Sediment Concentration And Soil Water-stable Aggregate Bulk Density Measured Based On Sedimentation Method

The sedimentation of soil particles directly affects the distribution of sediment in the water body,and then has a great influence on the distribution of pressure at each point in the water body and the transport of erosion sediment.The purpose of this study is to find a new method for rapid and accurate determination of sediment concentration in field.In the process of erosion,part of soil is eroded in the form of aggregates,and the transport of aggregates is quite different from that of traditional soil particles(sand and gravel).Aggregates are not dense soil particles,and their bulk density directly affects the transport of aggregates.In this paper,the settlement velocity of water-stable aggregates with different grain sizes was measured by experiments,and the bulk density of soil aggregates was calculated,which could serve for the analysis of hydraulic erosion mechanism in soil erosion.Test leads to the principle of differential pressure change,based on subsidence from four kinds of soil,(cinnamon soil,sandy soil,coastal saline soil and paddy soil)precast 11 sediment concentration gradient of water containing sand respectively,using the digital differential pressure gauge to determine water samples containing sand the difference in value between the pressure and atmospheric pressure(differential pressure),establishing the function relation between the sediment concentration and differential pressure.Five typical soils(red soil,yellow cultivated loessial soil,black soil,cinnamon soil and purplish soil soil)were tested for dry sieving,wet sieving,scouring and free settlement of water-stable aggregates.The distribution characteristics,mechanical stability,water stability,settlement velocity and bulk density range of water-stable aggregates were obtained.The sediment concentration was positively correlated with the pressure difference at the level of 0.01.For three kinds of sand-bearing water samples(cinnamon soil water samples,coastal saline soil water samples and paddy soil water samples),the method combining pressure difference with theoretical formula to calculate the sediment concentration is feasible,and the maximum absolute relative error is less than 14%.However,the absolute value of the maximum relative error is as high as 33%.Therefore,in order to reduce the error,the test established a correction equation by measuring the pressure difference between pure water and water sample with sand content of 500 kg/m3,and then calculated the sand content by combining the measured pressure difference value.It was found that the maximum absolute relative error was less than 8%.In order to further confirm the general applicability of sediment concentration determination based on the settlement method combined with the correction equation,the black soil correction equation was established by measuring the differential pressure of 500 kg/m3black soil water sample containing sand.It is found that the maximum relative error is less than 9%in the determination of black soil water samples with different sediment content.It shows that the sedimentation method can accurately measure the sediment content of water samples,and provides a new idea and method for the convenient determination of sediment content in the field in the field of soil and water conservation monitoring.Compared with dry screening,the mean weight diameter(MWD)and geometric mean diameter(GMD)of aggregates decreased after wet screening,while the content of micro-aggregates increased significantly,while the content of large aggregates was opposite.Correlation analysis showed that soil erodibility factor(K)was positively proportional to soil organic matter content.The aggregate failure rate(PAD)decreases with the increase of sand content and increases with the increase of water stable large aggregate content.There was a significant positive correlation between soil anti-scour coefficient and mean weight diameter(MWD)of mechanically stable aggregates at 0.05 level,but no correlation between soil anti-scour coefficient and mean weight diameter(MWD)of water-stable aggregates at 0.05 level.The sedimentation velocity of five kinds of soil water-stable aggregates is closely related to their diameters,especially the velocity increase of 0-2 mm water-stable aggregates is more obvious.The sedimentation rates of water-stable aggregates with the same diameter of different soil types were also different.When D<2 mm,the sedimentation rates of water-stable aggregates with the same diameter of red soil and purplish soil were very close.When D>2 mm,the water stable aggregates of purplish soil settle fastest.The free settlement test of water-stable aggregate shows that the settlement velocity increases and the bulk density decreases with the increase of the diameter of water-stable aggregate.According to correlation analysis,the weighted average velocity(V 12,12)of water-stable aggregates is correlated with the soil erodibility factor(K),which is inversely proportional,and has no correlation with soil anti-scour ability.The mechanical stability of soil aggregates is beneficial to enhance soil erosion resistance,and the increase of sand content is beneficial to reduce aggregate failure rate.As a whole,the average bulk density of the five water-stable aggregates was in the order of red soil>purplish soil>black soil>cinnamon soil>yellow cultivated loessial soil.The bulk density of the five water-stable aggregates(1.003×10~3-1.612×10~3 kg/m~3)was much lower than the average soil particle density(2.65×10~3kg/m~3).The bulk density of different soil water-stable aggregates is different.Even for the same soil water-stable aggregates,the bulk density of aggregates with different diameters is also different due to different loose pores.