Cohesive Sediment Flocculation And Settling Processes And The Controlling Mechanisms

Flocculation is one of the most important properties and also a major challenge in cohesive sediment transport dynamics.Knowledge of flocculation and associated settling processes is necessary to better understand,simulate and predict cohesive sediment transport dynamics and relevant environment impacts regarding transport of heavy metals and organic pollutants.Therefore,it is of great significance to have basic understanding of cohesive sediment transport behavior in projects such as river regulation,minimizing harbor and channel siltation,tidal flat development and habitat conservation,and ecosystem restoration,etc.In order to explore the floc properties,flocculation processes and their controlling mechanisms under various hydrodynamic and sediment dynamic conditions,a series of laboratory experiments and in situ surveys have been conducted.In the laboratory experiments,the LabSFLOC-2 system was used to obtain floc data,and the effects of turbulent shear rate,suspended sediment concentration and salinity on flocculation were investigated.In field surveys,three representative environments were selected,including freshwater river,the turbidity maximum zone of estuary and the muddy tidal flat.In situ floc particle size distributions and floc volume concentrations were obtained by LISST-100 C,together with tidal currents and turbulence,suspended sediment concentration,salinity etc.data.The main findings are summarized as follows:1.Turbulent shear plays a major role in controlling flocculation development,with respect to both floc size and floc structure.Mean floc size decreases with increasing turbulent shear rate in natural waters(river and estuary).Our data do not show a medium shear rate benefiting maximum flocculation as that found in laboratory experiments.It maybe because(1)the hydrodynamics and flow structures in natural waters are much more strong and complicated than those in laboratory experiments,and(2)multiple factors are able to cause flocs break-up,thus the crushing effect of turbulent shear on flocs have always been dominant.We find that flocs formed under strong turbulence are more condensed in structure(fractal dimension Nf=2.0-2.6)compared to those under weaker turbulence(Nf=1.5-2.3).Because flocs with loose structure are more fragile and will be easily destroyed by strong turbulent shearing,thus the overall floc structures are more compact under strong turbulence environments;2.Primary particle sizes of suspended sediment is another key factor affecting flocculation,while suspended sediment concentration is a secondary influencing factor,and the existence of salinity could promote flocculation and affect floc structure.Aggregation of suspended sediment occur mainly within the fine particles below 16 ?m,and the higher the content of these fine particles,the larger the flocs will be.Floc sizes tend to increase with the increase of suspended sediment concentration,while the destruction of flocs caused by particle collisions will also increase significantly when the concentration is high.The correlation between mean floc size and suspended sediment concentration is poor in natural waters,but flocs formed under larger suspended sediment concentration are overall larger in size when the turbulence is weak,thus we conclude that suspended sediment concentration is a secondary influencing factor compared to turbulence.The existence of salinity plays a significant role in promoting flocculation,with flocs 2-3 times larger in salt water than in freshwater.Salinity could also affect floc structure,the flocs formed in freshwater are more condensed than those formed in saline conditions.However,the relationship between mean floc size and salinity in tidal cycles is not well established;3.Variations of mean floc size in flood and ebb tides were asymmetry,and the asymmetric pattern behaved inversely between the estuary turbidity maximum zone and the muddy tidal flat.In the North Passage of the Changjiang Estuary,flocs during ebb tides are slightly larger than those during flood tides,while in the Kapellebank tidal flat of the Scheldt Estuary in Holland,flocs during flood tides are generally larger.The former is due to the "hysteresis effect" of flocculation,i.e.,floc break-up processes are faster than aggregation processes,therefore flocculation dominated by aggregation during flood tides cannot reach equilibrium and flocs are overall smaller.The latter is mainly due to rapid settling of large flocs during high water slacks,leading to a significant decrease in floc sizes in suspension.Meanwhile,differences in primary particle composition and suspended sediment concentration between flood and ebb tides could also contribute to the tidal asymmetric feature of flocculation on the muddy tidal flat;4.Flocculation plays an important role in the settling and deposition of suspended sediment.In the freshwater river,flocculation could also happen and floc settling velocity increases from water surface to near bottom.In the middle and lower Changjiang River,the averaged floc settling velocity near bottom is approximately twice as large as that in the surface.Flocculation stimulates settling of very fine particles,leading to accumulation of fine sediment within the bottom layer.In the estuary,vertical profile of floc settling velocity changs with hydrodynamics within tidal cycles.It shows a decrease tendency from water surface to near bottom during maximum floods and ebbs,while it changs to an increase tendency with water depth during slack waters.In the turbidity maximum zone of the Changjiang Estuary and a muddy flat in the Scheldt Estuary,the largest settling flux during a tidal cycle occurs during high water slacks,leading to rapid settling of suspended sediment from the upper and middle water layers to the near bottom,and causes significant sediment deposition within a short period of time.