.95%)have some applicability to predict sediment transport capacity of overland flow,but the prediction accuracy is not satisfactory.The basic expression of the selected sediment transport capacity of river has certain applicability to the overland flow,the resulted equation of the sediment transport capacity can be used to predict the transport capacity of overland flow(P.O.0.5-2=90.48%,R2=0.79,NSE=0.78).The basic expression for sediment transport capacity of overland flow was obtained through the dimensional analysis and substituted into the measured data in this study.Fitting results agree well to measurements（R2=0.98,NSE=0.88）,and the validation results showed that the equation derived by dimensional analysis for the sediment transport capacity based on the sediment characteristics and hydrodynamic parameters has good prediction accuracy(P.O.0.5-2=97.14%,R2=0.89,NSE=0.84),and the accuracy of the basic expression is better than that of the basic expression of the selected sediment transport capacity of river. |
