Study On The Mechanism And Benefits Of Fine Mesh Net For Runoff And Sediment Reduction Of Spoil Deposits

As an important temporary protection measure,which is widely used in soil erosion prevention and control of production and construction projects,and it is of great significance to quantitatively reveal the runoff and sediment reduction benefits and mechanism of covering with the fine mesh.In order to analyze the characteristics of soil and water loss on the slope of spoil deposits,to investigate the effectiveness of regulating runoff and sediment by covering with the fine mesh net,establishing a reasonable and economical arrangement pattern of protective measures against the spoil deposit of production and construction projects.In our study,typical spoil deposits in micro-water erosion areas in the Fen-Wei Valley on Loess Plateau were considered.The main contradictions facing the temporary covering measures layout under different underlying surface conditions are identified,explored the differential influence of the fine mesh net on spoil deposits’runoff and sediment yield,combined the layout of the different fine mesh net and proposed the targeted temporary measures mode.Hence,indoor artificial simulated rainfall tests were used to explore the hydrodynamic mechanism of fine mesh net cover regulating the soil erosion process of spoil deposits,and evaluate the runoff and sediment reduction benefits of fine mesh net cover under different rainfall intensity and slope combinations.The main conclusions are as follows:（1）Covering with the fine mesh net can prolong the initial runoff production time on the slope of the spoil deposit,reduce the sediment yields of the spoil deposit,and effectively control the sediment yield peak and stable rate.Rainfall intensity is the key factor affecting the initial runoff production time of the spoil deposit.The restraining effect of the fine mesh net cover on the initial runoff production duration of the slope is weakened with the increase of rainfall intensity,and the restraining effect slope range of the initial runoff production duration can be decreased significantly.The minimum runoff production delay benefit was10.1%,and the maximum reached 54.08%.The sediment yield rate of the fine mesh net on spoil deposits was reduced by 30.99%～45.83%compared with the bare ground,and the sediment reduction benefits（N_P）were 49.95%(60 mm·h^-1),35.84%(90 mm·h^-1)and31.93%(120 mm·h^-1),respectively.Compared with the sediment reduction benefits,the runoff reduction of fine mesh net on spoil deposits was poor.The runoff reduction benefit（L_P）was the highest under a rainfall intensity of 60 mm·h^-1,while reached the lowest of 120 mm·h^-1,which were 16.55%and 1.92%respectively.（2）Two runoff pattern were observed on covering with and without fine mesh net spoil deposits,which were subcritical-laminar runoff and supercritical-laminar runoff under different rainfall intensity and slope.However,covering with fine mesh net promoted the transition from rapid to slow on the slope of the spoil deposits.Under the rainfall intensity of0～120 mm·h^-1,the Darcy—Weisbach runoff resistance coefficient of covering with fine mesh net on the spoil deposits is lower than that of the bare ground.Compared with CK treatment,the Darcy—Weisbach runoff resistance coefficient of covering with fine mesh net on the spoil deposits increased by 6.81～24.85(60 mm·h^-1),4.63～21.92(90 mm·h^-1)and 1.72～4.79(120mm·h^-1),respectively.Spoil deposit development process of slope surface erosion is the result of comprehensive function such as the Reynolds number Re,the Ru De Number Fr,Darcy—Weisbach runoff resistance coefficient f and runoff velocity v.In contrast,the runoff power w had a better characterizing effect on the sediment production of covering with and without fine mesh net on spoil deposits.（3）The benefit of runoff and sediment reduction of the fine mesh net were decreasing with the rainfall duration and then tend to be stable.The benefit of runoff and sediment reduction of the fine mesh net could be described well by the power function model under different rainfall intensification and slope combination,and the benefit attenuation coefficient b was increasing with rainfall intensity.The runoff reduction and sediment reduction benefits of the fine mesh net can be divided into high efficiency stage,decreasing stage and stable stage with the rainfall duration.However,the optimal runoff reduction benefits are not simultaneous with the optimal sediment reduction benefits.The optimal runoff reduction benefits are generally less than 30 min,while the optimal sediment reduction benefits are around 20 min.From the perspective of economics,the soil erosion intensity of the spoil deposit was a direct factor determining the selection type of the fine mesh net.From the perspective of economics,the soil erosion intensity of spoil deposit is a direct factor determining the selection of the fine mesh net type.With the increase of soil erosion intensity of the spoil deposits,the more type of fine mesh net can be laid,but the increase of sediment reduction benefit after laying was low,and the economic value of the temporary measures（covering with fine mesh net）decreased.In summary,this study emphasized the sediment regulatory benefits of the fine mesh net on the spoil deposits,revealed the driving role of runoff power in the erosion development process,and provided a reference scheme for solving the difficulties faced by the control of soil and water loss on spoil deposits.