Hydrodynamic Characteristics In Eroding Rill Of Purple Soil And Comparison Study With Loess Soil

Purple soil as a special soil resource, has the good characteristics of soil texture and higher potential fertility. Therefore it becomes a unique, valuable soil resource in agriculture. But purple soil also has the characteristics of easy erosion, easy degradation and weak adverse resistance. So if the purple soil uses in improper or excessive way, its fertility will be greatly diminished. And eventually it leads to the problems such as soil drought, thin, low fertility, easily eroded. The sloping lands widespread distribute in Yangtze River Basin, which is usually abundant in rainfall and the ground surface. Consequently, runoff can easily cause soil and water loss. The soil erosion amount of rill erosion generally accounted for the entire slope soil erosion amount more than 70%. In the loess soil area, researches on rill erosion have made fruitful achievements. Then, the purple soil area also carried out more researches about rill erosion. But the erosion process researches of purple soil water dynamics were less. Therefore, it is necessary to research for further understanding of mechanism about rill erosion of purple soil. These studies can provide a theoretical basis to the prevention and control of rill erosion. In this study, the soil trough scouring method was used to study rill erosion while the loess area has been widely used. These experiments were on five slope gradients（5 °, 10 °, 15 °, 20 ° and 25 °） and three flow rates（2,4 and 8L/min）. The specification of the experimental flume was 12 m length, 0.1m width（limiting the rill width of 0.1m with PVC boards）, and 0.3m depth. The hydrodynamic characteristics of purple soil slope were studied by taken continuous samplings, measurement runoff velocity and rill depth along the flumes with segmental method. And the correlations of sediment concentration, soil detachment rate, runoff velocity, runoff shear stress, soil erodibility and soil critical shear stress were discussed. The results obtained with the previous loess soil were compared, and the results showed that:1) With the increase of slope gradient and flow rate, the sediment concentration of purple soil increased with rill length under all experimental conditions. At the beginning part of the rill the slopes of the sediment concentration curves were steep, which indicated a rapid and initial increasing in sediment concentration. The increasing rate gradually decreased to approximately zero at a certain rill length when the sediment concentration approached the limiting value at the maximum sediment delivery. The rill detachment rates of purple soil along the rill length exponentially declined. The decreasing rates were decreased fast at the beginning, and then decreased slowly. The steeper the slope gradients were and the greater detachment rates were. The detachment rates became greater with the flow rates increasing.2) The rill depth of purple soil was the maximum at the range 0-4m of the flumes, which indicated that the soil erosion was intensive. So the rill head was the key area of the layout of water loss and soil erosion prevention and control measured. The rill depth changed in volatility with the slope length, and indicated that soil erosion was a complex process with erosion, transport and deposition of alternating and recycle.3) The relationship between runoff velocity of purple soil and slope length can express by power functionnv =mL. The regression coefficient values of m and n was 0.30-0.72, 0.06-0.22, respectively. The values of m represented the minimum velocity at the entrance to the rill. The values of m increased with the slope and flow rate increasing. The runoff velocity of rill rapidly increased with slope length in 0-4m of the flumes, and in the 4-12 m slowly increased. Comparing with loess soil, the rill length range of the increasing runoff velocity was shorter. The runoff velocity was influenced by slope gradient and flow rate. But the flow rate had greater influence on runoff velocity than slope gradient.4) The runoff shear stress along the rill lengths of purple and loess soils showed decreasing trend. The runoff shear stress of rill was the maximum at the entrance to the flumes. The relationship between runoff shear stress and slope length can be used power function 0Ï„=D₀e^Eoc to express. The runoff shear stress of rill decreased with the increase of sediment concentration and the relationship can be fitted by the exponential function. Under the conditions of same slope gradients, the runoff shear stress, sediment concentration and the sediment content in the final exit were greater with the flow rate increasing. Under the same flow rate, the runoff shear stress and sediment concentration of runoff were greater with the slope gradient increasing.5) The soil erodibility parameters and soil critical shear stress can be calculated by the regression of the maximum detachment rate and runoff shear stress. The soil erodibility parameter Kr of experimental purple soil was 0.0469[kg/（m²Â·s·Pa）], and the loess soil was 0.1361[kg/（m²Â·s·Pa）]. The value of Kr of loess soil was 2.90 times of purple soil. The critical shear stress Ï„c of purple soil was 0.3875 Pa, and the loess soil was 0.2367 Pa. The Ï„c of purple soil was 1.64 times of loess soil, which indicated that under the same conditions, the loess soil was more susceptible to erosion and loss.