Response of runoff and sediment production and sediment and water benefits of engineering spoil heaps to vegetation characteristics

Engineering spoil heaps as a severely eroded geomorphic unit in production and construction projects have generated widespread attention to soil erosion. Vegetation was the most direct and effective means for ecological restoration of engineering disturbed underlying surfaces. Conducting quantitative research on the effects of different near-surface characteristics of vegetation on the runoff and sediment production and water storage and sediment reduction benefits of spoil heaps under field simulated rainfall experiments was of great significance for deeply revealing the internal mechanism of vegetation protection disturbance slope erosion. This study selected mixed soil and rock spoil heaps (10% gravel mass fraction, slope 30°) as a typical representative of engineering erosion underlying surface, and used bare slope (BS) as a control. Four sets of eight engineering accumulation models were established in the field, with dimensions of 3.46 m×2.00 m×0.60 m (length), and the projected slope length was 3 m. Quantitative analysis was conducted on the characteristics of runoff and sediment production and the benefits of water storage and sediment reduction of the spoil heaps under three types of near-surface characteristics of vegetation (intact plant IG, no leaf NL, only root OR) through field rainfall experiments (set rainfall intensity as 0.8, 1.2 and 1.8 mm/min). The Cynodon dactylon (CD) as a grass species for vegetation restoration in the study area was a typical type of grass species with strong growth ability and belongs to the perennial grass species, which was also widely used in engineering construction. Further, it reveals the internal mechanism of vegetation regulation of spoil heaps erosion from the perspective of its impact on water and sediment relationships. The results showed the following: 1) The occurrence of runoff begin time on the gentle slope surface of intact plants (IG) resulted in a delayed benefit of 50.74%-188.98%, as compared to BS, while the lack of vegetation canopy (NL, OR) accelerated the occurrence of runoff begin time, causing it to occur 2.19%-70.12% earlier than BS. 2) The instantaneous runoff rate of vegetation under NL and OR treatments was 0.20%-185.58% higher than that under IG treatment, while the instantaneous runoff rate of OR and NL treatments under heavy rainfall conditions (1.8 mm/min) was 1.20%-169.10% higher than that of BS. The instantaneous soil loss rate of vegetation under IG and NL conditions decreased by 0.91%-98.71% compared to BS. They even increased erosion under OR conditions when the rainfall intensity reached 1.8 mm/min, making it 6.76%-75.63% higher than BS. 3) As the rainfall intensity increased, the sediment reduction benefits of vegetation under NL and OR treatments decreased from 95.18% and 68.31% to 46.58% and -68.13%, respectively. However, the impact on sediment reduction benefits under IG treatment was small (benefit difference<2%), and the average water storage benefits decreased with increasing rainfall intensity. There is a significant linear correlation between water and sediment in both BS and vegetated spoil heaps (R² ranging from 0.40 to 0.88), and a runoff rate of 4 L/min significantly changed the protective effect of vegetation on the slope surface of the spoil heaps. The vegetation and its near-surface characteristics had significant impacts on the average runoff and sediment yield and per unit runoff sediment yielding (P<0.05). It is proposed to choose vegetation with a rich canopy when carrying out ecological restoration of spoil heaps, while avoiding external factors that may cause damage to vegetation. The research results are of great significance for revealing the erosion mechanism of vegetation protection disturbance slopes, and can provide practical guidance for the restoration of accumulated vegetation.