Water Erosion Processes And Sediment Sorting Associated With Transport Mechanisms On Steep Slopes

Soil erosion by water involves the detachment,transport,and deposition of soil materials due to the erosive forces of raindrops and runoff.Particle properties of sediment link to sediment transport during water erosion processes.However,previous studies have measured the particle size distribution of sediment with and without dispersion treatment,leading to different results,thus sediment sorting and transport mechanisms during erosion processes need to be investigated systematically for better understanding of water erosion processes.We conducted rainfall simulation experiments to quantify particle size distribution of sediment for investigating impact of rainfall kinetic energy,overflow characteristics and soil properties on water processes and sediment sorting.The main results are as follows:(1)Rainfall kinetic energy(KE)associated with water erosion processes and sediment sorting on steep hillslopes was investigated.The six KE values were obtained(76,90,105,160,270,and 518 J m-2 h-1)by covering wire screens located above the soil surface with different apertures to change the size of raindrops falling on the soil surface,while maintaining the same rainfall intensity(90 ± 3.5 mm h-1).The comparison of the effective particle size sidtribution(PSD)and ultimate PSD of sediment was used to investigate the detachment and transport mechanisms involved in sediment mobilization.The effective/ultimate ratios of clay-sized(<0.002 mm)particles were less than 1,whereas those of sand-sized(>0.05 mm)particles were greater than 1,suggesting that these particles can be transported as aggregates.The ratios of silt-sized(0.002~0.05 mm)particles were closed to 1,indicating these particles were transported as primary particles.Moreover,the higher KE,the closer of the ratios to 1,indicating that sediments may be transported as primary particles at higher KE owing to an increased severity of aggregate disaggregation for the silty clay loam soil.The percentage of clay-sized particles and the relative importance of suspension-saltation increased with increasing KE when KE was greater than 105 J m-2 h-1,while decreased with increasing KE when KE is less than 105 J m-2 h-1.The value of 105 J m-2 h-1 can be as the threshold level beyond which the disintegration of aggregates was severe,influencing erosion processes and sediment sorting may change.(2)The effect of land cover on water erosion processes and particle size selectivity was evaluated through various mulch-cover levels applied on soil surface.Both rainfall kinetic energy and overflow characteristics were changed by different mulch-cover levels.Compare with the bare soil,the mean runoff rate reduced to 10.5%、20.5%、35.2%、68.8% and 71.5%,and the effective-ultimate ratios of clay-sized(<0.002 mm)particles reduced to 6.5%,12.0%,18.7%,20.9% and 29.7% when the soil coverd by 0.075,0.15,0.25,0.35 and 0.45 kg m-2 straw mulch,respectively.As a result,sediment contained less primary particles when the soil covered by more straw mulch,owing to the reduction of rainfall kinetic energy and streampower by mulch-cover which increased aggregate breakdown.Comparison of the ultimate particle-size distribution of the transported sediment with that of the parent soil provides a measure of the particle-size selectivity of sediment mobilisation.Enrichment ratio(ER=percentage of particles in a given size class in eroded sediment/ percentage of particles in a given size class in original soil)of clay,silt and sand was analyzed.The results illustrated that ER of clay was less than 1,indicating that clay was prone to deposit rather than being eroded during erosion processed,probably because clay particles were aggregated as larger aggregates harder to be transported than other small size particles due to larger gravity.Conversely,ER of silt was larger than 1 that is,silt was easy to be eroded by rainfall and runoff so sediment contained more silt than other size particles.ER of sand reduced with increasing mulch-cover levels,because the erosive force and transport capacity of raindrops and runoff decreased when mulch-cover levels enhanced,sand was prone to deposit on the ground during erosion processes due to its larger particle density.(3)The influence of soil texture and aggregation on the particle size distribution of sediment on steep hillslopes was detected,as well as the variation in the particle size distribution before and after rill initiation.We selected four different soils from Yangling(YL),Changwu(CW),Ansai(AS),and Suide(SD)on the Loess Plateau of China.The results showed that the degree of sediment aggregation decreased with decreasing clay content in the original soils,as reflected in the effective/ultimate PSD ratio close to 1.Before rill initiation,the median particle size of sediment from YL,CW,AS and SD soils was 0.027,0.023,0.037 and 0.039 mm,respectively,while after rill development,the median particle size of sediment from YL,CW,AS and SD soils was 0.040,0.029,0.040 and 0.044 mm.The sediment after rill development was coarser than that before rill development owing to the large flow depth and runoff energy of the rill flow.Rill width/depth ratios decreased with increasing clay content in original soils due to the higher shear stresses and more stable aggregates which limited sidewall collapse.These may increase flow depth and streampower and correspondingly enhance the runoff entrainment capacity.As a result,coarse particle content increased with decreasing rill width/depth ratios.Moreover,soils that contained more heavy particles,including large aggregates and coarse primary soil particles,more easily rolled on steep slopes than soils containing fewer heavy particles due to the strong gravity and inertial forces in the direction of the slopes,which may increase the relative importance of the bed-load transport mechanism.To summarize,we noticed the mobilization of particles by sheet flow and rill flow and found that the mass fraction of different sediment sizes was distributed bimodally,including the peaks for finer particles and coarser particles.Furthermore,we detected the influence of the variation of aggregates during erosion processes on sediment sorting.Aggregates were mainly disrupted by slaking and raindrop impact in our study,and the degree of aggregate disintegration may determine the particle size distribution of sediment that is,sediment contained more fine particles if more aggregates were disrupted.Additionally,sediment transport can be divided into suspended,saltating and contact(rolling)loads.We confirmed that fine particles were prone to being transported by suspension/saltation,while coarse particles were prone to being transported by rolling.Medium size class particles were hard to be eroded.The relative importance of each transport mechanism may vary with soil properties and erosive forces.Coarse particles were easy to roll on the steep slopes,but this situation may not occur on gentle hillslopes.Consequently,results of the study may improve the accuracy of water erosion prediction on steep hillslopes by consider rolling transport mechanism.