Effects of typical ecological restoration measures for engineering accumulation on sediment yield and hydrodynamic process

This study aims to explore the temporal variations in the sediment yield and hydraulic characteristics during soil erosion on steep slopes of engineering accumulation in the Longmenshan fault belt under different measures of typical ecological restoration. The optimal ecological restoration measures were utilized for the soil and water conservation on engineering accumulation. The steep slopes were selected in the engineering accumulation for the No.2 cross-tunnel area of the Yangjiaping tunnel of the Chengdu-Lanzhou railway in western China. The ecological measures were constructed using the ecological restoration plants (Medicago sativa, Avena sativa, Robinia pseudoacacia) and ecological restoration materials (sheep manure and PVAc (Polyvinyl Acetate)). The scouring tests were conducted with a flow discharge of 60 L/min, in order to investigate the temporal variations in the soil sediment concentration and hydraulic characteristics (runoff regime, flow velocity, Darcy-Weisbach resistance coefficient, and runoff power) on the steep slopes of engineering accumulation under different restoration measures. It was found that the soil sediment concentration showed a clear initial dynamic stage, followed by a relatively stable stage after a period of adjustment. The time-averaged steady-state concentration of soil sediment were 15.57, 4.51, 9.84, 1.09, 4.37, 2.55, 2.07, 9.84, 15.93, 6.91, 6.28, 7.06, and 6.39 g/L in treatments with the control (CK), Medicago sativa + sheep manure (MM), Medicago sativa + PVAc (MP), Medicago sativa (M), Avena sativa + sheep manure (AM), Avena sativa + PVAc (AP), Avena sativa (A), Robinia pseudoacacia + Medicago sativa + sheep manure (RMM), Robinia pseudoacacia + Medicago sativa + PVAc (RMP), Robinia pseudoacacia + Medicago sativa (RM), Robinia pseudoacacia + Avena sativa + sheep manure (RAM), Robinia pseudoacacia + Avena sativa + PVAc (RAP), Robinia pseudoacacia + Avena sativa (RA), respectively. There were increasing trends in the temporal variations of runoff velocity and runoff power at the initial stages of the scouring tests, while the decreasing trends were observed in the Darcy-Weisbach resistance coefficient. The steady-state runoff velocity, runoff power, and Darcy-Weisbach resistance coefficient were attained in the period of adjustment. The runoff remained still in the turbulent zone, during the whole experiment. Different ecological restoring measures were also utilized to effectively reduce the soil sediment concentrations. Notably, the measures of M, A, AP, or RMM significantly increased the Darcy-Weisbach resistance coefficient, but decreased the runoff velocity and runoff power (P≤0.05). The herb layer played an important role in the soil and water conservation after 1-2 a of ecological restoration on the steep slopes of engineering accumulation, while the trees, sheep manure, and PVAc needed more time to demonstrate the soil and water conservation capacity. Consequently, the trees cannot effectively realize the soil conservation in the process of short-term ecological restoration, due to the short planting period and species competition. Meanwhile, the organic matter in sheep manure cannot be fully decayed, resulting in the soil aggregates formed by sheep manure or PVAc being prone to disintegration under water immersion. Overall, the yellow soil + Medicago sativa and yellow soil + Avena sativa can be the most important ecological restoration modes for the soil and water conservation at the initial stages of ecological restoration for the steep slopes of engineering accumulation on the Longmenshan fault belt.