Soil Erosion Processes And The Size Selectivity Of Eroded Sediment On Hillslope

Soil erosion by water involves the processes of detachment, transport and deposition of soil materials by the erosive forces of raindrops and runoff. There is a need to describe sediment sorting to characterize the on-site and off-site effects of soil erosion. A better understanding of sediment sorting will improve understanding of erosion and sedimentation processes, which in turn will improve erosion modeling. Sediment may serve as the delivery mechanism for soil nutrients and contaminants to streams. Nutrient distribution is non-uniform over different sizes of sediment particles; fine sediments are usually richer in soil-sorbed nutrients than are coarse sediments. Knowledge of sediment sorting and its dynamics can also provide the basis for understanding and modeling the transfer of nutrients and contaminants from hillslopes to water bodies.Based on the current weakness of research on sediment particle characteristics and erosion evolution process in hillslope erosion process, simulated experiment of two kinds of soil (Lou soil and loess soil) was conducted to explore the development of soil erosion process, from sheet erosion to interrill erosion then to rill erosion, and the process of sediment yield in slope erosion and the redistribution of sediment particle. According to the theory of soil erosion, hydraulics, sediment movement mechanics, geomorphology, and combining with measurements of slope surface erosion form and analysis of sediment particle characteristic, the effects of rainfall intensity, coverage, gradient and slope length on slope erosion styles evolution process was revealed. On the basis of sediment particle characteristics and with the purpose of a furthermore understanding on the dynamical mechanisms of slope sediment yields process, details on the feedback mechanism between the soil structure and slope erosion processes were demonstrated to interpret the inherent law of soil erosion and to provide the theory basis for the models establishment of soil erosion. The main themes of the paper are as follows:(1)The study has researched that the variation of clay content of Lou soil and Loess soil in hillslope erosion under different rainfall intensity and slope gradient, and compared the difference of particle size distribution between effective sediment particle (non-dispersed) and ultimate sediment particle(dispersed). The results showed that the clay content of erosion sediment increased with the slope length and rainfall intensity, while decreased with slope gradient. The clay content of Loess soil increased8.8%with the slope length and3.9%with rainfall intensity, while in Lou soil, the increase was only2.8%and0.4%, respectively, moreover, the clay content of the two soils decreased with slope gradient which was4.9%and3.9%, respectively. Both of Lou soil and Loess soil, aggregation ratio was less than1and enrichment ratio was greater than1, which indicated that most clay particles were transported in the form of aggregates.(2) The study has investigated the sediment particle characteristics of Lou soil during the process of sheet erosion and rill erosion. The results suggested that the intensity of sediment yield and precipitation showed exponential relationship under different rainfall intensity (60mm/h,90mm/h and120mm/h). Hillslope erosion process was divided into three stages:splash-sheet erosion, interrill erosion and rill erosion.2～50μm particle content increased significantly in the stage of splash-sheet erosion comparing with the other two stages,<2μm particle content increased while2-50μm particle content decreased in interrill erosion,<2μm and>50μm particle content didn’t change significantly, and2～50μm particle content increased gradually in rill erosion. Additionally, the result showed a highly significant linear correlation among the effective sediment particle, the intensity of sediment yield, runoff rate, overland flow rate, precipitation and the ultimate sediment particle (R2=0.8, n=20).(3) The study has researched the effect of the effective sediment particle and sediment process under six different coverage (0,15%,30%,50%,70%,90%). The results showed that runoff intensity of straw mulch reduced14%-84%than bare land, and sediment intensity reduced49.8%-95.3%. With the extension of rainfall time, clay content was gradually reduced and accorded with the power function, the correlation coefficients were0.75and0.98(n=18), sand aggregation ratio of90%has achieved the minimum, sand aggregation ratio of bare land has achieved the maximum. It is showed that the ultimate sediment particles of bare land was much than the straw-covered slope.(4) According to the erosion rainfall distribution law in Loess Plateau, I has selected a rainfall intensity (90mm/h) and four slopes (10°,15°,20°,25°) to research the effective particle in time-varying size distribution and its separation characteristics and handling mechanisms. The results showed that clay content increased with the rainfall kinetic energy, accorded with linear regression (R2=0.452, P<0.01). Suspension-saltation<54μm size sediment was the most important erosion mechanism at work during inter-rill erosion processes. However, after rill development on hillslope, bed-load transport by rolling of medium to large-sized sediment particles (>152μm) became an increasingly important transport mechanism, and it were also enhanced by increased slope.