Study On Effect Of Slope Preparation And Cover Measures On Erosion And Its Mechanism

Slope treament measure is an important move for soil and water conservation in Loess Plateau which is significant for ecological environment protection and agricultural sustainable development in area.Slope preparation and cover measures,as the main measures in the process of slope treatments,have been widely used in the allocation of regional soil and water conservation measures.However,the spatial characteristics of each measure on the slope surface are different which resulting in the difference of runoff and sediment control mechanism.In order to systematically analyze different spatial distribution slope measures,fish scale pit,level terrace and grass were taken as typical measures and study object which represented point,banded and planar measures.Artificial simulated rainfall experiment was applied to research on infiltration characteristic,runoff and sediment characteristic,and benefit of soil and runoff reduction.Runoff scouring experiment was applied to explore the characteristic of slope erosion,and its relationships with hydraulic characteristic which clarified the slope erosion mechanism for different measures.The main research of the study included:(1)The results of infiltration characteristic in different measures indicated that the increase of grass coverage can enhance the soil infiltration rate.The relationships between mean infiltration rate and slope in grass slope under the same coverage can be predicated and fitted by logarithmic function.The soil infiltration rate in level terrace generally showed that small level terrace was higher than big level terrace.The comparison between grass measures and level terrace measures indicated that the infiltration rate in 80% coverage grass was largest and the small level terrace was second.The characteristic of infiltration in fish scale pit slope showed obvious difference before and after the threshold of measure control.Water storage was main process which included depression filling and infiltration before fish scale pit damaged.The depression would lose storage ability which infiltration became main process after fish scale pit damaged.Kostiakov model,Horton model and Philip model were used to simulate the infiltration process in grass and level terrace process,and root mean square error(RMSE)was used to test the models.The results indicated that Kostiakov model was the most suitable for simulate the grass slope and level terrace slope infiltration process which can better predict the change of infiltration rate with time.The exponential function empirical model can be used to simulate the variation regularity of slope water storage and infiltration rate with time.(2)Grass planting,level terrace and fish scale pit allocation can change the underlying surface properties and affects the runoff process and sediment change characteristics which possess water storage and soil conservation function.Initial runoff time was put off by three measures,and initial runoff time was longest for 80% grass coverage slope and the initial runoff time of big level terrace slope was shortest.Compared with the smooth slope,the grass covered slope has better benefits of sediment and runoff reduction,and the difference is significant in different coverage.With the increase of rainfall intensity,the sediment reduction benefit and runoff reduction benefit of 80% coverage slope were 1.25~1.60 times and 1.60~2.03 of that of 40% coverage slope.Mean benefits of runoff reduction and sediment reduction level terrace measures were 21.72%~40.78% and 60.54%~71.38%.The benefits of runoff and sediment were higher before fish scale pit damaged which the reason was related to the runoff and sediment storage by fish scale pit.Runoff and sediment amount obviously increased while fish scale pit was damaged under bigger rainfall intensity and longer time.Compared with the sediment amount of smooth slope,the benefits showed negative value in certain time.Through analyzing the different measures,the benefits of runoff and sediment reduction 80% grass coverage slope and fish scale pit slope were better while measures could control runoff and sediment amount.According to the different soil erosion amount between measure and smooth slope,the value of cover-management factor with grass cover and soil and water conservation factor which represented by level terrace and fish scale pit could be calculated.The values of C factor under 40% and 80% grass coverage were 0.332~0.682 and 0.150~0.311.The values of P factor based on big level terrace and small level terrace were 0.28~0.47 and 0.24~0.41.The mean values of P factor based on fish scale pit were 0.17 and 0.14(3)Mean runoff rate and mean sediment rate could be decreased by grass measure.The average runoff intensity and average sediment rate were positively correlated with slope and rainfall intensity,respectively,and their changes with coverage showed that the average runoff intensity and average sediment rate of 40% coverage slope were significantly higher than that of 80% coverage slope.The runoff intensity and sediment rate increased with rainfall intensity and slope gradient increase in level terrace slope.The runoff intensity and sediment rate showed significant difference before and after control threshold.The sediment concentration of grass,level terrace and fish scale pit measures had positive correlation with slope and rainfall intensity and its sediment concentration increased obviously after measure damaged.The main reason was that the edge of fish scale pit was damaged which leaded to runoff and sediment was poured out quickly.By linear fitting the relationships between runoff rate and sediment rate and the rake ratio of linear model reflected the change speed of sediment with runoff.The values of linear model were influenced by rainfall intensities and slope.The rake ratio of linear model reflecting runoff and sediment relationships in 40% and 80% grass coverage slope were 0.009~0.023 and 0.006~0.016 under 70mm/h and 100mm/h rainfall intensities.The rake ratio of linear model reflecting runoff and sediment relationships in level terrace and small level terrace were 0.010~0.044 and 0.027~0.050 while rainfall intensities were 70mm/h and 100mm/h.The rake ratio of linear model reflecting runoff and sediment relationships in fish scale pit slope showed large difference before and after measure damaged.The slope sediment reduced mainly due to the runoff reduction or decrease of the sediment concentration which could be analyzed quantitatively by calculating indices of sediment reduction caused by decreasing runoff(SRR)and sediment reduction caused by flow-sediment relationship changes(SRS)for clarifying the internal mechanism of sediment regulation.Compared with 40% grass coverage slope,the percentage of SRR for 80% coverage slope was bigger and the SRS was lower.The contribution rate for sediment reduction of level terrace slope showed that the percentage of SRR increased and the percentage of SRS decreased gradually.The effect of sediment reduction was mainly depended on SRR.Based on the analysis of soil anti-scourability characteristics under different grass coverage,the soil anti-scourability index of 80% coverage slope is significantly higher than that of 40% coverage slope,and similar rules are also showed among different soil layers.Soil anti-scourability indices showed a downward trend with the increase of slope.The increase values of anti-scourability index were 0.13~0.83 in grass cover slope.The mean soil detachment rates of level terrace slope were lower than smooth slope which were related to spatial distribution of level terrace.The mean soil detachment rates of fish scale pit slope were higher than smooth slope which were mainly caused by soil detachment rate increasing obviously while fish scale pit measure damaged.(5)When the discharge is constant,velocity,flow shear stress,stream power,unit stream power and unit energy of water-carrying section are positively correlated with slope.The results indicated that soil detachment gradually decreased with the increase of above values of indices by analyzing the soil anti-scourability characteristic response to hydrodynamic parameters.Linera function was used to predict and simulate the relationships between soil anti-scourability indices and hydrodynamic indicators by Nash-Sutcliffe efficiency(NSE)coefficient test,and the fitting accuracy results showed unit energy of water-carrying section>velocity>stream power>unit stream power>flow shear stress.Linear function were used to fit the relationships between soil detachment rates and velocity,flow shear stress,stream power,unit stream power and unit energy of water-carrying section.NSE coefficients were applied to test the model accuracy and results showed that the accuracy of models were better between soil detachment rate and flow shear stress,stream power and unit stream power for level terrace slope.The accuracy of models was better between soil detachment rate and velocity and unit energy of water-carrying section for fish scale pit slope.