Preliminary Study On Flocculation And Settlement Characteristics Of Hydrodynamic Flow In The Three Gorges Reservoir

This paper,taking the problem of the silt of the Three Gorges Reservoir as the breakthrough point,discusses the research on the flocculation settlement of the very fine particles and sediment in the flow turbulence,which is of great significance to the regulation of the silt of the reservoir.Due to the limitation of the existing measuring instruments and technical means and the looseness of floc itself,there are few studies on flocculation and sedimentation of reservoirs at present.From two directions of field observation and laboratory test,the flocculation and settling of fine-grained sediment is studied.A set of unique transverse vibration grid system is designed,which is divided into dynamic water and hydrostatic flocculation settlement with the characteristics of bend and wide valley deposition.The floc size and settling velocity are obtained by the flocculation measurement system,and the different workers are analyzed.According to the rule of flocculation and sedimentation,the physical and chemical properties and sedimentary profile of the deposit were observed and analyzed,and the following conclusions were obtained:(1)The turbulent flow characteristics of the grid are related to the size of the grid.Through the calculation of the mean square root velocity of the turbulent flow in the water,the equation of the turbulent mean square root of the flow is obtained.Under the condition of less sediment concentration,the low flow turbulence promotes the flocculation of sediment.When the turbulence intensity is 3.027cm/s,it is likely to be closer to the best flocculation turbulence intensity under the experimental device.When the flow turbulence is reduced,the turbulence intensity of the flow is not enough to maintain the energy required for the larger flocculation group to suspend,and the settlement of the larger floc increases.When flocculation is affected by flocculation,the settling velocity of cohesive fine particles has been changed.When flocculation reaches dynamic balance,the flocculation settling velocity has a certain relationship with the turbulence intensity and sediment concentration,and the empirical formula of sediment settling velocity is derived.(2)The surface sediment is partly derived from suspended sediment,and the particle size is relatively fine.The median particle size is about 0.01 mm,which satisfies the threshold condition of sediment flocculation.Mineral composition contains clay minerals such as chlorite and illite,which determine particle size,particle shape and the ability to adsorb heavy metals.(3)The fine sediment in the suspension is flocculated and deposited on the surface of the river bed,which makes the clay content of the surface sediment higher than the suspended sediment,and is limited to the supply of fine sediment and the hydrodynamic environment.This part of the suspended sediment does not belong to the wash load,but also exchange with the bed sand to participate in the process of making bed.