Study On Sediment Transport Capacity In Eroding Rill Of Purple Soil Slope

Purple soil concentrately distributes in the low mountain areas and hilly areas of Sichuan Basin. The distribution areas are plenty of rainfall and the rainfall is heavy concentrated and frequent. The land vegetation coverages are poor, and physical weathering of purple rock is strong. Parent rocks weather a layer to peel a layer. Then a lot of loose debris with weak cementation capacity is formed, resulting in serious purple soil erosion. The serious extent of purple soil erosion ranks only second to loess soil. The main land use pattern of purple soil areas is slope land. It is easy to form the rills, so the rill erosion is extremely serious in purple soil area. The rill erosion is not only restricting the sustainable development of agriculture but also a serious threat to people’s life and property safety. Sediment transport capacity of rill erosion is very important in water erosion sediment and sediment transport process, and is an important model parameter to soil erosion prediction. Sediment transport capacity has been highly concerned to the research field of soil erosion. Calculating the sediment transport capacity of rill erosion is the key to build soil erosion model based on physical development. A well-developed purple soil was sampled from runoff plot experiment station of Southwest University,Beibei District, Chongqing City(E 106°25’45 ",N 29°49’18").The soil contains 43.03% clay(< 0.005 mm), 39.27% silt(0.005- 0.05 mm), and 17.70% sand(> 0.05 mm).Soil materials were air dried and crushed before passing through an 8 mm mesh sieve. The experiments used a flume platform(12.0 m long and 3.0 m wide, to form rills(0.1 m wide and 12.0 m long) and separated by upright PVC boards. The bottom 5 cm of the rill was packed with clayey–loamy soil at a bulk density of 1.5 g cm-3 to imitate the plow pan layer. On top of the plow pan, the experimental soil materials were packed to a depth of 20 cm in 5 cm-thick layers at a bulk density of 1.3 g cm-3. Similar soil materials were glued to both sides of the PVC boards to ensure similar roughness of the soil surface and decrease the boundary effects on water flow and erosion. In addition, the imitated rill surface was packed slightly lower in the middle to form a V-shape to converge the water flow to the middle of the rill and reduce the boundary effect. Prior to each experimental run, the soil materials were pre-wetted with a rainfall simulator and drained for 24 h to ensure the same soil moisture content in all experimental runs. The experimental runs involved five slope gradients of 5°, 10°, 15°, 20°, and 25° and three flow rates of 2, 4, and 8 L/min with three replicates. After erosion, the flume was lowered to the horizontal position to measure eroded rill volumes at different rill segments through the volume replacement method. Thin plastic sheets were used to line the eroded rill bottom before filling the rill segments with water. The volumes of water required to fill the eroded rill up to the brim at each rill segment were recorded as the eroded volume. This study uses the sediment load process data along the eroding rill to estimate the maximum possible sediment concentration under different hydraulic conditions before the transport capacity was estimated. Three methods were used to calculate the sediment transport capacity by the actually measured maximum sediment concentration, the fitted maximum value and the sediment concentration when detachment rate is 0. The results showed that:1. Sediment transport capacity of purple soil was calculated by the actually measured maximum sediment concentration and the fitted maximum value. When the slope gradients were 5°, 10° and-15°, sediment transport capacity increased with the slope gradient and flow rate increasing. When the slope gradients were 15° and 20°, the sediment transport capacity increased with slope gradient and flow rate increasing. But the extent of increasing was less. When the slope gradients were 20° and 25°, the sediment transport capacity didn’t nearly increase with slope gradient and flow rate increasing. Therefore, a critical slope gradient was presumed between 15°-20° on the rill erosion of purple soil. Correlation analysis showed that a good correspondence between sediment transport capacity measured by the actually measured maximum sediment concentration and the fitted maximum value.2. Under the same experimental conditions, the comparison was between the sediment transport capacity of purple soil and the loess soil measured by the actually measured maximum sediment concentration. Under the same hydraulic conditions, the sediment transport capacity of loess soil was about 1.59~2.56 times to the sediment transport capacity of purple soil. The transport sediments of purple and loess soils are increasing with the rill length increasing, but the magnitude of increasing is gradually decreases with the rill length increasing, finally tend to a stable values, and the decreased extend of transport sediment of purple soil flow is greater than the decreased extend of transport sediment of loess soil flow.3. The transport sediments of purple soil were finally measured by the actually measured maximum sediment concentration and tended to sediment transport capacity when detachment rate was 0. It proved the method’s correctness of disposing the date using the volume displacement method. Three methods were used to calculate the sediment transport capacity. There was a very good consistency among the fitting relationships of three methods of calculating the sediment transport capacity. Under the same hydraulic conditions, the sediment transport capacity of loess soil measured by the analytical method was about 1.38~2.14 times to the sediment transport capacity of purple soil measured.4. The isolines that sediment concentrations of purple and loess soils were becoming sparse with rill length increasing. In larger slope gradient, the trend was more obvious. Under the same hydraulic conditions, and at the same rill lengths, the changing of sediment concentration of loess soil was greater than purple soil. It indicated that the loess soil is more prone to be eroded than purple soil.5. The sediment transport capacity of loess and purple soils increased linearly with flow rate. In larger slope gradients, the influence of flow rate on sediment transport capacity was greater. The sediment transport capacity of loess and purple soils increased logarithmically with slope gradient. In greater flow rates, the phenomena were more obvious, and the influence of the slope gradient and flow rate on sediment transport capacity of loess soil was greater than the purple soil.6. The sediment transport capacity of loess and purple soils increased logarithmically with water power, but the increasing rate of sediment transport capacity of loess soil was faster than the purple soil.