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Purpose: Spoil tips are an important source of newly increased water and soil loss in production and construction project areas. However, limited information focuses on the combination of rainfall and inflow effects on soil erosion of spoil tips. Therefore, this study aimed to quantify the effects of rainfall intensity, inflow rate and the interaction of rainfall and inflow on the slope erosion of spoil tips. Materials and methods: The runoff plot dimensions were 5 m × 1 m × 0.5 m (length × width × depth), and the slopes were set as 32° and 36°. A series of field experiments was conducted to study the effect of rainfall intensity (1.5, 2.0 and 2.5 mm min−1), inflow rate (8, 10 and 12 L min−1) and the interaction between rainfall and inflow on the soil erosion of spoil tips. Results and discussion: The results showed that rainfall intensity and inflow rate contributed 22.94–49.87% and 9.24–27.90%, 30.96–50.38% and 13.97–69.73%, respectively, to the runoff and soil loss. The interaction between rainfall and inflow contributed 6.18–38.65% to runoff and 20.72–70.83% to soil loss. The unit stream power, runoff shear stress and stream power can be considered the best hydrodynamic parameter to characterise the dynamic mechanisms of soil erosion under rainfall alone, inflow alone and rainfall + inflow, respectively. The corresponding critical values were 0.013 m s−1, 4.230 N m−2 and 0.774 N m−1 s−1. A dimensionless model for predicting slope soil erosion of spoil tips under the combination of rainfall and inflow was established based on the comprehensive consideration of rainfall, inflow, topography and runoff factors, which showed satisfactory prediction results (R2 = 0.887, NSE = 0.758). Conclusions: Inflow rate was the key element affecting the soil erosion of spoil tips. Therefore, in the management of soil erosion in spoil tips, the focus should be on how to effectively regulate runoff from the platform and slope. [ABSTRACT FROM AUTHOR]

• Quantified the effect of the slope, inflow rate and scouring times on soil loss of spoil tips. • Rill depth was the best rill morphological parameter for evaluating rill erosion of spoil tips. • Shear stress was the best hydrodynamic parameter for predicting rill erosion of spoil tips. The soil erosion of spoil tips seriously threatens the safety of people's lives and property and the surrounding ecological environment. Rill erosion is an important cause of water and soil loss in spoil tips. This study investigated the rill erosion on spoil tip slopes, the morphological changes of rills and the mechanisms of rill erosion. A field runoff plot (5 m long, 1 m wide and 0.5 m deep) with three inflow rates (1.6, 2 and 2.4 mm min−1) and three typical slopes (28°, 32° and 36°) was used for runoff simulation experiments. Results showed that inflow rate was the most important factor affecting soil erosion, which contributed 68.87% and 60.54% to the runoff rate and soil loss rate, respectively. Rill development has three stages: formation, development and adjustment. The overall predominance of parallel-shaped rills suggested that rill formation was dominated by concentrated runoff. Inflow rate exerted the greatest effect on the average rill width and average rill depth, with contribution rates of 73.11% and 60.26%, respectively. Meanwhile, slope exerted the greatest effect on the rill width-to-depth ratio, with a contribution of 60.45%. The average rill depth was the best indicator of rill morphology for evaluating rill erosion. Reynolds number was the best hydraulic parameter for predicting rill erosion. Flow shear stress was the best hydrodynamic parameter to describe rill erosion mechanism, with a critical value of 1.98 Pa. These results contributed to elucidating the rill erosion mechanism of spoil tip slope and provided a scientific basis for its soil erosion control. [ABSTRACT FROM AUTHOR]