|Macrovlokken en hun bijdrage aan de slibtransporten in estuaria|
van Leussen, W. (1994). Macrovlokken en hun bijdrage aan de slibtransporten in estuaria. National Institute for Coastal and Marine Management: Den Haag. ISBN 90-369-0483-8. 477 pp.
Estuaries; Flocculation; Sediments; Brackish water
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Laboratory experiments and field measurements have been conducted to obtain more quantitative knowledge on the properties of the so-called estuarine macroflocs, particularly their settling behaviour, and to understand their role in the dynamics and transport of fine-grained sediments in estuaries. Turbulence is shown to be of particular importance due to its limitation of the floc growth. However to understand the effect of turbulence, regions of low and high turbulence intensity should he distinguished. In fact the flocs are continuously moving through zones of varying turbulence intensity they participate in a continuous process of aggregation and breakup.
The suspended sediment concentration is an important parameter in determining the settling velocity of mud floes, but particularly at low levels of turbulence. At high levels of turbulence the limiting properties of turbulence dominate. This explains why no effect was observed of the suspended sediment concentration on the sizes' and settling velocities of the macroflocs in the Ems Estuary.
An in situ under water videosystem VIS (=Video In Situ) has been developed, with which the sizes and settling velocities of the macroflocs can be determined directly in nature without destroying these very fragile aggregates.
In the Ems Estuary an abundance of macroflocs was observed shortly after maximum current velocity. Generally these macroflocs had sizes in the range of 200 to 700 µm, and sometimes more than 1 mm, and survived high current veloeities (> 1 m/s). The settling velocities were in the range of 0.5 to 8 mm/s.
These fragile macroflocs consist of the more firmly bound microflocs. Although biological processes are the primary factor in the formation of the microflocs, the formation of the macroflocs is principally physical in nature.
Owing to their fragile structure, the macroflocs are generally destroyed by usual sampling methods, including field settling tubes. This means that in the case of large aggregates the field settling tubes give values of settling velocities which are too low, and can not explain the often observed phenomenon of "rapid settling".
The short-term vertical transports (erosion /sedimentation cycles) are the "building blocks" for the ultimate long-term transports of fine-grained sediments under tidal action. This means that the settling behaviour of the mud flocs, which is generally governed by the macroflocs, together with a number of other relevant processes, such as resuspension and vertica1 mixing, plays an important role in the fine-grained sediment transport in estuaries.