EurOCEAN 2000 - Vlaams Instituut voor de Zee

processes of CBS. This is particularly relevant in case of spatially confined occurrences of
CBS/fluid mud, as often encountered in nature.
Flocculation
Because, as far as cohesive suspensions are concerned, laboratory experiments cannot scale the
turbulent properties of natural flows adequately, field measurements were obtained of floc
characteristics. The measurements were taken in September 1998 in the upper reaches of the
Tamar Estuary, and covered the neap and spring tide conditions. In that location the turbidity
maximum is well developed and suspension concentrations of order g/l are present. The aim of
the experiment was to measure floc size and settling velocity and their dependence on salinity,
concentration and turbulent shear.
Two stations about 1km apart in the centre of the channel were deployed simultaneously. At
both stations frequent profiles of velocity, salinity, temperature, and suspended sediment
concentration were obtained, and Owen tube measurements of settling velocity were taken. At
the lower station profiles of floc size were taken with a Lasentec P-100 system (Law et al.,
1997), and near bed measurements of floc size, settling velocity, and effective density obtained
with the INSSEV instrument (Fennessy et al., 1994). Also turbulence parameters were
measured with miniature electromagnetic flowmeters above and below the INSSEV. At the
upper station a LISST laser diffraction particle sizer was deployed, video measurement of floc
size and settling velocities were observed within Owen tube samples. Additionally, samples
were taken for laboratory analysis of the carbohydrate and chlorophyll a contents and CHN
ratios.
Suspended near bed sediment concentrations ranged from 16 mg/l on a neap tide to 7 g/l on a
spring tide. Maximum velocities were 0.5 and 1.5 m/s, respectively. Typical spring tide shear
stresses were 0.68 N/m 2 at the level of the INSSEV. The number of flocs per INSSEV sample
varied between 14 and 1150. Floc sizes up to 600 microns were observed. Examination of the
settling velocity spectra of the flocs has indicated that the settling velocity Ws can be
represented in simple linear correlation by: Ws = 5.784 (Shear) 0.606 and Ws = 0.524 (SPM) 0.249 ,
where Ws is in mm/s, shear is in N/m 2 and SPM in mg/l. Also the ratio of macroflocs to
396
Settling velocity (Ws) mm/sec
10
1
0.1
Selected spring & neap tide INSSEV mean macrofloc (greater than 160 microns)
settling velocities plotted against both shear stress and SPM (0.5 m above the seabed)
Ws = 5.7841SHEAR 0.6063
R 2 = 0.7331
Shear stress SPM
Ws = 0.5242 SPM 0.2489
R 2 = 0.7757
0.01 0.1 1 10 100 1000 10000
Shear stress (N/m^2) Suspended Particulate Matter (SPM) mg/l