Experimental Study On The Influence Mechanism Of Typical Grass Cover On Gully Headcut Erosion Processes In The Loess Tableland Region

The loess Plateau gully region is an important geomorphic type area and grain and fruit base of the Loess Plateau.Since the project of“Grain for Green”and the“Controlling gully for loess-tableland protection”has been implemented,the coverage rate of forest and grass has increased greatly,and the soil and water loss in the loess tableland has been curbed obviously.In recent years,climate change has led to frequent heavy rainfall events,and storm runoff in Loess tableland often leads to serious gully headcut erosion.Vegetation can effectively inhibit gully erosion and change the development process of gully erosion.However,it is unclear how vegetation affects hydraulic and gravitational erosion during gully headcut erosion.In view of this,a series of in situ inflow scouring experiments were conducted on bare land plots and vegetation plots（including the upstream area,headwall,and gully bed）planted with Bothriochloa ischaemum,Agropyron cristatum,and Medicago sativa with clustered stems and Artemisia gmelinii with erect stems and different inflow discharges(3.6-8.4 m³ h^-1)conditions to investigate the influence of vegetation on the process of gully headcut erosion processes.The objectives of this study are to（a）elucidated the root distribution,root mechanics,and basic physical and mechanical characteristics of gully head soil,（b）clarify the vegetation and inflow discharge of the upstream area,gully head and gully bed runoff hydrodynamic characteristics,（c）reveal the influence of vegetation on the morphology of gully headcut erosion and sediment yield in gully head system,（d）Finally,the mechanism of vegetation on hydraulic and gravity erosion was revealed.The research results can provide a theoretical basis and technical support for the ecological construction of the“Controlling gully for loess-tableland protection”of the Loess Plateau and ecological environmental protection of the Yellow River Basin,and have important scientific and practical significance.The main results are as follows:（1）The vegetation roots morphological distribution and mechanical characteristics at the gully head and their effects on basic physical and mechanical properties of gully head soil were investigated.The root indexes showed a gradual decrease with the increase in soil layer depth.The root length density of vegetation decreases by 5%-81%,and root surface area density was reduced by 30%to 82%,respectively,when the soil layer deepens from 0-20 cm to 80-100 cm.The root system of each vegetation was mainly composed of 0-0.5 mm diameter roots,accounting for 78%-96%of the total root length.The friction resistance between roots and soil of different diameter classes decreased with the increase of soil water content,showing a linear function relationship.The root friction resistance characteristics of different vegetation were as follows:the root friction resistance of Bothriochloa ischaemum with a diameter of 0-0.5 mm was small,and the root friction resistance was not significantly different among Agropyron cristatum,Artemisia gmelinii,and Medicago sativa,ranging from 2.24 N to 2.79 N.For roots with a diameter of 0.5-3 mm,the friction resistance between the Agropyron cristatum roots and soil was the highest,followed by Artemisia gmelinii and Medicago sativa under the same water content.On the whole,the soil shear strength of vegetation plots was significantly higher than that of bare land,and that of Medicago sativa plots was the maximum.Under the same soil layer,soil cohesion decreases with the increase in soil moisture content.The soil antiscourability of all vegetation plots was higher than that of bare land plots,ranging from 4.61 L/g to 29.15 L/g,and ranked highest to lowest on Medicago sativa,Bothriochloa ischaemum,Agropyron cristatum,Artemisia gmelinii,and bare land.With the deepening of soil depth,the soil antiscourability coefficient showed a decreasing trend.（2）The influence of vegetation on hydrodynamic characteristics in the headcut erosion in the gully head system was expounded.Compared with the bare land,vegetation can be significantly increased the upstream area runoff resistance coefficient,significantly reduces the upstream area velocity and jet flow velocity at the gully brink.Bare land plots upstream area runoff velocity is 1.2-3.3 times of the vegetation plots.the resistance coefficient of each plot ranked highest to lowest on Medicago sativa,Bothriochloa ischaemum,Agropyron cristatum,Artemisia gmelinii,and bare land.Under the same inflow discharge,there is no significant difference in jet flow velocity at gully brink.The influence of vegetation with clustered stems on roughness and runoff velocity was greater than that of vegetation with branched stems.On the whole,the velocity of gully bed was higher in bare land plots than in vegetation plots.Except for Medicago sativa plot,the Darcy-Weisbanch friction factor of gully bed in vegetation plot and bare land plot showed an increasing trend with time.Under different inflow discharge condition,the bare land plot upstream area runoff kinetic energy significantly greater than the vegetation communities,is 1.5 times to 10.0 times of the vegetation communities.The jet flow kinetic energy at gully brink of the bare land plot was significantly greater than that of the vegetation plots,and the greater the discharge of inflow discharge,the greater the difference between the bare land plot and the vegetation plots.The energy consumption of gully bed in Agropyron cristatum and Artemisia gmelinii plots was higher than that in Medicago sativa and Bothriochloa ischaemum plots.（3）The influence of vegetation on the development process of erosion,gravitational erosion and sediment yield of gully head was ascertained.On the whole,the scour hole depth in the gully headwall in the vegetation plots presents a cyclic change process of“gradually enlarging-suddenly dropping–stabilizing”over time.The depth of he scours hole increases gradually with the test time.When the depth reaches a certain degree,the depth of the scour hole in thegully head drops abruptly due to the gravity erosion and collapse,and the frequency of the sudden drop increases with the increase of inflow discharge.Compared with the vegetation plots,the scour hole depth of Agropyron cristatum plot was the largest,followed by Bothriochloa ischaemum plot,while the scour hole depth of Artemisia gmelinii and Medicago sativa plot was relatively small.There are obvious differences between vegetation gully head and bare land gully head.Vegetation plots gully head retreat in a stepwise way,showing a trend of“steady-enlarging-stable”.The way of the bare land plot gully head retreating basically changes with the increase of the test time.Vegetation can significantly inhibit the gully head retreat distance,and the gully head retreat distance of bare land plots is1.1 times to 10.8 times of that of vegetation plots.The occurrence time of gravity erosion in vegetation plots and bare land plot is obviously different under different inflow discharge.The frequency of gravity erosion in bare land plot increases first and then decreases with the increase of inflow discharge.Under the experimental conditions,no gullehead cantiledral collapse event occurred in the bare land plot and Agropyron cristatum plot.The size of critical gullehead cantiledral collapse cavity in the Artemisia gmelinii plot was the largest,followed by Bothriochloa ischaemum plot,and the Medicago sativa plot was the smallest.Vegetation can significantly reduce the upstream area runoff erosion,Agropyron cristatum and Medicago sativa on upstream area erosion inhibition effect is better than that of Agropyron cristatum and Artemisia gmelinii.Vegetation can significantly reduce the soil loss rate of gully head system by 31%～95%,and Medicago sativa has the best effect.Compared with the soil loss rate before the collapse,the soil loss rate increased rapidly and significantly by 9%to 2872%compared with that before the collapse.（4）The mechanism of hydraulic and gravity erosion of vegetation was revealed,and the stability model of root-soil complex was explored.The vegetation affected the upstream area hydraulic parameters,which affect the on-wall flow and jet flow conversion.Under the3.6～8.4 m³ h^-1 inflow discharge,the proportions of on-wall flow and jet flow are 7%～92%and93%～8%,respectively.With the increase of stem diameter,the proportion of on-wall flow flow increased,while the proportion of jet flow decreased.The upstream area runoff velocity,Reynolds number and runoff kinetic energy can significantly affect the allocation proportion of on-wall flow and jet flow.The upstream area runoff velocity is parsed upstream area optimization parameters of runoff sediment concentration,both the power function relationship.The upstream area runoff velocity,jet flow velocity at gully brink and gully bed velocity and kinetic energy of the upstream area runoff were the optimal upstream area hydraulic parameters,jet flow specific parameters,gully bed hydraulic parameters,and runoff energy parameters for analyzing the soil loss rate of the gully headcut system.Based on the torque balance analysis method,the stability model of the root-soil complex was established based on the change process of the shear strength characteristics and the friction characteristics of the root-soil interface,the change process of the erosion shape parameters of the gully head（the soil antiscourability of gully headwall）and the change of the root density of each diameter class with the soil depth of gully head.Vegetation had significant direct effects on soil properties and hydrodynamic parameters,and the morphology of gully head was significantly affected by soil parameters and hydrodynamic parameters.The morphology of cavity in gully head significantly affected the retreat distance of gully head（gravity erosion distance）.In addition,the frequency of gravity erosion significantly affects the sediment production rate and the retreat distance of gully head.