Experimental Study On The Dynamic Process Of Particle Size Selectivity In Interrill Erosion On The Loess Hillslope

The Loess Plateau of China is the most serious area of soil erosion globally,resulted in the extremely fragile ecology here and leads the Yellow River to become the world-famous"suspended river"which always threatens both sides of the lower reaches.The economic and social development of the Loess Plateau and the middle and lower reaches of the Yellow River are seriously restricted due to the serious soil erosion and the resulting sediment disaster and non-point source pollution.The interrill erosion process is one of the most important erosion processes and causation of serious sediment yield with the widest area on the hillslope in the Loess Plateau.Particle size selectivity during interrill erosion process is a vital and typical feature of interrill erosion,and it implies rich scientific connotation and important scientific issues of erosion process.The soil solid particles and chemical substances carried by the particles resulted from the selective process of interrill erosion have a serious influence on the soil quality,sustainable development and water environment of the Loess Plateau,as well as the middle-lower reaches of the Yellow River.The study on the dynamic process of particle size selectivity during interrill erosion on the Loess hillslope could reveal the particle size selectivity during interrill erosion process,as well as its dynamic mechanism,deepen the understanding of the mechanism in interrill erosion process,further promote the development of slope erosion theory,and provide important scientific basis for the soil erosion control and water environment regulation in the Loess Plateau,as well as the middle-lower reaches of the Yellow River.In this paper,simulated rainfall experiments were conducted to study the dynamic process of particle size selectivity during interrill erosion on the loess hillslope using a clayey loess soil on a three-area pan under complete combinations of five rainfall intensities(0.7,1.0,1.5,2.0 and 2.5mm min^-1)and five slope gradients?12.28%,17.63%,26.79%,36.40%and 46.63%?.The process,dynamic mechanism and intensity of particle size selectivity during interrill erosion process,as well as the coupling relationships between particle size selectivity and sediment yields from interrill erosion process were investigated and analyzed.The main results are as follows:1.The particle size selectivity during interrill erosion process on loess hillslope were revealed.?1?During the rainfall process,the percentage of<0.002 mm,0.002-0.05 mm,0.05-0.25 mm and 0.25-2 mm effective particles,as well as the effective mean weight diameter,of sediment detached selectively by raindrops were 9.09%?19.01%,46.34%?65.09%,15.27%?33.08%,0.76%?13.86%and 63.28?217.41?m,respectively.With the increase of rainfall intensity,the percentage of<0.002 mm and 0.002-0.05 mm effective particles in sediment firstly decreased and then increased,and its minimum value was reached at the rainfall intensity of 1.5 mm min^-1.Inversely,the percentage of 0.05-0.25 mm and 0.25-2 mm effective particles,as well as the effective mean weight diameter,of sediment firstly increased and then decreased with rainfall intensity,and its maximum value was reached at the rainfall intensity of 1.5 mm min^-1.Relationships between the percentage of each size effective particles,as well as the effective mean weight diameter,of sediment detached selectively by raindrops and rainfall intensity can be described by parabola equation,respectively.With the increase of slope gradient.the percentage of<0.002 mm and0.002-0.05 mm effective particles in sediment decreased,while the percentage of 0.05-0.25mm and 0.25-2 mm effective particles,as well as the effective mean weight diameter,of sediment increased with slope gradient,which can be described by logarithmic equations,respectively.The factor equations for particle size selectivity in raindrop detachment process on loess hillslope includes two sets of factor equations,one is the composite equation of binary power-exponential function of relationships among the percentage of each size effective particles in sediment detached selectively by raindrops,rainfall intensity and slope gradient,and another is the composite equation of binary power-exponential function of relationships among the effective mean weight diameter of sediment detached selectively by raindrops,rainfall intensity and slope gradient.The contribution rate of rain intensity and slope gradient to the effective mean weight diameter of sediment detached selectively by raindrops were 50.86%and 38.16%,respectively.?2?The enrichment ratio of<0.002 mm and 0.002-0.05 mm effective particles detached selectively by raindrops were greater than 1,being 1.27?1.8 and 1.13?1.31,respectively,inversely,the enrichment ratio of 0.05-0.25 mm and 0.25-2 mm effective particles detached selectively by raindrops were lower than 1,being0.59-0.75 and 0.23-0.94,respectively,which indicated that the process of particle size selectivity in raindrop detachment was the prior detachment process of<0.002 mm and0.002-0.05 mm effective particles by raindrops.?3?During the rainfall process,the percentage of<0.002 mm,0.002-0.05 mm,0.05-0.25 mm and 0.25-2 mm effective particles,as well as the effective mean weight diameter,of sediment transported selectively by sheet flow were 10.28%?27.84%,52.37%?67.76%,3.93%?32.28%,0.44%?7.01%and29.07?135.10?m,respectively.With the increase of rainfall intensity and slope gradient,the percentage of<0.002 mm and 0.002-0.05 mm effective particles in sediment decreased,inversely,the percentage of 0.05-0.25 mm and 0.25-2 mm effective particles,as well as the effective mean weight diameter of sediment increased.The factor equations for particle size selectivity in sheet flow transport process on loess hillslope include two sets of factor equations,one is the binary power function equation of relationships among the percentage of each size effective particles in sediment transported selectively by sheet flow,rainfall intensity and slope gradient,and another is the binary power function equation of relationships among the effective mean weight diameter of sediment transported selectively by sheet flow,rainfall intensity and slope gradient.The contribution rate of rain intensity and slope gradient to the effective mean weight diameter of sediment transported selectively by sheet flow were 53.30%and 40.96%,respectively.?4?The enrichment ratio of<0.002 mm and 0.002-0.05 mm effective particles transported selectively by sheet flow were greater than1,being 1.29?2.20 and 1.23?1.42,respectively,inversely,the enrichment ratio of 0.05-0.25mm and 0.25-2 mm effective particles transported selectively by sheet flow were lower than1,being 0.40-0.76 and 0.05-0.49,respectively,which indicated that the process of particle size selectivity in sheet flow transport was the prior transport process by sheet flow of<0.002 mm and 0.002-0.05 mm effective particles that pre-detached by raindrops.?5?The process of particle size selectivity in interrill erosion was the prior detachment of<0.05 mm effective particles by raindrops and then the prior transport by sheet flow of<0.05 mm effective particles that pre-detached by raindrops.2.The dynamic mechanism of particle size selectivity in interrill erosion on loess hillslope was clarified.?1?The combination of raindrop kinetic energy and sheet flow velocity?v?is the best rainfall physical and sheet flow hydraulic parameters to describe the percentage of<0.002 mm effective particles in sediment detached selectively by raindrops.The combination of raindrop kinetic energy,sheet flow depth?h?and sheet flow velocity is the best rainfall physical and sheet flow hydraulic parameters to describe the percentage of0.002-0.05 mm,0.05-0.25 mm,0.25-2 mm effective particles,as well as the effective mean weight diameter,of sediment detached selectively by raindrops.Considering sheet flow hydraulic parameters can greatly improve the precision of equation when establishing dynamic equations of particle size selectivity in raindrop detachment process.?2?The dynamic equations of particle size selectivity in raindrop detachment process on loess hillslope include two sets of dynamic equations,one set is the dynamic equations that describing effective particles size distribution of sediment detached selectively by raindrops,and another set is the dynamic equation that describing effective mean weight diameter of sediment detached selectively by raindrops.The former includes four equations,being the composite equation of binary exponential-power function of response relationship of the percentage of<0.002 mm effective particles in sediment detached selectively by raindrops to raindrop kinetic energy and sheet flow velocity,and three composite equations of triple exponential-power function of response relationships of the percentage of 0.002-0.05 mm,0.05-0.25 mm and 0.25-2 mm effective particles in sediment detached selectively by raindrops to raindrop kinetic energy,sheet flow velocity and sheet flow depth,respectively.The latter is the composite equation of triple exponential-power function of response relationship of the effective mean weight diameter of sediment detached selectively by raindrops to raindrop kinetic energy,sheet flow velocity and sheet flow depth.The five equations were as follows:P_r?<0.002 mm?=9.854e^0.0004KEKE^-0.105v^-0.343P_r?0.002-0.05mm?=218.75e^0.0004KEKE^-0.200v^-0.037h^0.043P_r?0.05-0.25 mm?=13.557e^-0.0002KEKE^0.083v^0.170h^-0.053P_r?0.25-2 mm?=1.65×10-⁷e^-0.004KEKE^2.424v^0.196h^-0.568MWD_r=0.033e^-0.002KEKE^1.184v^0.209h^-0.267;?3?The combination of unit stream power and raindrop terminal velocity?V_t?is the best sheet flow hydrodynamic and rainfall physical parameters to describe the percentage of<0.002 mm effective particles in sediment transported selectively by sheet flow.The combination of stream power and raindrop kinetic energy is the best sheet flow hydrodynamic and rainfall physical parameters to describe the percentage of 0.002-0.05 mm,0.05-0.25 mm,0.25-2 mm effective particles,as well as the effective mean weight diameter,of sediment transported selectively by sheet flow.Considering rainfall physical parameters can effectively improve the precision of equation when establishing dynamic equations of particle size selectivity in sheet flow transport process.?2?The dynamic equations of particle size selectivity in sheet flow transport process on loess hillslope include two sets of dynamic equations,one set is the dynamic equations that describing effective particles size distribution of sediment transported selectively by sheet flow,and another set is the dynamic equation that describing effective mean weight diameter of sediment transported selectively by sheet flow.The former includes four equations,being the composite equation of binary exponential function of response relationship of the percentage of<0.002 mm effective particles in sediment transported selectively by sheet flow to unit stream power and raindrop terminal velocity,and two composite equations of binary logarithmic function of response relationship of the percentage of 0.002-0.05 mm and 0.05-0.25 mm effective particles in sediment transported selectively by sheet flow to stream power and raindrop kinetic energy,and the composite equations of binary power function of response relationship of the percentage of 0.25-2 mm effective particles in sediment transported selectively by sheet flow to stream power and raindrop kinetic energy.The latter is the composite equation of binary exponential-power function of response relationship of the effective mean weight diameter of sediment transported selectively by sheet flow to stream power and raindrop kinetic energy.The five equations were as follows:P_t?<0.002 mm?=21.447e^{-7.149U-0.017V_t}P_t?0.002-0.05 mm?=-21.47ln?+0.217ln KE+51.634P_t?0.05-0.25 mm?=5.502ln?-2.635lnKE+56.899P_t?0.25-2 mm?=39.833?^0.777KE^-0.020MWD_t=21.47e^2.817??^0.279KE^-0.0617;?5?The dynamic equation of particle size selectivity in interrill erosion,which includes dynamic equation of particle size selectivity in raindrop detachment process on loess hillslope and that in sheet flow transport process,clarified the dynamic mechanism of particle size selectivity in interrill erosion.3.The intensity of particle size selectivity in interrill erosion on loess hillslope was quantified.?1?The average value of selective ratios of effective particles detached by raindrops during a rainfall event were 0.40?0.90,which firstly increased and then decreased with the increase of rainfall intensity,reaching its maximum at rainfall intensity of 1.5 mm min^-1,and increased with the increase of slope gradient,indicating that the intensity of particle size selectivity firstly weakened and then intensified with the increase of rainfall intensity,with the weakest at the rainfall intensity of 1.5mm min^-1,and weakened with the increase of slope gradient.The effect of rainfall intensity and slope gradient on selective ratio of effective particles detached by raindrops can be described by a composite equation of binary exponential-power function.The contribution rates of rainfall intensity and slope gradient to selective ratio of effective particles detached by raindrops were 50.86%and38.16%,respectively.?2?The average value of selective ratios of effective particles transported by sheet flow during a rainfall event were 0.45?0.94,which firstly decreased and then increased with the increase of rainfall intensity,reaching its minimum at rainfall intensity of 1.5 mm min^-1,and has no significant changes with the increase of slope gradient,indicating that the intensity of particle size selectivity firstly intensified and then weakened with the increase of rainfall intensity,with the most intensified at the rainfall intensity of1.5mm min^-1,and has no significant changes with the increase of slope gradient.?3?The average value of selective ratios of effective particles of sediment yields from interrill erosion during a rainfall event were 0.24?0.61,which increased with the increase of rainfall intensity and slope gradient,indicating that the intensity of particle size selectivity weakened with the increase of rainfall intensity and slope gradient.The effect of rainfall intensity and slope gradient on selective ratio of effective particles of sediment yields from interrill erosion can be described by a binary power function.The contribution rates of rainfall intensity and slope gradient to selective ratio of effective particles of sediment yields from interrill erosion were 53.30%and 40.96%,respectively.?4?The average contribution rate of particle size selectivity of sediments yielded by raindrop detachment to that yielded from interrill erosion during a rainfall event were 18.18%?88.33%,with a mean value of 54.24%.The average contribution rate of particle size selectivity of sediments yielded by sheet flow transport to that yielded from interrill erosion during a rainfall event were 11.67%?81.82%,with a mean value of 45.76%.The two indicated that the contribution rate of particle size selectivity of sediments yielded by raindrop detachment to that yielded from interrill erosion was larger than that yielded by sheet flow transport in general.With the increase of rainfall intensity,the contribution rate of particle size selectivity of sediments yielded by raindrop detachment to that yielded from interrill erosion firstly decreased and then increased,reaching its minimum at rainfall intensity of 1.5 mm min^-1,and correspondingly,with the increase of rainfall intensity,the contribution rate of particle size selectivity of sediments yielded by sheet flow transport to that yielded from interrill erosion firstly increased and then decreased,reaching its maximum at rainfall intensity of 1.5 mm min^-1.With the increase of slope gradient,the contribution rate of particle size selectivity of sediments yielded by raindrop detachment to that yielded from interrill erosion decreased,and correspondingly,the contribution rate of particle size selectivity of sediments yielded by sheet flow transport to that yielded from interrill erosion increased.The response relationships of contribution rate of particle size selectivity of sediments yielded by raindrop detachment and sheet flow transport to that yielded from interrill erosion with rainfall intensity and slope gradient can both be described by composite equation of binary exponential-power function.Rainfall intensity showed a greater influence on the contribution rate,with a contribution rate of influence of 75.25%,when compared to slope,which showed a contribution rate of influence of 12.31.Critical rainfall intensity existed within the range of 0.7?1.5 mm min^-1 and 1.5?2.5mm min^-1 rainfall intensity,which can transform the dominant process of particle size selectivity in interrill erosion process?contribution rate of particle size selectivity of sediments yielded by raindrop detachment or by sheet flow transport to that yielded from interrill erosion larger than 50%?.?5?Totally,intensity of particle size selectivity in interrill erosion on loess hillslope showed up as intensity of particle size selectivity in raindrop detachment greater than intensity of particle size selectivity in sheet flow transport,and contribution rate of particle size selectivity of sediments yielded by raindrop detachment to that yielded from interrill erosion was larger than that yielded by sheet flow transport.4.The coupling relationships between particle size selectivity and sediment yields from interrill erosion on loess hillslope was quantified.?1?Under different combinations of rainfall intensity and slope,the average interrill erosion rate during a rainfall event was5.33×10^-6?1.03×10^-4 kg m^-2 s^-1;?2?Coupling response equations of interrill erosion rate to particle size selectivity of sediment yielded from interrill erosion on loess hillslope is the power function equation of interrill erosion rate?E_r?with the effective mean weight diameter of sediments yielded from interrill erosion(MWD_Er):E_r=4×10^-11 MWD_Er^1.317.?3?The integration of coupling response equation of interrill erosion rate to particle size selectivity of sediment yielded from interrill erosion,dynamic and factor equations of particle size selectivity in sheet flow transport process on loess hillslope built the particle-size-selectivity-based dynamic and factor equations of interrill erosion.