| The problem of sediment deposition in the Three Gorges reservoir area has been widely concerned,and it is generally believed that the main reason is sedimentation.In the past,researchers focused more on flocculation of sediments in the estuary area.Few studies have focused on the flocculation of sediments in such freshwater environments in the Three Gorges reservoir area.It is of great significance to study the flocculation settlement of cohesive sediments in turbulent water bodies in the Three Gorges Reservoir Area,to understand the sedimentation laws of the Three Gorges Reservoir sediments,to rationally adjust reservoir operation schedules,and to rectify waterways in the reservoir area.This paper mainly studies the flocculation characteristics of sediments in different reaches of the Three Gorges reservoir area under different turbulent shear rates,analyzes the variation of the floc size distribution with the shear rate,and defines the maximum floe size and critical shear rate in the turbulence process.The main work carried out and results are as follows:(1)A flocculation settling device was developed,and the motor was used to drive the multi-layer grid to vibrate up and down to produce an approximate isotropic uniform turbulent flow.Using ADV to measure the three-dimensional pulsating flow velocity under different grid operating parameters S2M1/2f H-3/2 in the flocculation settling system,to establish the quantification of the turbulent shear rate G and S2M1/2f H-3/2 of the water in the flocculation settling system.The relationship is that when the amplitude is 3 cm,G=10.854 f 1.5.(2)The floe image analysis system was developed and based on the image analysis method,the equivalent particle size of each floe was calculated,and the number of flocs in different particle size segments was counted to obtain the floc size distribution(ie,The percentage of floes per particle size).(3)Samples of river core sediments from the Changshou,Zhongxian,and Fengjie rivers in the Three Gorges Reservoir area were collected to make experimental sand samples.The concentration was 0.3 g/L?0.6 g/L?1.0 g/L and the shear rate was 3.84 s-1?10.85 s-1?19.94 s-1?30.07 s-1 for flocculation sedimentation experiments,the distribution of floc size after flocculation of cohesive sediments in the Three Gorges reservoir area was analyzed and the results show that:(1)The effect of flocculation at each section of the Three Gorges reservoir area is obvious,and the maximum size of the floe is comparable to the maximum size of the floc in the Yangtze Estuary;(2)When the concentration is constant,as the shear rate increases,the content of large-sized flocs(greater than 0.096 mm in diameter)gradually increases,it can reach 3.1%~11.3%,0.9%~10.2%,and 1.1%~11.4% when G is 19.94 s-1;(3)The floe maximal particle size Df,95 in the three stages of Changshou,Zhongxian and Fengjie shows a trend of increasing first and then decreasing with increasing turbulent shear rate,at a concentration of 1.0 g/L and the maximum shear rate was 19.94 s-1,Df,95 is 0.126 mm,0.126 mm and 0.114 mm.(4)The critical shear rate of sediment flocculation in the Changshou,Zhongxian and Fengjie reach of the Three Gorges reservoir area is both 19.94 s-1,and the floc size distribution is greatly affected by the concentration.Under the critical shear rate,when the concentration of sediment is small(0.3 g/L,0.6 g/L),the maximum floc size and the percentage of flocs with large floc size are the highest after the sediment flocculation balance in the Zhongxian reach.When the concentration of sediment is large(1.0 g/L),the maximum floc size and the percentage of flocs with large flocs are the largest after the flocculation balance of sediment particles in the Changshou reach. |
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