Hydraulic Characteristics Of Soil Detachment Capacity In Terrace Banks With Different Plants In Purple Hilly Area

It is a typically measure uses terrace banks to protect slopes because of the lack of stones materials and costly transportation expenses in middle part hilly area of Sichuan province.The technology of use hexagonal grid concerte terrace banks fully utilizes the advantages of engineering measures and plant measures to reduce the status of soil erosion effectively and improve the agroforestry ecosystem.As the most active factor in the prevention and control of soil erosion,plants improve the soil structure,nutrient content and distribution in the hexagonal grid concerte terrace banks.In order to clarify the characteristics of the dominant species under the terrace banks with different plants and the hydrodynamic parameters of soil detachment in the purple purple hilly area.we collected four kinds of the terrace banks planted with Ophiopogon japonicus,Allium tuberosum,Vegetable soybean and bare land(control)in situ respectively.The soil particle characteristics and organic matter content were determined,and the undisturbed soil was taken in the field with 500 cm~3 ring cutters.Flume scouring experiments were carried out under the combinations of 3 slopes(15°,20°,25°)and 5 flow rates(39.86,59.98,79.67,100.45,121.16 L/min)with 3 replicates(i.e.,a total of 135 scouring experiments).Investigate the hydrodynamic characteristics of the hitlslope sheet flow and the soil detachment capacity under different plants,and scientifically eliminate the data(the data of the Vegetable soybean soil detachment capacity is invalid).The mathematical relationships between soil detachment capacity and hydrodynamic parameters were analysised.Such as soil rill erodibility.The main conclusions are as follows:(1)In the same soil layer,the soils covered by different vegetations have significant differences in the same particle size,and the vegetation can effectively improve the soil texture.The mechanical composition and the micro-aggregates of the soil covered under Allium tuberosum,Vegetable soybean are larger,and the effect of improving soil structure are better.The fractal dimension is significantly negatively correlated with the content of fine sand and significantly positively correlated with fine silt,crude clay and fine clay.The fractal dimension reflects the degree of particles differently.The fractal dimension of mechanical composition is the most reflective of fine sand,and the fractal dimension of soil micro-aggregate is the most reflective of fine clay.The soil sand content decreased and the clay content increased with the increase of soil layer,and the soil particles were dominated by fine sand and crude silt.There were differences in soil organic matter content under the different plants,and the soil organic matter content was higher in surface layer.Positive correlation between organic matter content with fractal dimension of soil mechanical composition was significant(P<0.01).Soil organic matter played a role in refining soil particles and changing soil properties.the soil sand content decreased,(2)Reynolds number increased with an increasing flow rate.This relationship was strong and could be robustly predicted by power function of flow rate.Flow pattern Froude number and slope gradient show a very significant power function positively.It is fluctuant between water flow resistance and slope gradient.The water flow resistance is linearly negatively correlated with the Froude number.There was a critical slope 20°in relation to the Reynolds number.The water flow resistance is less than 20°and the Reynolds number is negatively correlated with a positive correlation of more than 20°.The average flow depth,Manning coefficient and water depth ratio both exhibit a power function relationship with the flow rate,and the slope gradient effect is relatively weak.The shear stress,stream power,the unit energy were correlated with slope gradient and flow rate positively.The flow rate effect was slightly higher than the slope.The unit stream power had a power function with slope gradient positively,and the slope gradient effect is much larger than the flow rate.(3)The average value of soil detachment capacity under various plants was as follows:D_c Ophiopogon japonicus(2.42 kg/(s·m~2))>D_c bare land(1.34 kg/(s·m~2))>D_c Allium tuberosum(1.09 kg/(s·m~2)).The soil detachment capacity was positively correlated with the slope gradient and flow rate with a power function.The soil detachment capacity of Ophiopogon japonicus was significantly higher than that of Allium tuberosum and bare land(P<0.05).The difference of soil detachment capacity between bare soil and Allium tuberosum was not significant.The average soil detachment capacity of Ophiopogon japonicus was 2.04 and 1.67 times than Allium tuberosum and bare land respectively.The average soil detachment capacity of bare land was 1.23 times than Allium tuberosum.(4)The soil detachment capacity and mean flow velocity,Reynolds number,shear stress,stream power,unit stream power and unit energy were related to the power function positively under different plants,and the ratio of Manning coefficient to flow depth showed a very significant negative correlation of power function,in which the shear stress(R~2=0.75)is higher than stream power(R~2=0.73)compared with other hydrodynamic parameters and soil detachment capacity power function relationship R~2.The shear stress and stream power were the better hydrodynamic parameters describing the soil detachment capacity.(5)Hydrodynamic parameters such that mean flow velocity,Reynolds number,ratio of Manning coefficient to flow depth,shear stress,stream power,unit stream power and unit energy soil erodibility coefficient were significantly different from zero(P<0.01),can better describe soil erodibility.The hydrodynamic critical value of ratio of Manning coefficient to flow depth and Darcy-Weisbach drag coefficient were extremely significant with zero(P<0.01).The critical values was largest in bare land,which smallest in Allium tuberosum.The soil under vegetation cover is difficult to occur soil erosion.Allium tuberosum has a better protective effect on the soil than Ophiopogon japonicas in terrace banks.(6)The soil detachment capacity increased significantly with the increase of sand content,and decreased with the increase of silt and clay content.The soil fractal dimension was correlated with sand content negatively,correlated with silt and clay content positively,soil detachment capacity and fractal dimensions were correlated negatively.The soil detachment capacity has a high correlation coefficient with the content of silt and soil organic matter.Stepwise regression analysis determined the linear equation of soil detachment capacity with percentage of silt content in soil mechanical composition and organic matter content:Dc=-0.148SOM-0.071Silt+6.160(R~2=0.854,P=0.003).