Experimental Study On Flocculation Critical Condition Of Fine Sediment In The Three Gorges Reservoir

The problem of sediment deposition in the Three Gorges Reservoir has been widely concerned.Existing domestic studies suggest that after the impoundment of the Three Gorges Reservoir,fine grain sediments will flocculate under the combined effects of slowing of the flow rate and thinning of incoming sand in the the wide valley section of the reservoir area.The existence of flocculation will cause great changes in the laws of sediment movement,especially in the Three Gorges Reservoir,the flocculation of sediment will accelerate the speed of sedimentation,thereby accelerating the deposition rate of sediment in the reservoir area.Therefore,the study of the critical conditions for flocculation of fine sediment in the Three Gorges reservoir area is a fundamental issue for river dynamics,and it also has important guiding significance for the rational operation and safe operation of reservoirs.In this paper,through the field test,indoor settlement test and theoretical analysis of Huanghuacheng and Fengjie reach in the reservoir area,the critical conditions for flocculation of fine particles in the reservoir area are studied and analyzed.The main works are as follows:(1)Based on the synchronous measurement of ADV,the instantaneous flow velocity and instantaneous sediment concentration are obtained.Wavelet analysis method and nuclear density estimation method are used to denoise and deburr the acoustic intensity signal,meanwhile,calibrating the relationship between backscatter acoustic intensity and instantaneous sediment concentration to measure the instantaneous sediment concentration of the scene;the sediment velocity of the sediment in the field is measured indirectly according to the sediment diffusion theory;The sediment particle size in the field is backcalculated according to the stokes' settling formula.Performing grading analysis of the sediments AMP of all the measuring points by laser particle size analyzer after discretization,and then,the ratio of the particle size in the field to the sediment median diameter after discretization is defined as the degree of flocculation,which reflected the degree of flocculation in the reservoir area.It is found that the most of the fine-grained sediments in the reservoir area occured flocculation,and most of them are medium-moderate flocculation,also,flocculation mainly occurred in the middle and lower layers of the water body;analyzing the relationship between the sediment concentration,particle size,flow velocity and the degree of flocculation.The analysis results show that the critical sediment concentration for fine sediment flocculation in the reservoir area was approximately 0.65kg/m3,the critical particle size was approximately 0.018 mm,and the critical flow velocity was approximately 0.75m/s.(2)The sediment of Huanghuacheng river section in reservoir area is selected for the laboratory sediment settling experiments,and the influencing factors of flocculation are analyzed: Based on PTV,a method for measuring particle size and settling velocity of fine sediment is explored,and the accuracy of this measurement method is verified.It is proposed a PTV technique suitable for the measurement of the fine particles to search the flocculation particle size and settling velocity of fine sediment;It is proved that the sediment is flocculated in the laboratory sediment settling test,and the variation of sediment velocity with the turbulence intensity is analyzed,and the critical turbulence intensity is found to be 5(mm/s)2.When the turbulence intensity is less than 5(mm/s)2,the turbulence intensity is promoted by the sediment flocculation.When the turbulence intensity exceeds 5(mm/s)2,the structure of floc flocculation is destroyed after the turbulence intensity is increased and the flocculation of sediment is blocked.The variation of sediment velocity with sediment content under different turbulence intensity ranges is analyzed,and the critical sediment concentration of sediment flocculation is about 0.5kg/m3,when less than the sediment content,the increase of sediment content promotes flocculation of sediment,and the increase of sediment content hinders the development of flocculation.(3)Based on the theory of colloidal stability,the expression formula of van der Waals force and double-layer force is derived.The corresponding Fan Dehua gravity and double layer repulsion force are calculated under different particle sizes.It is found that during the close proximity of fine particles,the Fan Dehua gravitation is far greater than the repulsion force of the double layer,which proves the flocculation of fine sediment in the reservoir area.At the same time,it is found that the flocculation effect of the reservoir area is obvious when the particle size is smaller than 0.018 mm.After particle size is greater than 0.018 mm,the flocculation effect of sediment in reservoir area is not obvious.