| Flocculation is an important process in fine cohesive sediment transport that still requires investigation as it is a complex process,involving the aggregation,break-up and reconformation of mineral and biological particles.Fine cohesive sediments are found in rivers,estuaries and their adjacent areas.With the rapid economic development in the Changjiang(Yangtze River)estuary,human activities have intensified,including the development of shipping,and the exploitation of water and soil resources.The erosion and deposition problems in waterways and tidal flats have become increasingly prominent,affecting the evolution of geomorphology and ecological environment.In estuaries under combined action of fluvial and marine processes,flocculation of cohesive sediment has many influencing factors involving complex mechanisms.At present,the occurrence of flocculation,the transport and settling processes of sediment flocs cannot be accurately predicted.Up to now,the research on sediment flocculation in estuaries mainly focuses on the influence of physical factors such as hydrodynamics,suspended sediment concentration and salinity,while the research on the influence of biological factors is insufficient.Fully understanding sediment flocculation and its effect on sediment transport is not only beneficial to sediment transport modelling,but also for the research on the transport of the material carried by sediment particles,such as heavy metals,organic pollutants and nutrient substances.Better understanding flocculation can help to solve many engineering problems associated to river channel management,channel siltation,mudflat evolution,and the corresponding ecological problems which are ongoing with changes in climate and environment.In this study,the influence of biology on sediment flocculation and the characteristics of flocs and transport processes of flocculated sediment under different conditions are investigated by combining laboratory experiments and in-situ observations.In the laboratory experiments,grab sediment samples from the Changjiang Estuary were analyzed by Static Light Scattering(SLS).This study was mainly meant to understand the characteristics of flocculation processes(timescales,floc sizes)for different environmental conditions at constant shear.These conditions were adapted by changing salinity and studying the effect of the presence of the microalgae Skeletonema costatum which is one the dominant algae species in the Changjiang estuary.The effect of extracellular polymeric substances(EPS)on flocculation was also studied.EPS is usually secreted by microorganisms.Mean floc particle size,particle size distributions,zeta potential and conductivity were recorded and correlated to each other.During the in-situ survey,the characteristics of flocculation in different tidal cycles,seasons and locations in the maximum turbidity zone of the Changjiang estuary were observed.In-situ laser diffraction technique(by means of a LISST-100C)was used to obtain in-situ volume concentration and particle size distribution.Combined with hydrodynamic data recorded synchronously,the effects of biology under different hydrodynamic conditions on sediment flocculation and transport was analyzed and discussed.The key conclusions obtained are as follows:1.The effects and mechanisms of algae,extracellular polymers(EPS)and salinity(Chapter 4).In the laboratory experiments,sediment flocculation is different in the presence of algae,EPS and dissolved salt:(1)Algae particles can form large flocs by themselves,the floc size distribution(PSD)presents a bimodal curve when algae and sediment particles are mixed.With the progress of flocculation,the bimodal curve gradually merged into a unimodal curve,and the overall floc size increased.(2)The effect of EPS needs the participation of cations.The floc size changes little with EPS in fresh water,while the flocculation is remarkable under the influence of salinity.(3)Salinity can promote sediment flocculation,and the flocculation effect of divalent cations(MgCl2 and CaCl2)is greater than that of monovalent cations(NaCl).Algae and EPS can significantly promote the flocculation of sediment,and the flocculation time is short,while salinity has a weak influence,with smaller flocs and longer flocculation time.The presence of salt ions in the water changes the surface potential of sediment particles and the flocs are formed by the DLVO interaction force between sediment particles.The algae change the floc's composition and form large algae-sediment mixed flocs by combining algae-formed flocs with sediment particles.EPS,as a connecting medium,has bridging effect between sediment particles under the action of cations.2.Coupling effect of algae and suspended sediment in flocculation process(Chapter 4,Chapter 5 and Chapter 6).Both in-situ observations and laboratory experiments show that algae and sediment are coupled.In field observations,a correlation was found between algae concentration and suspended sediment concentration(SSC)in estuary area.In this study,the ratio of chlorophyll concentration(CC)to suspended sediment concentration is taken as the parameter to represent the algae effects,which is abbreviated as algae-sediment ratio(CC/SSC).Both chlorophyll concentration and suspended sediment concentration show a trend that increases with the increase of water depth in an estuarine area.The algae-sediment ratio not only reflects the role played by algae in the formation of flocs,and therefore can help to study sediment flocculation in the water column.In laboratory experiments,increasing algae-sediment ratio promotes the flocculation process,and the flocs formed have larger particle size and faster flocculation rate.In the field observations,the algae-sediment ratio increases significantly in the middle and bottom layers of the water column during the short period after high water slack in summer,which reflects the fact that algae are settling during the sediment flocculation process(differential settling).3.Biological effects on sediment flocculation in estuarine area(Chapter 5 and Chapter 6).Most of the algae particles in the Changjiang Estuary are distributed in the part where the floc population has an average size larger than 100 ?m.When the concentration of algae is high with respect to the sediment concentration(the algae-sediment ratio is high),the floc particle size distribution(PSD)will display a bimodal curve with one peak associated to sediment particles and one to algae particles.The bimodal PSD and unimodal PSD are interchangeable due to variation of tidal hydrodynamics and the main peaks are located in the range 10–100 ?m.The effective density and settling velocity of flocs in this size range are related to the flocculation time.Flocculation time influences settling velocity,as particles grow and have a time-dependent density upon flocculation.This is very important for sediment transport models.Instead of using constant sediment settling velocity,sediment transport models should account for a change in settling velocity of flocs.In line with previous studies,it was found that flocs can be divided into three components according to their particle size distribution:Single-grain(< 5 ?m),Microflocs(5–200 ?m)and Macroflocs(> 200 ?m).The main flocculation mechanism is related to the change of microflocs into macroflocs.The algae-sediment flocculation process in estuaries is mainly affected by shear rate.During the high water slack period,a low shear rate is maintained for about 2–3 hours,and the algae particles and sediment particles are fully mixed under relatively stable hydrodynamic conditions to form unimodal large flocs with a particle size of about 40–80 ?m.Flocculation is then close to steady-state.Salinity and other factors have no significant influence on algae-sediment flocculation process in estuarine area.4.Temporal and spatial variation of biological effects and tidal asymmetry of sediment settling(Chapter 6).The effect of algae is of more importance in summer due to higher algae concentration than in winter.The variation of flood and ebb tide indicates different flocculation processes in summer: during flood tide,the algae-sediment ratio increases in the upper layer of the water column and algae-sediment flocculation occurs in the early flood period,forming large flocs with low density and high settling velocity.In the following 2–3 hours,the large flocs gradually settled to the middle and bottom layers of the water column,and the floc size distribution changes from bimodal to unimodal.In the short period after high water slack(within 1–2 hours),the suspended sediment concentration decreases significantly as many sediment particles have settled to the bed.During ebb,the flocculation efficiency decreases due to the decrease in suspended sediment concentration and seawater retreat(which contains most algae),so there are no double peak PSDs and the mean floc size and settling velocity is smaller than that during flood.Consequently,it can be concluded that algae promote the asymmetry of sediment settling during ebb and flood tide,which is an important factor causing sediment transport toward the estuary in a tidal cycle.To sum up,the combination of laboratory experiment and in-situ observation results is helpful to deeply understand flocculation kinetics.This study establishes the relationship between sediment transport and ecological cycle in estuarine area.The coupling phenomenon between biology and sediment showed that sediment flocculation not only affects the evolution of estuarine geomorphology,but is also an important process in estuarine ecological cycle. |
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