| The Three Gorges Project is the largest water conservancy project in the world currently, and the problem of sedimentation after The Three Gorges reservoir operation has been a focus of attention. One of the main factors affecting sedimentation is flocculation which has a huge impact on sediment deposition and distributions. Unlike seawater, the Three Gorges Reservoir has a freshwater environment that has been lack of depth observation and research to make sure flocculation phenomenon. In order to study impacts of water environment on sediment flocculation in The Three Gorges Reservoir, and summarize the reservoir sediment flocculation laws, this paper analyzes flocculation conditions and characteristics through data analysis, research laboratory, and theoretical analysis. The main contents and results are:(1) Sediment flocculation and settling characteristics are analyzed based on field measured data in the Three Gorges Reservoir: â‘ Reservoir field measurements shows that, in the 819 sampling points, there are 99.4% of the points having varying extent of flocculation, and points where floc diameters are 2 to 8 times the single particle size take up 83.6%, simultaneously, flocculation extent has an attenuation along the flow direction, and the extent of Miaohe reach are much lesser than that of Fengjie reach, that shows forming floc has been sinking into the bed along the way; â‘¡ Critical particle size of flocculation in the Three Gorges Reservoir is about 0.02mm, and the particle sizes smaller than this are likely to form flocculation; â‘¢ In the Three Gorges Reservoir, critical velocity of flocculation is approximately 0.7m/s. When the flow velocity is less than 0.7m/s, sediment can form relatively stable flocculation in the reservoir.(2) Effects of flocculation to sedimentation velocity, volume and specific quantitative relationship in the Three Gorges Reservoir are proposed based on laboratory test results: â‘ Flocculation extent is proportional to sediment concentration, and sedimentation efficiency after flocculation is significantly higher than the single particle. With sediment concentration increasing, collision probability increases, and equivalent particle sizes of floc grow toward to critical particle size which is about 0.02-0.024mm. The amount of sediment those sizes less than the critical particle is approximately 83% of all; â‘¡ Most of the sediment in the reservoir will be affected by flocculation. Flocculation factors and material sedimentation factors are proportional to concentration, and with the concentration increasing, material sedimentation factors increases. When sediment concentration increases to 1.5kg/m3, the corresponding sedimentation volume is up to 66% compared with single particle; â‘¢ Sediment particle settling velocity is gradually increases with increasing concentration no matter it is under the action of Fe3+or Fe3+and Ca2+, however, No significant increase in settling velocity under the action of Fe3+and Ca2+compared with the Fe3+alone, this means effect of Ca2+ on the flocculation is very small. Under the influence of metal ions, the peak of particle group accounting change after flocculation is about 0.04 mm, and almost all the sediment will flocculate under the action of metal ions; â‘£ Increasing humic acid concentration does not accelerate fine sediment settling velocity, so flocculation settling efficiency is hardly affected by humic acid concentration.(3) Flocculation probability and characteristics are analyzed theoretically based on reservoir water environment:â‘ Energy provided by reservoir water environment cannot make fine particles cross the barrier potential to form strong flocculation, but it can make particles form relatively weak flocculation near the second minimum value; â‘¡ The average particle diameter of suspended sediment taken from reservoir is 0.009mm, belonging to cohesive sediment. The results of Fuzzy Comprehensive Evaluation based on the analysis of background values in the reservoir shows that flocculation probability of fine particles is 60%.
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