Laboratory studies show that turbulence controls the size of flocs by
disrupting those flocs which exceed a critical diameter. Estuarine floc sizes have
been shown to vary with the spring/neap cycle and turbulence has been suggested
as the mechanism.
A survey of the tidal variations of cohesive sediment floc size
distributions and turbulence parameters has been undertaken in the Tamar estuary
in south-west Britain. In-situ particle size distributions have been obtained using a
'marinised' version of the 'Malvern' laser diffraction sizing system. Turbulent
current speeds were obtained using 10 cm diameter annular electromagnetic
current meters. Velocity data is analysed using the inertial dissipation method to
provide turbulent dissipation rates.
Turbulence and size data, along with profiles of current, salinity,
temperature and suspended solids concentration, record the passage of turbidity
maximum and salt intrusion over four complete tidal cycles.
Time series of observed particle size distributions vary smoothly over
timescales of about one hour and these variations are linked to the flow
conditions. Eight subsections of the tidal cycle were selected over which size
distributions and flow conditions were slowly varying and the size distributions
were time averaged over these subsections, and the resulting distributions
compared.
Size distributions in the turbidity maximum are strongly influenced by
the mean current speed and this is found to be due to the different resuspension
characteristics of newly formed aggregates and consolidated primary particles.
Distributions are less dependent on tidal range at other stages during the tidal
cycle.
The size dependence of settling velocities strongly influences the size
distribution of particles reaching the bed during the final stages of erosion of the
salt intrusion, when the salt/fresh interface descends at a rate less than the
settling velocity of large flocs but greater than that of small particles. This tends
to sharpen the downstream edge of the turbidity maximum and preferentially retain
floc aggregates in the upper reaches of the estuary.
Date of Award | 1991 |
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Original language | English |
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Awarding Institution | |
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Observations of Estuarine Turbulence and Floc Size Variations
McCabe, J. C. (Author). 1991
Student thesis: PhD