Runoff, sediment yield and rill development characteristic of unpaved road in mining area under field artificial simulated rainfall condition

Abstract: Serious man-made soil and water loss are common on unpaved roads in the mining area of Shenfu Coalfield. In general, unpaved roads are frequently rolled by transport machinery and result in massive mud after rainfall. A certain thickness of loose surface soil layer is generated after the mud is air-dried and rolled, which negatively impacts the ecological environment. An artificially simulated rainfall experiment was carried out in this study. The law of runoff and sediment yields and the characteristic of rill morphology development of unpaved road in mining area were studied at different rainfall intensities and slope degrees. According to the results of field investigations, the slope of roads usually ranged from 2° to 20° in the area and thus we set the slope of plots to 3°, 6°, 9°and 12°. Six rainfall intensities were tested, varying from 0.5 to 3.0 mm/min with an interval of 0.5 mm/min. The thickness of loose surface soil layer was set to 0.5 cm in our experiments. Plot used in the experiment was laid out to be 3 m×1 m and two flow sections were set for the measurement of runoff and sediment. Before each experiment, rainfall intensity was calibrated repeatedly until the rainfall uniformity coefficient reached 80% or higher. Soil particle composition was determined using Mastersizer 2000, whilst soil bulk density and moisture content were measured using the oven-drying method. During each experiment, flow velocity was measured with the dye tracing method. Flow width and depth were measured with a point gauge system. From 3 minutes before the runoff generation, runoff samples were taken once a minute. Other samples were taken every 3 minutes after runoff generation. All of the experiment was repeated twice. Results showed that: 1) Runoff rates varied from 1.12 to 8.24 L/min and had a significant linear relationship with rainfall intensities and no correlation with slope degrees. The Reynolds number and Resistance coefficient were 232.38-2 073.0 and 0.45-4.47, respectively. The turblence intensity of runoff increased with the increases of rainfall intensities and slope degrees. Resistance coefficient varied significantly with slope. 2) The soil detachment rate of unpaved road ranged from 0.92 to 324.46 g/(m2·s) and was a significant power function of rainfall intensities, slope degrees and runoff rates. Critical shear stress and stream power activating sediment detachment of unpaved roads were found to be 2.15 N/m2 and 0.41 W/(m2·s). 3) Sheet erosion was the main erosion form on unpaved roads with a slope degree of 3°. The rill formed on the road surface when the slope degree was greater than 3°. The rill breadth depth ratio, rill complexity degree, rill split degree and rill density were 1.80-3.75, 1.07-1.55, 0.20%-10.33% and 0.067-2.01 m/m2, respectively. Overall, the rill density increased with rainfall intensity increased, rill breadth depth ratio decreased with slope degrees increased and rill complexity degree and split degree were positively correlated with the interaction of rainfall intensities and slope degrees. 4) Ratios of rill soil erosion yield to the total soil erosion yield were 18.0%-57.16%. The power function could be used to describe the relationship between total erosion yield and rill split degree and rill density. Exponential and linear functions were found to be useful to express the relationship between total erosion yield and rill breadth depth ratio and complexity degree. The results provide key parameters to the implementation of engineering soil and water conservation measures on unpaved roads and are therefore meaningful for the production safety in the mining area.