Effect of hydrodynamics factors on sediment flocculation processes in estuaries (original) (raw)
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Effect of hydrodynamics factors on flocculation processes in estuaries
2016
Cohesive sediment flocculates under certain conditions to form flocs which are larger than individual particles and less dense. The flocculation has an important role in sediment transport processes of settling, deposition and erosion. In this study, well controlled laboratory experiments were performed to investigate the effect of turbulence on floc size and settling velocity. Experimental research was conducted in a 1L glass beaker of 11 cm diameter using suspended sediment samples taken from the Severn Estuary. A PIV system and image processing routine were used to measure the floc size distribution and settling velocity, as well as the flocs density. This study found that turbulent shear stress in a range from 0.57 to 8.5 N/m 2 would cause a breakdown in floc structure. The settling velocity of the samples was found to range from 0.4 to 1.4 mm/s. The average settling velocity increased with the increase of the turbulent shear stress up to the maximum value (1.1 mm/s), and then d...
Sediment transport: Physical modelling of flocculation
Environmetrics, 2007
A study is made of the behaviour of cohesive sediment in turbulent flowfields, such as are found in strongly tidal river estuaries. A model is developed which incorporates the fact that cohesive sediments, usually clays, consist of particles which can flocculate because of the electrical charges on them. During the cycle of erosion and deposition that occurs in tidal estuaries, the degree of flocculation changes. An equation is formulated for the evolution of the size distribution of the particle aggregates in suspension, taking into account the effect of turbulence both on the rates of flocculation and breakup. Solutions of this equation are obtained using an extension of the quasi-stationary approximation. The results are used to investigate the interpretation of the output of electro-optical turbidity meters. These are important sources of field measurements, but they require calibration before their data can be used for the testing and construction of sediment transport equations. It is shown that the transformation of turbidity-meter readings into density measurements requires statistical information about the sizes of the suspended aggregates. This can be calculated using the size distributions obtained from the model presented here, after they have been combined with an experimental determination of some empirical constants.
Time Variations of Sediment Floc Size and Density by using Settling Column Data
Civil Engineering Research Journal, 2021
The current study was conducted to examine the sediment floc size and density changes over time in quiescent water by experimenting with a plexy glass settling column with 0.19 m in inner diameter and 3 m in height. The experiments were done with 5 initial concentrations of 3, 5, 10, 15 and 20 g/l and suspended sediment concentration was measured at different time and height intervals. Mclauglin differential equation was used to obtain the settling velocity of the sediments and Kranenburg's equation, and Stokes' Law relationship were solved to estimate the geometrical characteristics of the flocs. In all experiments, the maximum settling velocity of particles occurred 15 minutes after the beginning of the settling process, which was in agreement with results obtained by other researchers. The results show that the maximum settling velocity of sediments is about 8 times the average settling velocity of the sediments. The results of floc density calculations show that the floc density gets to the minimum value after 15 minutes of the start of the experiment, and it is concluded to the primary particle density. Also, the floc diameter reaches to the maximum value after 15 minutes of the start of the experiment because the flocs reach their maximum size at this time.
Flocculation and its effect on the vertical transport of fine-grained sediments
Hydrobiologia, 1992
Recent experimental and theoretical work on flocculation and settling speeds of flocs is reviewed. On the basis of this work, an accurate and computationally efficient model of the aggregation and disaggregation of fine-grained sediments is proposed. This model is then used to predict flocculation times and steady-state floc sizes for a wide range of environmental conditions. The predicted flocculation times are smaller, sometimes by as much as two orders of magnitude, than those predicted by mono-disperse theory. The model is also used to show that the disaggregation of flocs due to increased shear near the sediment-water interface may be a possible mechanism for the increased concentrations often observed near this interface .
A comparison of in situ techniques for estuarine floc settling velocity measurements
Journal of Sea Research, 1996
An Intercomparison Experiment carried out In the turbidity maximum of the Elbe estuary aimed to determine the relative performance of a number of methods of measuring the settling velocity of estuarine floce. These comprised several Owen Tubes, with different sampling protocols, side withdrawal tubes, a settling box, and two in situ video systems. There were significant differences between the results which may partly relate to small-scale spatial and temporal patchiness in the turbidity field. Owen Tubes generally give settling velocities an order of magnitude smaller than the direct video measurements, which may Indicate that the tubes disrupt floce on sampling. Between Owen Tubes different methods of calculating the results may lead to different median settling velocities, especially at low concentrations. However, a well-controlled sampling protocol with the settling tubes gave consistent results. The direct video methods appear to give comparable results, and need to be evaluated further.
Continental Shelf Research, 2004
Measurements of flow, turbulence and suspended sediment concentration were carried out in a tidal creek that acts as a conduit for water and sediment to an intertidal salt marsh. A laser diffraction system (LISST), optical backscatter sensors (OBS) and acoustic Doppler velocimeters (ADV) were used to measure suspended sediment concentration and particle settling velocity variability throughout various tidal cycles for spring and neap conditions. Results suggest that ADV is a very useful system capable of measuring both sediment concentration and settling velocities. During neap tides, sediment in the tidal creeks is primarily in the form of flocs with a settling velocity ranging between 0.02 and 0.20 mm/s and a mean particle size of 25-75 mm. This population of sediment is enriched by both larger flocs and larger individual grain particles, during spring tides. The results indicate that in the tidal creek, erosion can occur only during the ebb stage of the spring tides, while during the flood tides the tidal creeks do not experience any significant erosion. On the contrary, they might act as temporary depositories for the small flocs. Sedimentation on the marsh surface occurs predominantly in the form of flocs, which seems to have the same settling velocities (0.24 mm/s) both during neap-spring tides. This indicates that the neap-spring tidal cycle is controlling marsh sedimentation through sediment availability for deposition and inundation time. Spring tides provide more sediment and more time for the flocs to be deposited and thus contribute more to marsh accumulation. r
Journal of Sea Research, 1998
The fate of fine particulate material in aquatic environments is closely linked to aggregation and disaggregation processes. Understanding the mechanisms controlling these processes is fundamental to the development of predictive models of fate and effects for particulate discharges in the coastal zone from such sources as offshore hydrocarbon exploration and development. One of the variables required for the development of these models is maximal floc size. Using a non-invasive imaging technique, the significance of turbulence, composition, and concentration on maximal floc size in an inverting column flocculator was determined for materials commonly discharged during offshore hydrocarbon development. The settling velocity of the suspension was determined from volume concentrations of samples obtained by pipette during still water settling in a manner similar to that of Owen tubes. After 20 h, both maximal floc size and settling velocity showed a highly significant dependence on turbulence and type of material in suspension, but showed no effect from concentration.
Significance of Aggregation of Fine Sediment Particles in Their Deposition
Estuarine, Coastal and Shelf Science, 2002
The significance of aggregation processes, by which the properties of suspended fine sediment particles or flocs change during transport, is examined for the simple case of deposition of estuarine sediments in a flume. A multi-class model for aggregation processes is combined with a one-dimensional, unsteady, multi-class sediment transport model to calculate the deposition rate for two flume experiments—one with no