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.

INTRODUCTION

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.

METHOD

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.

In order to ensure the results are comparable, the field and indoor sediment samples were both collected at the same location and at the same time. Figure 1 shows a comparison between the result of indoor particle gradation and that of field particle gradation measured at 8 km upstream of the Three Gorges Dam during a flood that occurred between 12 July and 21 July 2013. The proportion of the small particle sediment (particle size <0.016 mm) of field samples is much less than that of laboratory samples, while the proportion of large particle sediment (particle size >0.031 mm) of field samples is much greater than that of indoor samples. These data suggested that there is sediment flocculation in the Three Gorges Reservoir. When sediment aggregates from small particles into larger floccules, the proportions of small particle sediment reduce and the proportions of large particle increase. The average particle size of single particle sediment measured in the laboratory was about 0.015 mm, while that measured in the field was around 0.05 mm. The particle size amplification coefficient of sediment Z is defined as 
formula
1
where DF and DL are the average particle size measured in the field and the average size of single particle measured in the laboratory, respectively.
Figure 1

Comparison of field data (including floccules) and laboratory data (primary particles). 170, 350 and 470, respectively, represent the distances of sampling verticals away from the river bank, i.e., 170, 350 and 470 m.

Figure 1

Comparison of field data (including floccules) and laboratory data (primary particles). 170, 350 and 470, respectively, represent the distances of sampling verticals away from the river bank, i.e., 170, 350 and 470 m.

Figure 2 shows the distribution of Z values on multiple verticals. Z value ranges from the minimum value of 1.2 to the maximum value of 15, with most values around 3. The average particle size increased obviously because of flocculation. However, to quantify the influence of flocculation on the sediment settling velocity in the Three Gorges Reservoir, it is necessary to find out the difference between the settling velocity of floccules and that of single particles. Generally, sediment floccules are relatively loose, the density of which is lower than that of single particles. Thus, the existing sediment settling velocity formulas established for primary particles cannot be directly applied to evaluate the settling velocity of floccules. In fact, there are only a few formulas available for calculating the settling velocity of sediment floccules and most of these formulas were established based on the estuary waters. As such, this study collected the suspended loads at the Three Gorges Reservoir and, by means of the indoor quiescent settling experiment, the change in the settling velocity of single particles affected by sediment flocculation in the Three Gorges Reservoir was measured. Due to the looseness of sediment floccules, they are extremely susceptible to damage in sampling, so it is basically impossible to measure the settling velocity of floccules directly (Eisma et al. 1997). In order to avoid damaging floccules and interfering with the settling process of sediment, optical backscatter sensor (OBS) was used to determine the sediment settling velocity by measuring the changes in sediment concentration.
Figure 2

Z values along vertical profiles.

Figure 2

Z values along vertical profiles.

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.

Sedimentation experiment

Devices of the experiment

In order to reduce the errors in calculating the sediment settling velocity and to eliminate the influence of side wall on sediment settlement, the indoor quiescent settling experiment was carried out in an organic glass tank, 2 m in height and 1 m in diameter. To ensure uniform distribution of sediment particles in the tank at the preliminary stage of the experiment, a hydraulic circulating device was introduced (Figure 3).
Figure 3

Sketch of the experiment system (1 denotes cylinder, 2 denotes outlet of water, 3 denotes the upper OBS, 4 denotes the lower OBS, 5 denotes pipe, 6 denotes pump).

Figure 3

Sketch of the experiment system (1 denotes cylinder, 2 denotes outlet of water, 3 denotes the upper OBS, 4 denotes the lower OBS, 5 denotes pipe, 6 denotes pump).

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.

Voltage is the original output data from OBS and, for this reason, a correlation between voltage and sediment concentration is required (Figure 4). Moreover, it is found that the relations between voltage and sediment concentration measured by the two probes differ slightly. According to the working principles of OBS, the calibration data should be divided into two levels of concentration: high level concentration and low level concentration, with 0.6 kg/m3 as the boundary between them. In the case of the low level concentration, the relation between the sediment concentration and the voltage is linear; in the case of the high concentration, the relation between the sediment concentration and the voltage is nonlinear. These relations are being used in this study.
Figure 4

Relation between voltage and sediment concentration. (a) Calibration of the upper probe. (b) Calibration of the lower probe.

Figure 4

Relation between voltage and sediment concentration. (a) Calibration of the upper probe. (b) Calibration of the lower probe.

Experiment principles

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.

The portion of sediment with a particle size smaller than d can be obtained: 
formula
2
Theoretical considerations show that the settling velocity of a floc from static to achieving 99% of its terminal need a distance of less than 1 mm (Soulsby et al. 2013). This is a very small distance compared with that between the water surface and the observation point, therefore, the settling velocity of sediment with equivalent particle size d can be determined as 
formula
3
The equivalent particle size d can be obtained through sediment settling velocity as 
formula
4
where , , , g and are the sediment settling velocity, the specific weight of sediment, the specific weight of water, gravitational acceleration and kinematic viscosity of water, respectively (Wu 2008).

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.

Figure 5 shows the variation of sediment concentration in the tank. It is shown that the settling processes can be divided into three stages: (1) between 0 and 90 min, (2) between 90 and 240 min and (3) >240 min. In the first stage, the sediment concentration was abruptly reduced, suggesting a relatively rapid settling of sediment. In the second stage, with the emergence of a turning point, the settling velocity of sediment began to reduce significantly, resulting in a much lower settling velocity of sediment at this stage. In the third stage, the sediment concentration changed relatively slowly, suggesting that it was difficult for the residual sediment in the tank to settle. In addition, both the sediment concentration and the time when the turning point occurs were not affected by the initial sediment concentration. The sediment concentration at the occurrence of the turning point was around 0.25–0.3 kg/m3, and the final sediment concentration declined to 0.12 kg/m3.
Figure 5

Changing process of sediment concentration.

Figure 5

Changing process of sediment concentration.

To reflect the settling efficiency of each sediment concentration, Figure 6 shows the changes of relative concentration with time. In the initial 30 min, the sediment in single particle state basically had the same settling efficiency as the sediment with flocculation, and settling efficiency had no relation to the sediment concentration. This suggests that the sediment settlement during this period was in single particle state and almost contained no floccules. Further, within the sediment concentration range of the water at the Three Gorges Reservoir, no matter how the sediment concentration had changed, a small portion of the sediment in the form of relatively large particles was not influenced by flocculation in the settling processes. It maintained its single particle settling throughout and did not participate in the formation of floccules. After the first 30 min, due to the difference between the degree of sediment flocculation, the settling efficiency curves became different as the sediment concentration changed. This suggests that this portion of sediment was influenced by flocculation, and the settling efficiency of sediment with flocculation was greater than that of sediment in single particle state. And the higher the sediment concentration is, the greater the change in relative concentration occurs. Due to the increased distance among the sediment particles and the reduced probability of particle collisions, when the sediment concentration was less than 0.3 kg/m3, it was difficult for floccules to take shapes. Hence, on the settling efficiency curve with the sediment concentration of 0.3 kg/m3, it shows the same settling efficiency as single particles.
Figure 6

Changing process of relative concentration.

Figure 6

Changing process of relative concentration.

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.

Figure 7 shows a comparison between the result of the equivalent particle gradation obtained in the quiescent settling experiment and that of the original single particle gradation. Under different sediment concentration conditions, all the equivalent particle gradation curves shifted to the left as compared to the original single particle gradation curve. This suggests that the particle size had increased and that there was a certain degree of sediment flocculation in the tank. Moreover, the higher the sediment concentration is, the further to the left the gradation curve shifted after flocculation. This suggests that there is a direct proportional relation between the degree of sediment flocculation and the sediment concentration. Further investigation shows that the particle gradation curves intersected at a point with particular particle size. For the sediment particles smaller than that particle, the proportion increased significantly. On the other hand, for the sediment particles bigger than this particle, the proportion remained unchanged. Based on these results, the critical particle size of sediment flocculation in the Three Gorges Reservoir should be around 0.022–0.024 mm, which was basically consistent with the results obtained through the theoretical analysis using flocculation kinetics (Wang et al. 2014), and was agreed with field data (Guo & He 2011). The sediment particles smaller than the critical particle size need to take the influence of flocculation into account. This portion of sediment accounts for 83% of the total sediment, so that most of the sediment in the Three Gorges Reservoir is subject to the influence of flocculation.
Figure 7

Comparison of sediment particles gradation before and after flocculation.

Figure 7

Comparison of sediment particles gradation before and after 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.

Figure 8 shows the change of the proportion of sediment affected by flocculation for different particle size groups. Compared with primary particles, the proportion of small particles in the flocs reduced and the proportion of large particles increased. Furthermore, the proportion of particle groups bigger than 0.024 mm is not influenced by flocculation. The peak ratio corresponds to the critical particle size of flocculation, and the peak value increases with increasing sediment concentration. That is to say, with the increase in particle collisions and combinations, the equivalent particle size of floccules moves towards the critical particle size of flocculation. In single particle state, the proportion of the sediment group with particle sizes ranging from 0.019–0.022 mm is 6.7%. For the sediment concentration of 0.5 kg/m3, the proportion increased to 10.0% after flocculation. For the sediment concentration of 1.0 kg/m3, the proportion increased to 21.9%. For the sediment concentration of 1.5 kg/m3, the proportion even increased to 34.1% and this is more than 5 times that of the primary particles.
Figure 8

Variation of each sediment group proportions (Pp is the proportion of the primary particles, Pf is that of the flocs).

Figure 8

Variation of each sediment group proportions (Pp is the proportion of the primary particles, Pf is that of the flocs).

The flocculation factor F is commonly used to quantify the change in the settling velocity of sediment before and after flocculation: 
formula
5
where is the settling velocity of average particle size or median particle size with flocculation, and is the settling velocity of the corresponding particle size of primary particles (Migniot 1968).
The settling velocity of sediment in still water can be expressed by the mass settling flux, which is equivalent to the change of sediment mass per unit time of the water, i.e., sediment concentration multiplied by the settling velocity (Manning et al. 2011). The influence of flocculation on the deposition rate of sediment can be described more accurately by the mass settling flux. Similar to the flocculation factor in Equation (5), the mass settling flux factor is defined as 
formula
6
where is the settling flux of sediment with the influence of flocculation, and is the settling flux of sediment in single particles.
Table 1 shows the influence of flocculation on the group settling velocity of sediment and the volume of sediment deposition in the reservoir. It can be seen that the flocculation factor Fp derived from the average particle size dp is smaller than the flocculation factor F50 derived from the median particle size d50. Moreover, the difference between Fp and F50 increases as the sediment concentration increases. For the sediment concentration <1.5 kg/m3, the difference between Fp and F50 is less than 50%. The flocculation factors, Fp and F50 can be expressed with the sediment concentration C0 logarithmically as (shown in Figure 9) 
formula
7
 
formula
8
Table 1

Influence of flocculation on sediment deposition rate

Characteristic Primary Sediment concentration (kg/m3)
 
article size particles 0.5 1.0 1.5 
dp(μm) 9.6 13.0 15.8 17.7 
Fp 1.0 1.79 2.68 3.36 
d50(μm) 8.6 12.9 16.9 19.3 
F50 1.0 2.24 3.87 5.03 
RTM 1.0 1.24 1.47 1.66 
Characteristic Primary Sediment concentration (kg/m3)
 
article size particles 0.5 1.0 1.5 
dp(μm) 9.6 13.0 15.8 17.7 
Fp 1.0 1.79 2.68 3.36 
d50(μm) 8.6 12.9 16.9 19.3 
F50 1.0 2.24 3.87 5.03 
RTM 1.0 1.24 1.47 1.66 
Figure 9

Relations of flocculation factor and sediment concentration.

Figure 9

Relations of flocculation factor and sediment concentration.

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.

CONCLUSIONS

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:

  1. 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.

  2. 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.

  3. 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.

  4. 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.

ACKNOWLEDGEMENT

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.

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