Characteristics Of Runoff,sediment And Non-point Source Pollution Control In Different Terrace Configurations In Beijing Yanqing Mountain Area

The soil layers in Beijing Yangqing mountainou area are relatively thin,and the soil structure is unstable,which can easily cause soil erosion.At the same time,the unreasonable use of fertilizers and pesticides in the mountainous areas of Beijing caused surface pollutants to flow into farmland and water bodies with surface runoff,which easily caused the problem of non-point source pollution.This article takes terraces as the research object,using regression analysis and comparative analysis to control the hydrodynamic characteristics,runoff and sediment control,and Total Nitrogen（TN）,Total Phosphorus（TP）,Ammonia nitrogen（NH₄⁺-N）,Chemical Oxygen Demand（COD）,Biological Oxygen Demand（BOD5）five types of non-point source pollutants control effect research,furthermore,in view of the soil erosion and non-point source pollution control in the mountainous areas of Beijing,the most reasonable allocation model of terraces is proposed.The results show:（1）The soil separation rate Dr increases with the increase of runoff q,decreases with the fluctuation of scouring time,and finally flattens.The relationship between slope resistance coefficient f and runoff R² is greater than 0.899,and the index of flow is between-0.679 ^-0.316;the relationship between resistance coefficient and soil separation rate is R² is greater than 0.931,the resistance coefficient is less than 0,and the resistance coefficient and runoff there was a negative correlation between discharge and soil separation rate.（2）With the increase of the number of terraces levels,the output flow,sediment production and nonpoint source pollution control rates have gradually increased.The average control rates of the first,second,and third-level terracess to the output flow were 35.84%,42.01%,45.57%;sand production is 75.77%,83.77%,85.83%;non-point source pollutants are 21.95%²6.36%,37.98%⁴3.03%,50%⁵8.33%.（3）The control effect of stone-made terraces on runoff and sediment production and non-point source pollution is better than that of soil-made terraces.The average control rates of stone-made terraces and ridge station field on production flow are 45.36%,39.64%,respectively;92.01%,71.75%;non-point source pollutants are 50% to 56.25%,22.65% to 28.18%.（4）The output flow rate,sediment yield,and control rate of non-point source pollution in the configuration of the terraces plus plant side ditch is higher than those of the single terraces configuration.The control rates are from low to high in order: soil-made terraces,stone-made terraces,soil-made terraces plus plant side ditch and stone-made terraces plus plant side ditch.The research results show that the soil and water loss and non-point source pollution control effects produced by different terraces configurations are different.Among them,the multi-level terraces mode is better than the single terrace mode,and the resistance of stone-made terraces is better than that of soilmade terraces mode.The terraces plus plant side ditch is better than the single terrace mode.The use of different terrace models based on local conditions can provide protection for soil erosion control and nonpoint source pollution control in Beijing’s mountainous areas.