Clues to flocculation development by comparing particle size distribution patterns of suspended matter in the water mixing zone of the Yangtze River Estuary
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Abstract: Particle size is an important characteristic of suspended matter, and it contains crucial information about the deposition process. Suspended particle samples in the water mixing zone of the Yangtze River Estuary were collected in December 2016. Untreated original grain size and the decentralized grain size of the suspended particles were measured via a laser particle size analyzer. Morphological characteristics and the chemical composition of the suspended particles were also studied systematically using a scanning electron microscope(SEM) with an energy dispersive X-ray spectrometer(EDS). Then, the flocculation and sedimentation of suspended matter in the water mixing zone were explored by combining them with the water mixing processes in the estuary. The average particle size of suspended matter in the mixing zone of the Yangtze River Estuary ranges from 5.73 Ф to 7.98 Ф. The particle size distribution pattern is an abnormal model with a mainly unimodal pattern. In the freshwater area that was dominated by runoff, the suspended matter is mainly composed of fine particles, the settling velocity is slow, and the flocculation is weak. Floc particles were often seen in the mixing zone, with the flocs having a relatively large particle size, a low density and a loose structure appearing at the weak mixing zone; the flocs had a compacted structure in most areas of the mixing zone. The changes of suspended particle size in the estuarine mixing zone promote the settling and deposition of suspended matter, which has an important influence on the bed geomorphology and preservation of the fine suspended particles in the estuary.
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Key words:
- water mixing zone /
- suspended matter /
- particle size /
- sedimentation /
- flocculation /
- Yangtze Estuary
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Figure 1. Marine environmental characteristics (left) and sampling stations and isobath in the study area (right). The depicted circulation system and isobath was modified from Wang (2014), Pang et al. (2018) and Fan et al. (2023). KC-Kuroshio Current, YSWC-Yellow Sea Warm Current, TWWC-Taiwan Warm Current, YSCC-Yellow Sea Coastal Current, SBCC-Subei Coastal Current, ECSCC-East China Sea Coastal Current, CDW-Changjiang Diluted Water.
Figure 2. The spatial distribution of the water temperature, salinity, turbidity and suspended matter concentration in the study area.
Figure 3. Changes in the average suspended particle size in the study area. a represents the untreated sample, b represents the treated sample.
Figure 4. Frequency distribution patterns of the suspended particle size. a,b,c and d represent the un-treated samples, e,f,g and h represent the decentralized samples. a(e) is the surface layer sample of station 5, b(f) is the surface layer sample of station 9, c(g) is the bottom layer sample of station 29, d(h) is the middle layer sample of station25.
Figure 5. The morphological characteristics of suspended floc particles in the study area. a,b and c represent the floc particles with loose structure and small particle size in the freshwater area, d,e and f represent the floc particles in the weak mixing zone and d is a loose floc, e is a compact floc, f is a floc rich in organic matter, g,h and i represent the floc particles with compact structure in the strong mixing zone.
Figure 7. C-M diagrams of suspended particle size in each zone. All represents C-M diagram of the suspended particle size in the study area, FA represents C-M diagram of the suspended particle size in the freshwater area, WMZ represents C-M diagram of the suspended particle size in the weak mixing zone, SMZ represents C-M diagram of the suspended particle size in the strong mixing zone. The red dots represent the suspended particles in the surface layer; the green dots represent the suspended particles in the middle layer; the black dots represent the suspended particles in the bottom layer.
Table 1. Particle size parameters of the suspended matter in different zone. FA represents the freshwater area, WMZ represents the weak mixing zone, SMZ represents the strong mixing zone
zone σ Sk Ku Mz untreated treated untreated treated untreated treated untreated treated FA Min 1.25 1.56 −0.97 −1.76 1.64 2.02 6.26 6.48 Max 2.18 2.09 1.12 1.42 2.66 3.00 7.98 7.63 AVG 1.69 1.90 0.73 0.58 2.18 2.43 7.06 6.96 WMZ Min 1.62 1.72 −1.97 −0.73 2.08 2.18 5.73 5.88 Max 2.73 2.29 1.66 1.84 3.38 2.85 7.09 7.04 AVG 1.83 1.93 1.08 1.25 2.36 2.49 6.48 6.45 SMZ Min 1.64 1.73 −0.39 −0.81 2.13 2.21 6.15 6.13 Max 1.87 2.12 1.71 1.80 2.45 2.94 7.01 6.92 AVG 1.73 1.86 1.19 1.23 2.25 2.44 6.66 6.55 
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