Study On The Change Of Soil Erosion In Small Watershed Based On Gully Consolidation And Highland Protection (Gully-head Landfill) Project

As a major soil and water conservation work,the Gully Consolidation and Highland protection Project（GCHP）has an important impact on slowing the expansion of gullies,protecting the highland,and is of great significance to local ecological construction and sustainable social and economic development.However,the current GCHP project,especially the gully head landfill project widely implemented in Dongzhi Loess Tableland,the largest highland in the world,still has some problems,such as obvious changes in spatio-temporal scale effects,easy to induce secondary disasters and other issues,which need to be studied in depth.Through the comparative analysis of soil erosion laws in the natural state of basin and under the conditions of the GCHP（gully head landfill）Project,the change of soil erosion law before and after the implementation of the project is revealed,which is of great significance to guide the scientific implementation of soil and water conservation measures in this area.In this paper,through numerical simulation and indoor physical model erosion test,the changes of soil erosion law before and after the implementation of GCHP（gully head landfill）project,as well as the change of gully development characteristics under different landfill measures and different flow conditions are studied.The main research contents and conclusions are as follows:（1）The implementation of the GCHP（gully head landfill）project can effectively prevent soil erosion within a certain period of time,and the soil erosion changes significantly before and after the gully head landfill.Based on the model database of Dongzhuanggou Watershed,the simulation results show that the correlation coefficient between the simulated value of the monthly average rainfall and the measured value for many years is 0.97,and the Nash efficiency coefficients of the runoff simulation value and the measured value in the correction period and the verification period are 0.76 and 0.90,respectively.The meteorological data generated by the Geo WEPP model climate generator and runoff simulation results have high accuracy and good model applicability.By simulating the changes of runoff sediment in the original state and the gully head landfill scenario,the soil erosion changes under different scenarios were compared and analyzed.It was found that under the gully head landfill scenario,the annual erosion modulus of the Dongzhuanggou watershed changed from 6.1×10³t·km²·a^-1 in the original state to 5.16×10³t·km²·a^-1.The erosion area of grade 7 and grade 8 decreased significantly from 46.1%to 29.8%,and the erosion of gully head decreased significantly.In addition,the simulation results under different rainfall conditions show that the runoff and erosion amount under the gully head landfill scenario show different degrees of reduction,and the implementation of the GCHP（gully head landfill）project can effectively prevent soil erosion..（2）Based on the physical model test results,under the test of different landfill measures,it is found that the total sediment yield increases with the increase of the flow,and with the occurrence of secondary disasters such as collapse.For the original channel,the total sediment yield increases with the increase of the flow rate,and when the flow rate increases to a certain extent（1.24m³/h,1.62m³/h）,the sediment yield rate no longer increases significantly.On the other hand,the total sediment yield of the gully model decreases obviously after the implementation of gully head landfill,but it increases significantly when cracks and collapses occur in the landfill project under a large flow rate.Taking the flow rate of 0.32m³/h as an example,the average sediment yield of each outlet collection bucket under the conditions of original channel,simple landfill,simple landfill+drainage are 2.82kg/barrel,2.06kg/barrel,0.61kg/barrel respectively.In addition,when the flow rate is 0.43m³/h,the total sediment yield within 10-20min under the three engineering scenarios is 49.9kg,36.09kg and 3.14kg,respectively,and the export sediment yield has a significant downward trend.（3）The generalized indoor physical model is used to simulate the process of hydraulic erosion,the results show that the gully head landfill measures can effectively slow down the progress speed of the gully head under the different flow rates,and the development characteristics of the gully after the landfill change significantly compared with that before the landfill.After the process of surface erosion-drop pits-pits-discontinuous rill-continuous rill-fracture collapse,the gully head has expanded significantly and the highland has been seriously damaged.After the gully head landfill,the highland runoff enters the gully in the form of falling water after the steps.The steps at the gully head can effectively reduce the energy of the runoff,and the headward erosion of the gully is significantly reduced,but both sides of slope toe are seriously affected by erosion.When the flow rate is large（1.24m³/h,1.62m³/h）,the bottom of the gully head and the slopes on both sides are suspended by erosion,and the whole landfill project will collapse on a large scale.After the addition of hidden drainage pipe,the runoff into the gully was reduced from the source and energy is reduced by an energy dissipator at the bottom of gully.Under different flow conditions,the development and expansion of the gully are not obvious,and the highland is better protected.Overall,the test results show that the implementation of gully head landfill measures can effectively reduce the soil erosion in a short period of time,the speed of headward erosion is significantly slowed down,and the highland is effectively protected.However,if the conditions change,such as increasing the spatio-temporal scale,the sediment yield changes and gully erosion characteristics under different slope and vegetation cover need to be further studied.In addition,the results show that when the secondary geological disaster occurs under the large flow,the overall protective effect of the project is greatly reduced,the highland and gully are seriously damaged,and the sediment yield is also significantly increased.It is recommended to do a good job in the inspection and maintenance of drainage measures in the later stage of the project,establish the ecological slope protection after landfill,and avoid the safety hazards caused by the erosion of bare soil without drainage measures.Avoid the potential safety hazard caused by bare soil erosion without drainage measures.