Rill erosion process on red soil slope under interaction of rainfall and scouring flow

Abstract: Soil erosion with surface runoff in red soil hilly in tropical and subtropical zones of central China cause severe soil quality degradation and environmental issues. It is driven by both rainfall and runoff flow that usually take place simultaneously during an erosion event. The objectives of this study were to determine the main process of rill erosion evolution and explore the main erosion parameters that affected the erosion rate during the process. In order to simulate the erosion process, an indoor experiment including the experimental combination of 3 slopes (5?, 8?, 12?), 2 discharges (5, 7.5 L/min) and 2 rainfall conditions (no rain and 90 mm/h rainfall intensity) was carried out on a red soil flume (3-m length and 0.5-m wide and 0.4-m height). The slit clay soil passing through a 2-cm sieve was used for the experiment. Before the experiment, the bulk density and moisture for the soil sample was controlled at 1.35 g/cm3 and 30%, respectively. The staining method was utilized to measure the flow velocity; the runoff width could be measured through using a ruler at 3 positions; the sediment yield and flow rate were be deduced and calculated from the water bottle used for collecting the runoff samples at the bottom of the flume during a set time interval. A single trial persisted 50 min. The hydrodynamic parameters including shear stress, unit stream power and stream power were calculated. Results showed that: 1) The erosion of red soil slope had obvious periodic development. The initial phase of erosion processes was mainly based on the layer erosion before the first 3-min and then followed by rill erosion. Steep grade, high flow rate and rainfall condition could greatly accelerate the development of rill erosion process and largely increase the erosion rate. 2) The mean erosion rate of each erosion phase was followed by: initial phase> rill development phase> rill stabile phase. In general, the total sediment yield of each erosion phase was followed by: rill development phase> initial phase> rill stabile phase. The erosion in the rill development phase accounted for 38.43%-84.84% of the total sediment yield. The relationship between the hydraulic parameters and the erosion rate of initial phase was closer by stream power> slope> shear stress> unit stream power = flow velocity > flow rate. The mean erosion rate of rill development phase was extremely significant (P<0.01) with stream power, and was significant (P<0.05) with shear stress and slope, while the erosion rate of rill stable phase was just related to flow rate and slope. 3) Stream power was the optimal predictor to predict erosion rate for both layer erosion and rill erosion with a linear relationship. The critical stream power for the layer erosion, the rill erosion in the flow scouring test, and the rill erosion under the interaction of rainfall and scouring flow was 0.091, 0.121, ?1.691 N/(m·s), and their corresponding erodibility was 19.95×10-3, 2.64×10-3, 3.59×10?3 s2/m2, respectively. The results are valuable for agricultural water-soil engineering and controlling soil loss of red soil, and also play a significant role in improving soil erosion models.