| The loess tableland is a typical landform type in the Loess Plateau,with a broad and flat surface,has extremely valuable land resources and agricultural production conditions,and is an important food and fruit production base on the Loess Plateau.However,long-term severe headcut erosion has reduced the area,which threatening the safety of farmland,villages,towns,factories and mines,and causing serious harm to the local ecological construction and sustainable economic and social development.In the past,the study on large-scale headcut erosion has been conducted through field surveys and remote sensing interpretation methods,while simulation experiments on the influencing factors to soil erosion mechanism was rare.This study was conducted on Qingyang city,Xifeng district,Gansu province.This study examined the headcut erosion processes using slope gradients ranging from 1°to 7°and inflow discharges ranging from 3.6 m~3/h to 7.2 m~3/h.The headcut height was set as 0.9 m and 1.5 m.The aim of this study was to clarify the influence of slope gradient on loess tableland(gully bed),the gully height and flow discharge on the runoff and sediment processes,hydraulic characteristics and morphological evolution characteristics of headcut erosion on the loess plateau.This paper conducted field experiments on the headcut erosion processes and morphological evolution characteristics of the gully head.The aim of this study was to provide effective management of land resources and soil erosion in loess plateau areas such as Dong Zhiyuan.The main results are as follows:(1)Illustrated the hydraulic characteristics of headcut erosion.When the gully height was0.9 m,the Reynolds number,shear stress,stream power,Darcy-Weisbach friction factor,and unit stream power on the loess tableland and gully bed increased with experiment time by power functions,while the Froude number decreased with the experiment time.As the concentrated flow dropped from the loess tableland to the gully bed,the flow velocity,Reynolds number,shear stress,stream power and unit stream power reduced by9.45%～22.46%,3.68%～58.86%,25.60%～77.55%,86.25%～96.96%and 9.45%～25.75%,respectively,under 3.6 m~3/h,4.8 m~3/h,6.0 m~3/h and 7.2 m~3/h,while the Darcy-Weisbach friction factor increased 17.14～39.53 times.This showed that compared with the loess plateau,the runoff kinetic energy and runoff turbulence of the are reduced,and the erosion energy of concentrated flow was weaker.The increase of the flow discharge significantly increased the hydraulic parameters such as the flow velocity,the kinetic energy,the velocity at bottom,the potential energy at bottom,the jet velocity,and the total energy consumption.The potential energy at bottom and energy consumption of the system increased with the increasing slope gradient.The pore water pressure gradually decreased as the experiment progressed.There was a liner or logarithmic function relationship between the pore water pressure and experiment time.The increase of pore water pressure was accompanied by the occurrence of collapse,that is,the increase of pore water pressure was one of the key factors affecting the occurrence of collapse.The existence of a 0.9 m gully head led to a decrease in flow energy and a decrease in sediment carrying capacity,while the existence of a 1.5 m gully head led to an increase in flow energy and sediment carrying capacity.In addition,the difference in hydraulic parameters also increased with an increasing headcut height,indicating that the increasing headcut height increased the runoff turbulence.(2)Clarified the headcut erosion.The main sources of sediment were the loess tableland and gully head,and the gully head accounted for 78%～93%of the total sediment yield of the system.The runoff and sediment transport capacity of the gully bed was weaker than the loess tableland and gully head;therefore,the sediment was deposited on gully bed.Only when the flow discharge and slope gradient were larger to a certain value,the gully bed will be eroded.The total sediment yield reached its maximum value within 0～30 min,and stabilized within120～180 min as the experiment conducted.The total sediment yield increased with the increasing slope gradient and flow discharge.The frequency of collapse increased from 3.81%at 0～30 min to 26.69%at 150～180 min,and the collapse increased about 22.75%～324.59%soil erosion.Compared with the collapse time,the emergence time of the sudden change point of sediment production was delayed.The sediment yield increased by 1.84～14.78 times as the gully head height increased from 0.9 m to 1.5 m,indicating that the gully erosion can be prevented by reducing the headcut height.The texture of eroded sediment was rougher than that of the original soil.As the experiment progressed,the fractal dimension of sediment particles was smaller than that of the original soil.The minimum fractal dimension of sediment particles appeared in 30～90 min.The maximum value appeared in 120～180 min,which was close to the original soil.It showed that the particle size distribution of the eroded soil is ultimately similar to that of the original soil.The critical value of for the concentration of erosion and sediment particles was 0.0326 mm,which means that when the particle size was greater than 0.0326 mm,the sediment particles were enriched,and when the particle size was lesser than 0.0326 mm,the sediment particles were deposited.(3)Ascertained the headcut erosion morphology evolution characteristics.The gully width,gully depth and gully length on the loess tableland increased exponentially,logarithmically and logarithmically,respectively,with experiment time.The spilt degree of the erosion gully on the loess tableland showed a significant exponential increase trend with the experiment time.The headcut retreat length and the drop water height increased logarithmically with the experiment time.There was no obvious correlation between the initial headcut retreat rate and the slope and discharge flow.The increasing in the headcut height greatly accelerated the retreat length of the gully head.The initial heacut retreat length increased by 1.83～3.13 times as the gully head height increased from 0.9 m to 1.5 m.The drop water height decreased with the increasing of slope gradient and flow discharge in general.As slope gradient increased 2°,the drop water height decreased 1～34 cm.As flow discharge increased 1.2 m~3/h,the drop water height decreased 2～28 cm.The headcut retreat length no longer decreased after the headcut was connected with the loess tableland gully or eroded to the critical value,forming a secondary headcut or even a tertiary headcut.The initial headcut height was 50～89 cm,64～80 cm,37～99 cm,and 44～80 cm,respectively,under 1°,3°,5°,and7°.When the slopes gradient was 3°and 5°,the initial headcut height decreased with the increasing flow discharge.(4)Revealed the dynamic characteristics of headcut erosion and sediment yield and morphological evolution.The soil erosion rate on the loess tableland were significantly related to gully depth,gully width and width-depth ratio.The gully depth and width-depth ratio could be used to predict the soil erosion characteristics on the loess tableland,and the main source of erosion on the loess tableland was undercutting of erosion gully.The jet flow velocity was the optimal indicators for evaluating the headcut erosion.The morphological parameters of erosion gullies on loess tableland were significantly related to shear stress,stream power,Darcy-Weisbanch friction factor and unit stream power.The drop water height linearly decreased with the increasing jet flow velocity,the kinetic energy of jet flow,the flow velocity at bottom and the maximum shear stress.The sediment yield of system increased with the increasing energy consumption,and there was a significant exponential function between the two parameters.The soil erosion rate on the gully head decreased with the increasing drop water height,and increased with the increasing of the initial headcut retreat length. |
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