| Since the implementation of the grain for green project,the land use/cover on the Loess Plateau has undergone tremendous changes.The characteristics of the regional soil erosion have evolved significantly,and the benefits of soil and water conservation are remarkable.However,in recent years,global climate change has caused frequent rainstorms in the loess tableland region.The gully head erosion under vegetation-covering conditions,driven by intense runoff from rainstorms,is still serious.Previous studies have focused on the gully head retreat rate and its influence factors as well as empirical model building.The gully head erosion processes and morphological evolution mechanisms under vegetation-covering conditions are still unclear,which have become one of the scientific bottleneck issues urgently needed to be addressed in the prevention and control of gully erosion in the region.Given this,the gully head erosion under vegetation-covering conditions in the loess tableland region was taken as the research object.An erosion system of"upslope of gully head-vertical headwall-gully bed"was built.A series of field in-situ flow scouring experiments were conducted,and close-range photogrammetry technology and artificial field measurement were applied.Gully head retreat processes on a grassland were artificially simulated under different slope gradients and flow discharges.We aim to investigate the hydraulic characteristics of the water flow and gravitational erosion process during gully head retreat,clarify the effects on gully head erosion and sediment yield of hydraulic and gravitational erosion,and reveal the processes and mechanisms of the gully head morphology evolution on grassland.The research can provide a theoretical basis for gully erosion control and vegetation restoration projects on the Loess Plateau.The following main conclusions are reached:(1)The flow proportions of the on-wall and jet flow and flow hydraulics and energy consumption characteristics during gully head erosion were elucidated.The upslope flow is transformed into on-wall flow and jet flow as it flows through the grass-covered gully brink.The proportions of the on-wall flow and jet flow are 24.6%-58.7%and 41.3%-75.4%,respectively under different test conditions.The on-wall flow proportion decreased with increases in flow discharge and slope gradient,while the jet flow proportion changed in the opposite direction.The on-wall flow dominated the water drop at the gully head brink under the slope of 1°and flow discharge of 3.0-3.6 m~3/h,while the jet flow was the main part under greater flow discharges and slope gradients.The on-wall and jet flow proportions had the most significant correlations with the Froude number of the upslope flow and varied in power function with a negative exponent and logarithmic function with Fr,respectively(P<0.01).In terms of flow hydraulics,the jet flow velocity at the plunge pool was much greater than the upslope flow velocity,jet flow velocity at gully brink,and gully bed flow velocity.Meanwhile,the maximum and inside shear stresses of the jet flow were much larger than the shear stress of the upslope and gully bed flows.Additionally,the runoff energy consumption of the gully head in the gully head system accounted for63.3%-91.3%,indicating that the runoff energy was mainly consumed in the processes of the drop water erosion and sediment transport of the gully head.(2)The gravity erosion process during gully head retreat and its influence on the flow hydraulic characteristics of gully head were illustrated.Plenty of mass movement events occurred on the gully headwall during the gully head retreat.The observed mass movements were divided into soil debris collapse,scour hole collapse,and gully head overhanging mass collapse based on the scale of mass movement and its resulted morphological changes on the gully headwall.The frequency of the soil debris collapse was the largest under each test condition(76.0%-92.9%),followed by the scour hole collapse(2.9%-16.0%),and the frequency of the gully head overhanging mass collapse was the smallest(3.6%-9.7%).The time when the first overhanging mass collapse occurred was shortened with the increase of the flow discharge for each slope gradient treatment.The overhanging mass event occurred when the cantilever length,height,and thickness reached the thresholds of 16-30 cm,57-73 cm,and 17-33 cm,respectively.After the occurrence of the overhanging mass collapse,the flow width at the gully brink was reduced by19.1%-42.6%,but the flow depth and velocity were increased;meanwhile,the plunge height of the jet flow was decreased;additionally,the flow width was reduced,but flow depth and velocity were mostly increased on the gully bed.(3)The sediment yield characteristics during gully head retreat and its responses to hydraulic and gravitational erosion were clarified.Driven by the single hydraulic erosion,the sediment yield rate of the gully headwall increased with the increase of the flow discharge but decreased with the increase of the slope gradient and showed a highly significant linear function relationship with the on-wall flow rate(P<0.01).The sediment yield rate of the gully head system increased with the increases in flow discharge and slope gradient.The sediment yield rate of the gully head system had a highly significant power function relationship with the jet flow rate(P<0.01),and a linear function relationship with the runoff energy consumption of the gully head system(P<0.01);The sediment concentration of the upslope flow was less than 1 g/L,accounting for only 2.2%-5.8%of the sediment concentration of the gully head system.The proportion of sediment from the gully headwall in the sediment from the gully head system could reach 17.6%-64.9%,which had a highly significant linear relationship with the on-wall flow proportion.(P<0.01).The sediment yielding from the gully headwall was dominant in the sediment yielding from the gully head system under 1°slope gradient and 3.0 m~3/h flow discharge,while the sediment yielding from the gully bed dominated under greater flow discharges and slope gradients.Furthermore,driven by the superposition of hydraulic and gravitational erosion,the sediment yield rate of the gully head system suddenly increased when the scour hole collapse and overhanging mass collapse occurred,and then quickly decreased to the original level of the sediment yield rate.An overhanging mass collapse could suddenly increase the sediment yield rate of the gully head system by 4.3-23.5 times compared with that before the collapse.The average sediment yield rate of the gully head system increased with the increase of the flow discharge and varied complicatedly with the increase of the slope gradient.The sediment yield rate of the gully head system had a highly significant linear relationship with the length of the mass movement block under each flow discharge,and the line slope increased with the increase of the flow discharge.(4)The evolution processes and mechanisms of the gully head morphology of grassland were revealed.Driven by the on-wall flow scouring and gravitational erosion,a scour hole developed on the gully headwall.The depth and width of the scour hole underwent a cycle process of―slowly increasing-suddenly dropping-slowly increasing‖and a process of"slowly increasing-suddenly increasing-keeping unchanged‖,respectively.The on-wall flow scouring caused moderate increases in the scour hole depth and width.Scour hole collapse had a limited effect on the scour hole depth but resulted in a sudden increase in the scour hole width.Overhanging mass collapse suddenly enlarged the scour hole to occupy the whole headwall.The hydraulic erosion and gravitational erosion contributed to50.0%-74.4%and 25.6%-50.0%of the ultimate scour hole width,respectively.Moreover,scour hole collapse and overhanging mass collapse contributed to 6.7%-26.1%and18.3%-29.4%of the ultimate scour hole width,respectively.Driven by a single hydraulic erosion,the deepening and widening rates of the scour hole increased with the increase of the flow discharge and decreased with the increase of the slope gradient,and had a highly significant linear relationship with the on-wall flow rate(P<0.01).The splashing of the jet flow increased the depth and width of the plunge pool,but the scour hole collapse and overhanging mass collapse could cause the depth and width of the plunge pool to drop suddenly.The stable depth and cumulative width of the plunge pool under single hydraulic erosion increased with the flow discharge and slope gradient,and both of them had the most significant correlations with the jet flow kinetic energy at the plunge pool and the jet flow rate(P<0.01).The development of scour hole on the gully headwall is the main factor leading to the advance of the gully head on grassland.The gully head retreat length experienced a cycle process of"remaining unchanged-suddenly increasing-remaining unchanged"over time.The retreat length of the gully head remained unchanged under single hydraulic erosion but abruptly increased when the overhanging mass collapse occurred.An individual overhanging mass collapse could make the retreat length of the gully head suddenly increase by 15-28 cm.The cumulative retreat length of the gully head reached 17-51 cm under different treatments.The cumulative retreat length of the gully head increased with the increase of the frequency of the overhanging mass collapse under higher flow discharge.The vertical distribution of the root system,anti-scour coefficient,water-stable aggregate content,and shear strength of the gully head on grassland makes it difficult for upslope flow to cut down the gully head,and a large volume of on-wall flow is formed on the gully headwall to erode the middle and lower parts of the gully headwall.As a result,a scour hole develops on the gully headwall.The above processes triggered the overhanging mass collapse,which eventually led to the advance of the gully head on grassland. |
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