Measurements of the sediment flocculation characteristics in the Three Gorges Reservoir, Yangtze River

Abstract

The flocculation of fine sediments in the Three Gorges Reservoir (TGR) was confirmed in previous studies, but the flocculation characteristics have yet to be fully clarified. In this study, field measurements were conducted in the TGR to investigate the sediment flocculation characteristics. First, the instantaneous flow velocity and sediment concentration were measured through Acoustic Doppler velocimeter and sediment sampling. Then, the effective settling velocity was calculated based on the sediment diffusion theory to deduce the floc size and flocculation degree. Finally, the influences of particle size, flow velocity, and sediment concentration on flocculation were analyzed. Results showed that flocculation occurred in more than half of the sediments in the TGR, and the maximum flocculation degree was between 10 and 30. Flocculation weakened as particle diameter increased, with the critical particle size being approximately 0.018 mm, meaning that flocculation was unlikely to occur when the particle size exceeded the critical value. As the flow velocity increased, the flocculation degree first increased and then decreased, with the critical flow velocity being approximately 0.7 m/s, but the critical flow velocity increased with an increase in sediment concentration and tended to be a constant. The flocculation degree also increased with increasing sediment concentration and tended to be constant when the sediment concentration exceeded approximately 0.5 kg/m3. The results provide new information on the flocculation characteristics of the TGR and should be useful for understanding and simulating fine sediment transport in the TGR.

Propagation characteristics of the flow and suspended sediment peaks in the three gorges reservoir, China: Insight from numerical simulations and measured data

“…Thus, the movement of coarser suspended sediment can be interpreted as the propagation of high SSC near the riverbed, which prolongs the propagation time of the SSC peak (Figure 18b). However, in circumstances where the suspended sediment is fine, the SSC is high and the flow velocity is low, the flocculation effect in the TGR could similarly extend the propagation time of the SSC peak (Li et al, 2020; Wang et al, 2016; Zhang, 1998).…”

“… The sediment transport capacity is calculated by the Ruijin‐Zhang's method (Tan et al, 2018; Zhang, 1998) where K and m are the coefficient and index, respectively, and their values are provided by the Yangtze River Scientific Research Institution and China Institution of Water Resource and Hydropower Research (Huang et al, 2012; Li et al, 2018), that is, K = 0.245 and m = 0.92; is the average settling velocity of suspended sediment (m/s), and it is calculated by the following formula: , where p k and ω k are the proportion (%) and suspended sediment settling velocity (m/s) for the k th grain size fraction, respectively, and ω k is calculated by the method proposed by Zhang (1998). Furthermore, recent studies have revealed suspended sediment flocculation in the TGR (Li et al, 2020), which significantly influences the sediment deposition process. To reflect the flocculation effect, the sediment setting velocity is revised with the following formula (Zhang et al, 2021).…”

“…where p k and ω k are the proportion (%) and suspended sediment settling velocity (m/s) for the kth grain size fraction, respectively, and ω k is calculated by the method proposed by Zhang (1998). Furthermore, recent studies have revealed suspended sediment flocculation in the TGR (Li et al, 2020), which significantly influences the sediment deposition process. To reflect the flocculation effect, the sediment setting velocity is revised with the following formula (Zhang et al, 2021).…”

Evolution of the main channel of the Yangtze River

“…The continuous meandering bends of the lower Jingjiang River exhibit a phenomenon called “chute cutoff” and in‐depth studies have been conducted on the causes of its formation (Liu et al, 2022; Lu & Zhu, 2019; Zhu et al, 2017). In the Jingjiang River, the development of short subordinate branch at bifurcation sections is dominant (Li & Hu, 2017), while in the river reach downstream of Chenglingji, the main branch develops, while the subordinate branch diminishes (Zhu et al, 2015), and the main reasons for the river regime local adjustments at bifurcation sections in the middle and lower reaches of the Yangtze River have been studied (Liu et al, 2020; Zou et al, 2020). With the gradual construction and operation of cascade reservoirs in the upper reach of the Yangtze River, significant adjustments have occurred in the flow and sediment processes in the middle and lower reaches of the river, and the future channel bed reformation process in the middle and lower reaches of the Yangtze River has been predicted (Lu, 2021).…”

Effect of the Three Gorges Reservoir on vertical distribution of suspended sediment concentration and estimation accuracy of depth‐average concentration in downstream reaches

“…Reservoirs and associated dams can have a tremendous effect on river runoff, changing the natural hydrological cycle and sediment transport processes in downstream reaches (Benn & Erskine, 1994). There is no exception that the water‐sediment conditions of the middle Yangtze River have changed significantly due to impoundment of the Three Georges Reservoir (TGR) since 2003 (Chen et al, 2020; Dai & Liu, 2013; Li et al, 2020; Xia et al, 2017). It is mainly manifested in the redistribution of the runoff process (Han et al, 2018; Li et al, 2009), the significant reduction in sediment load (Mei et al, 2015), and the adjustments of sediment particle size (Lyu et al, 2018; Zhang et al, 2017).…”