| The purple soil is the main sediment source in the southern China. It is an eroded lithologic soil with high productivity. There are a lot of characteristics about it, such as soft quality, loose structure, easy weathering, poor corrosion stability, weak soil and water conservation capacity and so on. It makes that the soil erosion area and intensity of purple soil region rank only second to the Loess soil region. Rill erosion is the important progress of slope water erosion and main source of small basin sediment, as one of the main forms in the soil slope erosion research. It has the important potential impact in development and evolutionary process of basin slope landform. In the recent years, the researches about rill erosion are richer, but the rill erosion of purple soil is less. Therefore, the research of purple soil rill erosion is very meaningful to the science and actual production.A comparative study on rill erosion between the two soils is important to increase research knowledge and exchange application experiences. Rill erosion processes of loess and purple soils were determined through laboratory experiments with the volume replacement method. It is used in this research that rill erosion is simulated by the flumes. Water flow rates in the experiment were set in the light of the critical intensities of the rainfalls that produce rill erosion in sloping croplands on purple soil. Sediment was collected in a sampling bucket placed at the rill outlet. After a certain period of water scouring, rill erosion occurred, and then the soil flume was adjusted from sloping to horizontal. Plastic film was folded into multiple layers to form eleven thin, waterproof baffle plates the same in width as the rill. The baffle plates were inserted at 0~0.5 m, 0.5~1 m, 1~2 m, 2~3 m, 3~4 m, 4~5 m, 5~6 m, 6~7 m, 7~8 m, 8~10 m, and 10~12m from the rill entrance, thus dividing the erosion rill into the 11 sections. Rill erosion was found to be quite low on a slope 5° in gradient with a flow rate of 8 L min-1, thus excluding the need of running the test with flow rates being lower than 8 L min-1, that is, 2 L min-1or 4 L min-1, on the slope. Similarly, the test with the flow rate of 2 L min-1on a slope 10° in gradient, because erosion was already very low with flow rate at 4 L min-1. According to the undertaking cumulative eroded mass in the end of rill calculating the segmented, it is used to analyze the progress of rill erosion. The experimental results show that:1) The cumulative eroded mass and sediment concentration of the purple soil rill erosion can preferably match with the rill length using the exponential function. The cumulative eroded mass and sediment concentration increase with rill length, but the increasing rates decrease exponentially with distance along the rill, eventually tending towards a stable sediment concentration. The variation tendencies of cumulative eroded mass and sediment concentration are more obvious in the condition of steep slope gradient and high flow rate. The stable values of them increase with slope gradient and flow rate increasing. The significant analysis of slope gradient and flow rate was done. The influence of slope gradient was greater than the influence of flow rate, and the tendency of sediment concentration to increase with rill length was more obvious for changes in slope gradient than flow rate. The variation tendencies of them are similar. However, the transporting sediments of loess soil stream are more than the purple soil because of the different soil types. The advancing velocity of sediments is similar in the two kind of water stream.2) Under the conditions of different slope gradient and flow rate, the critical slope lengths of rill erosion are inferred to calculate, through the relationship between the sediment concentration and rill length. When the slope gradient is slow(5°and 10°), the critical slope lengths are still longer and the length range is 11~19m, even if the flow rate is greater. When the slope gradient is steep(15°, 20°and 25°), the critical rill lengths are more stable and concentrate on 4~5m.3) The rill detachment rate of purple soil exponentially decreases with rill length increasing, and linearly decreases with sediment concentration increasing. The rill detachment rate decreases with sediment concentration increasing, and the stable peak value increases with slope gradient and flow rate increasing. The velocity of decreasing to the minimum accelerates. When the soil type of rill underlying surface is confirmed, the most of energy of rill original segments is used to detach sediments, under the condition of certain slope gradient and flowrate. Then the water stream reached the greatest detachment capacity, and the detachment rate reached the maximum. With rill length increasing, the most of energy gradually transforms to half detach and half transport sediments from using to detach sediments. Reaching the end of rill segments, the most of energy is used to transport the detached sediments, and approaching the maximum sediment concentration. At this moment, the detachment capacity is the minimum, and the detachment rate tends to 0.At the same time, the characteristic principle of rill detachment progress is well obtained, through the formulas show that rill detachment rate is the great function of slope gradient, flow rate, rill length and sediment concentration. The variation tendency of rill detachment progress is similar in both of them. The clay content of purple soil is more than the loess soil, and it could easily form the aggregate structure to lead to difficultly transported by the water stream. Therefore, the detaching sediments of loess soil are more than purple soil, but two kind of soil could reach the saturated sediment content(the rill transport capacity) in the same rill length.4) The analysis values of rill detachment rate are calculated in accordance with the results of rill sediment transporting progress. The analysis values are used to compare with the experimental values using the volume replacement method. The result shows that the experimental values are similar as the analysis values. It can demonstrate the validity of analysis formula inferring, and obtain a new way to calculate the rill detachment rate. |
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