Study On Flocculation Mechanism Of Cohesive Sediment In Flowing Water

Flocculation of fine cohesive sediment is an important subject of sediment dynamics.In recent years,with the reservoir sediment deposition and bed sediment endogenous pollution has become increasingly prominent,the flocculation of fine sediment in freshwater environment has attracted more and more attention.At present,most of the research in laboratory are concentrated in the field of static water settlement.Due to the complexity of flocculation in dynamic water,there is less work on this field.Based on a large number of reading and summarizing the literatures,this paper designs and implements the indoor experiment on flocculation in dynamic water test for cohesive fine sediment.The test results are analyzed by using single variable control method and statistical method.The internal mechanism of fine sediment flocculation under dynamic water conditions are discussed,and some conclusions and results are obtained.The main contents and results of this paper are as follows:(1)The theoretical basis,research methods and main research results of fine-grained sediment flocculation research are comprehensively combed.The existing researches are based on flocculation dynamics theory,colloidal stability theory and fractal theory,using indoor experiments,field observations,and model simulation methods.Great progress has been made in the factors affecting the flocculation of fine-grained sediments,the sedimentation characteristics of flocs,and the flocculation mechanism of fine-grained sediments.Combining with engineering practice and environmental problems,this paper clarifies the research direction of fine sediment flocculation.The research on flocculation microstructure,influencing factors of fine sediment flocculation still needs to be further strengthened,and the research on inorganic flocculation and biological flocculation needs to be expanded.(2)Based on the influencing factors of flow movement,sediment concentration and initial gradation of sediment,the flocculation experiment of fine sediment under hydrodynamic conditions was carried out.The effect of flow velocity on the change of average particle size and gradation composition of flocs was analyzed under the condition of lower flow shear velocity gradient(G<50s-1).The flocculation phenomenon of fine sediment under hydrodynamic conditions is confirmed by comparing with the characteristics of single sediment.It is clear that the flocculation process of fine sediment is a process of rapid development and then stability.Based on the experimental data analysis,the corresponding relationship between flow intensity and sediment flocculation is obtained.When the shear velocity gradient of water flow is less than 50s-1,with the increase of flow shear strength,the average particle size of flocs increases,and the flocculation degree of fine sediment increases.(3)The single variable method was used to analyze the effects of flow,sediment concentration and initial particle size distribution on the average particle size and floc size distribution in the process of sediment flocculation.For this test result:?The critical flocculation size of fine sediment is between 29 and 34 microns,and no flocculation occurs for sediment larger than this size.?The flocculation degree of fine sediment is between 1.2 and 11.5 under hydrostatic condition,and between 1.2 and 3.5 under hydrodynamic condition.Flow movement reduces the flocculation degree of fine sediment.?When the shear velocity gradient is low(G<50s-1),there is a positive correlation between the flocculation degree of sediment and the shear velocity gradient and sediment concentration.The power function relationship between the flocculation degree of sediment and the shear velocity gradient,and the logarithmic relationship between the flocculation degree of sediment and the sediment concentration.Under the same flow intensity and sediment content,the finer the initial gradation,the greater the flocculation degree of fine sediment,(4)Taking the flocculation degree of fine sediment and the percentage change of mass fraction of particle size as evaluation indexes,the correlation and variance of flow shear gradient,sediment concentration and sediment gradation with the degree of sediment flocculation were analyzed,and the main influencing factors of sediment flocculation were obtained.Under this experimental condition,the main factor of fine sediment flocculation is the initial gradation of sediment,followed by the sediment content,and the influence of flow movement is relatively small.