By comparing the original particle gradation of sediment from the Three Gorges Reservoir with the single particle gradation, the differences in these two particle gradations showed that there is sediment flocculation in the Three Gorges Reservoir, which can accelerate the sediment deposition rate in the reservoir. In order to determine the influence of flocculation on the sediment settling velocity, sediment was collected at the Three Gorges Reservoir, and the indoor quiescent settling experiment was performed to study the mechanism of sediment flocculation. The experimental results showed that sediments aggregated from single particles into floccules in the settling processes. The single particles smaller than 0.022 mm will participate in the formation of floccules, which accounts for 83% of the total amount of sediment in the Three Gorges Reservoir. Moreover, the degree of sediment flocculation and the increase in sediment settling velocity were directly proportional to the sediment concentration. Taking the average particle size and the median particle size as the representative particle size, respectively, the maximum flocculation factors were calculated to be 3.4 and 5.0. Due to the sediment flocculation, the volume of sediment deposition will increase by 66% when the mass settling flux factor of total sediment had a maximum value of 1.66, suggesting that flocculation has a significant influence on the sediment deposition rate in the Three Gorges Reservoir.
The Three Gorges Reservoir located in the upper Yangtze River is the largest reservoir in the world. Most suspended particulate matter, with the median grain size of about 0.01 mm, in the reservoir region are composed of cohesive sediment and clay, which are prone to form floccules (Van & Van Bang 2013; Wang et al. 2014). Due to the construction of Three Gorges Dam, the backwater area in the Three Gorges Reservoir gets bigger and the corresponding flow velocity gets slower. More importantly, changes in flow regime have increased the possibility of sediment flocculation in the Three Gorges Reservoir.
Flocculation is regarded as one of the primary behaviors to the process of sediment transportation, erosion and deposition (Mehta 1989). In light of the previous experiments on clay minerals, flocculation and suspended-sediment concentration are almost as important as bottom shear stress for sediment deposition (Schieber et al. 2013). According to the dynamic of flocculation, positive ions are the most fundamental driving force for sediment flocculation. In addition, different ion concentrations may lead to different degrees of flocculation and form different floccule sizes (Winterwerp & Van Kesteren 2004). There is a relatively high concentration of positive ions in estuary region, while the ion concentration in fresh water is relatively low, and for this reason the sediment flocculation in estuaries are significantly greater than that in fresh water. That is why most studies on sediment flocculation are carried out in estuary regions (Winterwerp & Van Kesteren 2004; Manning et al. 2010). Nevertheless, flocculation is a characteristic of fine suspended particulate matter, and it should exist in both fresh water and seawater (Winterwerp & Kranenburg 2002; Manning et al. 2011; Andrew et al. 2013). Besides saltwater intrusion, other factors, like flow regime, sediment concentration and organic material, may contribute to the flocculation mechanism (Krishnappan 2000). Hence, sediment flocculation is not a phenomenon unique in estuary regions but also exists in rivers or lakes.
Researches have shown that there is sediment flocculation in the Three Gorges Reservoir. Guo & He (2011) used laser in situ scattering and transmissometry (LISST) to measure the sediment gradation along the mainstem of the Yangtze River and showed that sediment floccules existed in the upper and middle Yangtze River. Dong et al. (2010) found that the sediment deposition volume derived from the numerical calculations could not agree with the observed sediment deposition volume at the Three Gorges Reservoir. They concluded that it was due to the influence of flocculation on the sediment deposition. Li et al. (2015) introduced an underwater camera system to record the bed morphology formed by sediment deposition in the Three Gorges Reservoir region, and they observed the existence of floccule-like structures. Further, by comparing the vertical distribution of suspended loads in the Three Gorges Reservoir with that using the Rouse formula, they showed that sediment flocculation existed in the Three Gorges Reservoir.
Notwithstanding the previous research, the existence of sediment flocculation in the Three Gorges Reservoir still need to be proven with much clearer evidence. Further, the influence of flocculation on sediment deposition in the Three Gorges Reservoir also needs to be evaluated. Among them, the crucial problem is to find out floc settling velocity. It relates to many factors, some of which are quite difficult to be determined, such as the fractal dimension of the floc, floc shapes, permeability, and so on (Winterwerp 1998; Strom & Keyvani 2011). In order to make the formulas simple, Mikkelsen and Pejrup assumed that, in a floc, water volume is much larger than particle volume (Mikkelsen & Pejrup 2001), but this assumption cannot be applied here since flocs in fresh water are usually micro flocs with less water in them. Due to the significant differences between the Three Gorges Reservoir and estuary regions in terms of the flow regime, ion concentration, etc., the settling velocity of sediment flocs observed in estuary regions cannot be directly applied in the reservoir.
In order to resolve those issues, firstly, the existence of sediment flocculation in the Three Gorges Reservoir is identified based on the comparison of the particle gradations between the original samples and the single samples. Further, the indoor quiescent settling experiment is performed to quantify the influence of flocculation on the sediment deposition in the Three Gorges Reservoir. From these experiment results, a formula was developed which can be used to calculate the sediment deposition rate at the Three Gorges Reservoir.
Field observation of sediment flocculation
According to the definition of sediment flocculation, the floccules formed after sediment flocculation and, for this reason, the floc diameter is greater than the corresponding single particle diameter. As such, the original particle gradation from the field with flocculation is compared with the single particle gradation measured in the laboratory. This comparison will show whether sediment flocculation exists in the reservoir. Since it has been proved that laser diffraction instruments are able to measure flocs, LISST-100X is used to determine the particle gradation from the field (Bale & Morris 1987). For the single particle gradation measured in the laboratory, the Malvern laser particle size analyzer was used by following the specifications of hydrology survey to maintain the single particle state of sediment. Prior to the measurements, all the instruments were calibrated so as to avoid any systematic error.
The main factors that influence flocculation are ion concentration, flow velocity and sediment concentration (Manning et al. 2010). Given that the ion concentration of the water in the Three Gorges Reservoir is relatively stable throughout the year, the average ion concentration was used. Since there is no flow in the indoor quiescent settling experiment, the influence of flow velocity was not examined. The quiescent settling experiment was therefore used to determine the relation between the degree of sediment flocculation and sediment concentration.
Devices of the experiment
In the experiment, two OBS probes were used: (1) to monitor the distribution uniformity of sediment concentration at the preliminary stage of the experiment; (2) to measure the variation of sediment concentration around them; and (3) to verify the data measured by each other.
The sediment concentration is measured in the indoor quiescent settling experiment at the fixed measurement points and at regular intervals. The particle gradation can then be deduced according to the existing formulas relating the particle gradation to the sediment settling velocity. The particle size of sediment obtained in this experiment is known as the equivalent particle size, i.e., the single particle size with the same settling velocity. Assuming the distance of the OBS probe below the water surface level is L, the initial sediment concentration is C0, and the sediment concentration at the time t is C, the sediment gradation can then be calculated as follows.
By comparing the differences between the equivalent particle gradation and the original single particle gradation, the characteristics of sediment flocculation at the Three Gorges Reservoir and the change in sediment settling velocity due to flocculation can be determined.
RESULTS AND DISCUSSION
Sediment deposition process
The sediment concentration of the water in the Three Gorges Reservoir is usually less than 1 kg/m3. In consideration of the interaction between sediment concentration and sediment settlement under the circumstances of high sediment concentration, the sediment concentration in the experiment was confined to less than 1.5 kg/m3.
Particle sizes influenced by flocculation
Flocculation only influences the settlement of sediment in fine particles. For different water bodies and different sediment sources, the definitions of fine particles may not be the same. Thus, in order to analyze the influence of flocculation on sediment settlement, the first step is to define the range of the particle size influenced by flocculation. The upper threshold particle size is the critical particle size of flocculation, and the single particles with a particle size greater than the critical particle size of flocculation are immune from the influence of flocculation.
Settling velocity influenced by flocculation
Although all the sediment particles smaller than the critical particle of flocculation will participate in the formation of floccules and accelerate the settlement of sediment, the influence of flocculation on the sediment particles are different for differently sized particles. Hence, it is necessary to classify them into different particle size groups for a detailed discussion.
|Characteristic||Primary||Sediment concentration (kg/m3)|
|Characteristic||Primary||Sediment concentration (kg/m3)|
The mass settling flux factor (Fp and F50) increases as the sediment concentration C0 increases. For the sediment concentration of 0.5 kg/m3, the mass settling flux factor of suspended sediment RTM, is 1.24, indicating that flocculation makes the volume of sediment deposition increase by 24%. When the sediment concentration increased to 1.5 kg/m3, RTM increased to 1.66, indicating that the volume of sediment deposition may increase by 66% compared with the case of single particles.
The flocculation factor reflects the influence of flocculation on the group settling velocity of sediment, while the mass settling flux factor can be employed to estimate the influence of flocculation on the deposition rate of sediment. By comparing the flocculation factor and the mass settling flux factor before and after flocculation, it is found that the average particle size is suitable for being the representative particle size because it possesses a more clear physical sense and the corresponding flocculation factor will give a more accurate degree of flocculation.
The field measurement and the indoor quiescent settling experiment were carried out in this study to investigate the influence of flocculation on the sediment deposition processes in the Three Gorges Reservoir. It shows the following:
Floccules exist in the Three Gorges Reservoir. By comparing the original particle gradation measured in the Three Gorges Reservoir with the single particle gradation, it shows that the proportion of small particles decrease and the proportion of large particles increase. This is consistent with the flocculation characteristic.
The minimum sediment concentration for sediment flocculation in the Three Gorges Reservoir is about 0.3 kg/m3. The settling efficiency of sediment is directly proportional to sediment concentration, and when the sediment concentration is less than 0.3 kg/m3, the corresponding sediment settling efficiency is the same as that in single particle state. It proves that the sediment with the sediment concentration of less than 0.3 kg/m3 basically does not flocculate.
The critical particle size of sediment flocculation in the Three Gorges Reservoir is about 0.022–0.024 mm, which is not affected by sediment concentration. Combining the calculation of the sediment settling velocity with the change in sediment concentration, the equivalent particle size gradation curves with flocculation is obtained. A comparison of those curves with the single particle size gradation curve shows that 83% of the total amount of sediment will be influenced by flocculation. The single particles smaller than the critical particle size of flocculation are obviously influenced by flocculation, and the sediment with the particle size around the critical particle size of flocculation increased significantly. The higher the sediment concentration, the higher the flocculation degree, with more floccules formed around the critical particle size of flocculation.
Both the flocculation factor and the mass settling flux factor are directly proportional to the sediment concentration. There is a logarithmic relation between the flocculation factor and the sediment concentration for the sediment concentration less than 1.5 kg/m3. Due to flocculation, the mass settling flux of suspended sediment can increase as high as 66%. This suggests that the flocculation has a significant influence on the deposition rate of sediment in the Three Gorges Reservoir.
The authors would like to express sincere thanks to the Three Gorges Dam Bureau of Hydrology and Water Resource Survey for their support in our field measurements, as well as indoor particle gradation measurements, in this study.