Sediment Incipient Motion Of Rainfall Runoff On Sloping Land

Sediment incipient motion mechanism of rainfall runoff is the important theoretical foundation for soil and water loss control on sloping land and comprehensive treatment of watershed. According to insufficient research of sediment incipient motion of rainfall runoff on sloping land, the law of sediment incipient velocity under the condition of rainfall runoff on sloping land was carried out, through the method of combining theoretical analysis with experimental investigation. By data analysis and experimental verification, following conclusions were obtained:(1)Using typical experimental conditions that experimental slope of 8m long flume with adjustable dip, uniform sediment with grain size class interval of 0.125mm-0.25mm in wind-water erosion crisscross region, gradient of 2°,5°,8°,10°,12°, rain intensity of 15,30,60,90,120mm/h, the laboratory artificial simulation of rainfall and water scouring experiment were conducted. The results showed that rainfall intensity, water depth and gradient had more effect on sediment incipient in the early formation of runoff, however, each factors in sediment incipient of rainfall runoff on sloping land had non-dominant status. Compared with sediment incipient in streamflow, velocity, water depth and sediment grain size were still the main factors affecting sediment incipient.(2)Sediment on sloping land was effected by rainfall and runoff, and the driving force included drag force, lifting force, subaqueous gravity, bond force and raindrop erosion force. This paper established conditions for sediment incipient motion of rainfall runoff on sloping land, based on sediment sliding mode, and thus gained the formula of the average vertical velocity for sediment incipient motion of rainfall runoff on sloping land. The formula showed that the main factors affecting sediment incipient motion of rainfall runoff on sloping land were flow, rainfall intensity, sediment grain size and slope.(3)The above formula was verified by using experimental data from rainfall and water releasing experiment under different slope gradients, rainfall intensity, drainage flow and sloping land with uniform sand. the results showed that: comparing calculated values with experimental values, the number of positive errors were equivalent to negative errors, the maximum positive error was 35.1%, the maximum negative error was 28.6%, comparing calculated values with experimental values of rainfall runoff condition, the number of positive errors were more than negative errors, the maximum positive errors was 43.2% and the maximum negative errors was 11.8%, which indicated that the formula was more suitable for calculating sediment incipient velocity of rainfall runoff, and that erosion force should be considered in the research of sediment incipient motion of rainfall runoff on sloping land.The formula was further verified by recognized laboratory and field data at home and abroad and experiment data of different bottom slope for non-cohesive uniform sediment flume experiment. Calculated through different bottom slope for non-cohesive uniform sediment flume experiment data, the number of errors mostly is negative, the biggest positive and negative error was 14.5% and 45.6%. By further compared with various measured data and the other various formulas, this formula was in line with the Tang's when the fine particles exist, in line with the Zhang's when the coarse particle exist, smaller than Sha's. Preliminary verification results also showed that this formula was not only suitable for sediment incipient velocity calculation under the condition of rainfall runoff, but also suitable for calculation of streamflow, not only suitable for sediment incipient velocity calculation of large bottom slope, but also suitable for small bottom slope. The results above also showed that the formula can be used to calculate soil and water conservation works and river runoff sediment incipient velocity.(4)Compared with streamflow, flow patterns of rainfall runoff on sloping land had unique characteristics. Owing to the combined effects of rainfall erosion, the velocity fluctuation tendency of upslope was higher than that of downslope velocity, and velocity was in order upslope < medium-slope < downslope; fluctuation of sediment content under rainfall condition was more severe than that of under water releasing condition, and compared to water releasing condition, sediment content under rainfall condition was generally higher, which indicated that rainfall had an important contribution to sediment yield. Re changed slightly with the increase of slope gradient, and increased with the increase of rainfall intensity, and Re was less than 500 in this article. Flow pattern was classified as laminar flow for traditional division of hydraulic flow region in our experiment. It was obvious to find sand rolling in water surface, therefore, flow pattern wasn't classified according to the traditional division of hydraulic flow region. The problem about flow pattern of rainfall runoff on sloping land and its effects on sediment incipient motion needed further investigation.