Influences of vegetation types and near-surface characteristics on hydrodynamics and soil erosion of steep spoil heaps under rainfall and overland flow conditions

Spoil heaps, are characterized by loose structure, high infiltration, steep slopes, etc, and have become a source of soil erosion and geological disasters. Vegetation was regarded as an effective way to curb soil erosion of spoil heaps. However, few studies have been conducted to explore the effects of vegetation types and their near-surface characteristics on hydrodynamics and soil erosion of steep spoil heaps. This study aimed to address this research gap by conducting field simulated rainfall and rainfall with run-on experiments using simulated spoil heap plots with 3.4 m × 2 m × 0.6 m (length × width × depth) and a slope of 30 degree. The material used in the spoil heaps comprised a uniform mixture of silt soil particles, constituting 90 % by mass, and gravel, constituting 10 % by mass. For this study, bare slope (BS), Artemisia gmelinii (A. gmelinii), and Cynodon dactylon (C. dactylon) treatments were designed. Additionally, the grass-covered underwent two treatments: intact grass (IG) and only root (OR). The simulation experiments consisted of two types: rainfall conditions with intensities of 0.8, 1.2, and 1.8 mm min⁻¹, and rainfall with run-on conditions with three intensities and a discharge flow rate of 15 L min⁻¹. The results showed that: under rainfall conditions, A. gmelinii had a higher soil loss reduction benefit (93.3 %) than C. dactylon (88.5 %), while the percentages reversed with A. gmelinii at 82.1 % and C. dactylon at 91.9 % under rainfall with run-on. Moreover, A. gmelinii exhibited higher runoff volume reduction benefits than C. dactylon under both experimental conditions, except for rainfall intensity of 0.8 mm min⁻¹ in IG treatments. The canopy of A. gmelinii accounted for 53–69 % of the reduction in average soil loss rate, whereas the root and canopy of C. dactylon contributed 58 % and 162 % under rainfall and rainfall with run-on conditions, respectively. During heavy rainfall events, vegetation could experience more severe soil and runoff loss compared to BS once its aboveground canopy was lost. Furthermore, A. gmelinii had a greater effect in reducing average velocity and increasing resistance coefficient compared to C. dactylon. Additionally, C. dactylon had twice the effect in improving shear stress and stream power than A. gmelinii. Vegetation influences slope sediment and runoff yield by modulating erosion dynamic parameters. This study can serve as a valuable reference and practical guidance for the ecological restoration of similar engineering spoil heaps in the future.