| Soil particles detachment and transport processes under different rainfall conditions are an important issue for hydraulic erosion study, and it is also the basis of understanding soil and erosion mechanism. This research was conducted to investigate soil detachment and transport processes and effects of erosion dynamic on soil loss process of leached cinnamon soil on hillslope in Beijing through varied simulated experiments on plot slopes and heraceous slopes in rainfall, based on relevant theories in the science of soil erosion, hydraulics, erosion dynamics, using the methods of field plots and indoor test soil bin system under the condition of simulated rainfall, selecting rainfall, slope gradient and vegetation coverage as the major factors which affect the raindrop splash and runoff and sediment yield on hillslope. The study had important theoretical and practical significance for establishing process-based soil erosion prediction model and managing soil and water loss on slope. The main conclusions are as follows:(1) The effects of rainfall intensity, slope gradient and vegetation coverage on the splash erosion were studied. The characteristics of rain splash erosion on slope had a positive relationship with rainfall intensity and slope gradient, and the amount of splashed soil particles had an exponentially decreasing tendency with the increae of vegetation coverage. The amount of splash soil particles had a negatively exponential function with splash distance. Splashed soil particles were mainly distributed in the range of 0 and 30 cm, and the distance of the splashed soil particles increased with the increae of rainfall intensity. Along with the increase of slope gradient, the amount of splash erosion increased in downward slope, and decreased in upward slope. The splash distance of soil particles were effectively reduced with the existing of herbaceous plants. In addition, raindrop splash had selection effect of particle size on eroding soil particles. The main particle size was in the range of 0.05-0.002 mm and fine sand (0.25-0.05 mm) was the easiest to be splashed. Splashed amount in each diameter class of soil particles had a negative correlation with splash distance. The mean weight diameter increased with the increase of rainfall intensity, and decreased gradually with the increase of splash distance.(2) The changing process of the size of soil particles detached by raindrop splash erosion and runoff erosion and the effect of raindrops splash on aggregate structure of sediment was revealed. Sediment particle size was mainly falls in the range of 0.1-0.002 mm. For the sediment induced by splash, the sand particles decreased and silt and clay particles increased with rainfall duration. For the sediment induced by runoff, the sand particles increased and clay particles decreased, and fluctuation of silt content was small. In the initial stage of runoff, coarse sand, fine sand, fine silt and clay size existed in the form of aggregation, and coarse silt was basically found to be in a form of primary particles, which shows an important influence on the soil detachment process. The Raindrops splash influenced the detachment degree of soil aggregate structure, and the effect of splash on soil surface accounted for 28.09% of the total detachment degree before the runoff generation. The extent of soil aggregate on slopes without splash was relatively low with its increment factual dimension was 48.43% of the increment of raindrop-splashed soil aggregate. In the sediment transport process, sand particles with low structural stability has been gradually decomposed to be tiny particles, which silt particles was transported in the form of primary particles and clay was gradually enriched in the erosion process.(3) The hydrodynamic characteristic and law of runoff and sediment yield with raindrops splash and vegetation covered condition were analyzed. By dividing the flow pattern of surface runoff based on different types of rainfall intensity, slope gradient and vegetation coverage, the surface runoff flow regime was detected in main types of slow flow, slow transition and transitional flow. On the surface of slope, the existence of rain splash effect and vegetation cover has a great influence on the dynamic characteristics of runoff and sediment yield. The slope runoff rate, sediment concentration and soil detachment rate were all significantly decreased when eliminating or reducing the raindrop impact. The change trend of sediment transport rate of vegetation covered was moderate, being in a state of "Increase first and then decrease and then tend to be stable". With the increase of vegetation coverage, the sediment transport rate in the slope was inclined to be stable in a short time.(4) The herbs structures control the detachment of soil erosion dynamic mechanism was confirmed. In different combination of trials, the effect of herbaceous plants on soil detachment was mainly reflected by the retarding effects of grass crown and root on rainfall and runoff. Sediment reduction rate of roots was higher than of canopy for both of the herbs. In the process of rainfall, the decreasing effect of leaves of herbaceous plants on the kinetic energy of falling raindrops was obvious, while the herb root system mainly played the role of fixing soils by the root diameter less than or equal to 1 mm. Study on the soil detachment resistance showed that the main resistance in bare slope included granule resistance, configuration resistance and wave resistance. For the slope with herbs coverage, granule resistance and wave resistance tended to increase with the increase of vegetation coverage. The detachment rate of the slope and resistance index was correlated by exponential function. Rainfall and runoff erosion was obviously weakened by the herbaceous plants on the slope, and the reduction coefficients were increased with the increase vegetation coverage, with the range of 22.75-82.43%and 19.68-63.97%, respectively. |
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