| The spoil heaps formed by large-scale engineering construction activities not only occupy limited land resources,but also causes serious soil and water loss,even causing landslides,debris flows and other geological disasters.It has become the main source of new soil and water loss in China at the current stage.A series of studies were conducted on the impact factors of erosion and the dynamic mechanism of bare spoil heaps.However,quantified research on the regulation effect and mechanism of vegetation restoration on spoil heaps erosion is still insufficient research.Given this,the study built field spoil heaps models based on a large number of field investigations.Studied the effects of three vegetation types(Artemisia gmelinii,AG;Medicago sativa,MS;Cynodon dactylon,CD)on runoff characteristics,erosion dynamics,and sediment yield on the slope surface of steep spoil heaps under both intact grass(IG)and root only(OR)conditions,which through simulated rainfall and rainfall with run-on experiments,as compared to bare slopes(BS).Quantified the contribution rates of vegetation types and components to the benefits of regulating effects on runoff and sediment yielding,analyzed the differences of rainfall and rainfall with run-on conditions on the vegetation effects on runoff and sediment production of spoil heaps,and further revealed the internal mechanism of vegetation protecting the spoil heaps from erosion.The results can provide a scientific basis and technical guidance for prevention and control of soil erosion in engineering spoil heaps.The main conclusions include following four parts.(1)Quantified the effects of vegetation types on the runoff process and the benefits of different components in reducing runoff volumes of the spoil heaps.Under rainfall conditions,both the three grasses delayed the runoff begin time by17.3%～141.2%than that of BS.However,the incomplete vegetation protection makes it 52.2%～93.0%earlier than that of the intact grass.The runoff rates on the slopes of AG,MS,and CD decreased by 40.4%～63.4%,36.4%～82.7%,and 38.4%～48.1%,respectively,as compared to BS.The runoff rate under OR treatment slopes increased by 51.0%～275.2%as compared to IG.The average runoff volume reduction benefits of AG,MS,and CD were 49.3%,60.1%,and 41.9%,respectively.Among them,the contribution rates of the canopy were 54.3%,55.6%,and 64.6%,which was 1.19-1.82times of the root system.Under rainfall with run-on conditions,the runoff rate on grasses covered slopes decreased by 18.8%to 42.0%,as compared to BS,and increased by 3.0%to 275.2%under OR treatment as compared to IG.The benefits of vegetation on runoff volume reduction were 14.7%～68.7%lower than those of rainfall conditions,and the contribution rates of the canopy to reduction benefits was 1.62-17.06 times that of the root system.(2)Clarified the impact of vegetation types and components on the erosion hydrodynamic parameters of spoil heaps.Under rainfall condition,the benefit of AG on flow velocity reduction benefits(50.5%)was higher than that of MS(35.6%)and CD(21.3%).The flow velocity under OR treatment increased by 42.6%to 141.5%as compared to IG.The flow of spoil heaps belonged to the laminar flow category(Reynolds number less than 120),but they changed the flow pattern from slow flow to transition flow and jet flow,due to the vegetation type,components,and different runoff sources changing.The runoff resistance coefficient of grasses covered increased by 13.4%to 317.2%compared to BS,which decreased by 16.2%to 86.3%under OR treatment as compared to IG.The benefits of MS and CD in reducing runoff shear stress(23.5%～34.2%)were greater than those of AG(11.4%),but their effects on reducing runoff stream power were similar(55.4%for AG,55.4%for MS,and 46.2%for CD).The runoff shear stress and stream power under OR treatments increased by18.1%～79.4%and 31.3%～211.7%,respectively,as compared to IG.Under rainfall with run-on conditions,the benefits of vegetation reduction in flow velocity increased to56%～59%,and the flow velocity under OR treatments increased by 35.5%～114.5%compared to IG.The effects of vegetation on the slope resistance coefficient increased significantly by 2.28-35.35 times,which still increased by 16.1%to 242.6%even under OR treatment.The variation in runoff shear stress of grasses covered slopes compared to BS was less than 20%,however,it had a significant impact on runoff stream power.(3)Clarified and quantified the impact of vegetation types on the sediment production process and sediment reduction benefits by different vegetation components.Revealed the sorting characteristics of erosion sediment in spoil heaps under the influence of vegetation.Under rainfall condition,the soil loss rate on the BS decreased with time,while they showed generally stable for grasses covered plots.The average soil loss amount reduction benefits by AG(93.3%)were higher than that of MS(92.3%)and CD(88.5%).Which was followed by AG(59.2%)>MS(26.0%)>CD(13.3%)under OR treatments.The contribution ratio of AG root and canopy to sediment reduction efficiency was 63%:37%,while they were 28%:72%and 15%:85%for MS and CD,respectively.The proportion of sand particles in eroded sediment particles was the largest,while clay particles were the smallest.The protection of AG reduced sand loss,but intensified silt loss under OR treatments.The protection by MS and CD reduced clay loss but exacerbated silt loss.Under rainfall with run-on condition,the soil loss amount reduction benefits by AG and MS decreased to 82.3%and 74.9%,respectively,but the benefits of CD increased to 91.9%.Under the condition of OR treatments,the soil loss amount reduction benefits of AG,MS and CD were 55.2%,61.2%,and 57.7%,respectively.The contribution rate of the vegetation root system for AG,MS,and CD to soil loss amount reduction benefits were 2.04,4.41,and 1.70 times that of the canopy.The loss of silt content increased under vegetation protection,while the loss of clay particles decreased.From the perspective of sediment reduction benefits,AG was the advantageous grass species for vegetation restoration in spoil heaps as there was no upper flow.However,CD was more effective in protecting the slope surface of spoil heaps and reducing erosion when there was upper flow adding the rainfall events.(4)Revealed the impact of vegetation on the dynamic mechanism of slope erosion in spoil heaps.The correlation between soil loss rates with runoff shear stress was greater than that of runoff stream power for BS,however,the correlation between soil loss rate with runoff power was more significant for grasses covered plots.Under rainfall conditions,the critical runoff shear stress of grasses covered plots(0.67～0.98Pa)was increased by 3.4%～50.1%compared to BS(0.65 Pa).The soil erodibility parameters of AG,MS,and CD based on soil loss rate and shear stress reduced by 85.5%,81.9%,and 81.4%,respectively,compared to BS.The critical runoff stream power of grasses covered plots was 0.0195～0.0331 W/m~2,which the soil erodibility parameters reduced by 9.7%～30.3%,as compared to BS.The critical shear stress and stream power under OR treatments were 1.18-1.97 times and 1.83-3.04 times higher than those of IG.Revealed that differences in vegetation roots and stems lead to changes in flow velocity,which in turn alters effective shear forces,resulting in changes in the erosion benefits of vegetation on the spoil heaps.The results by the structural equation model(the goodness of fit is 0.66)indicated that vegetation directly affects the basic physical and chemical properties(0.90),hydrodynamic parameters(-0.51),and runoff(-0.45)of the spoil heaps,thereby indirectly affecting the erosion of the spoil heaps. |
