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Flocculation Dynamics in San Francisco Bay
 Measurement of in situ particle size is critical to
understanding the dynamics of fine-sediment flocculation and settling. The
methods used to evaluate size distribution range from video camera-based
(Manning and Dyer, 2002) to laser-diffraction, e.g., Laser In-Situ Scattering
Transmissometer -100 (LISST-100; Agrawal and Pottsmith, 1994). The LISST-100
system is an autonomous unit that measures laser diffraction from suspended
particles, and assigns each particle to 1 of 32 logarithmically spaced size
classes (1-250 µm). The output of the LISST-100 is total volume concentration
in each size class. While these results are helpful in analyzing particle-size
evolution, the evolution of mass distribution can be estimated only with
knowledge of the average mass density of each size class. Mass distribution is
necessary to estimate mass concentration and settling flux for each size class,
both of which have ramifications for numerical modeling and contaminant
transport.
Converting a volume distribution to a mass distribution requires knowledge of
the floc density for a given size class. Kranenburg (1994) suggests that
sediment flocs aggregate in a fractal manner, requiring that a given fractal
dimension and primary particle size will yield a specific density for a
corresponding floc size. Varying fractal dimension and primary particle size
will alter the floc density, though actual floc size may remain unchanged. If
one assumes that fractal dimension and primary particle size remain unchanged
over space and time for a given system, then Kranenburg's floc size-density
relation also will hold. This relation then can be applied to
volume-distribution data to yield the mass distribution.
Tidal cycle, synoptic, and continuous measurements of floc size and SSC provide
the means to investigate the floc size-density relation. This relation is
validated with measurements from several sites throughout San Francisco Bay.
The constancy of this relation implies a uniform primary particle size
throughout the Bay, as well as uniform aggregation/disaggregation mechanisms
(which modify fractal dimension). The exception to the relation is identified
during near-bed measurements, when advected flocs mix with recently resuspended
flocs from the bed, which typically have a higher fractal dimension than
suspended flocs. The constant relation for suspended flocs simplifies
monitoring and numerical modeling of suspended sediment in San Francisco Bay.
Publication
Ganju, N.K., Schoellhamer, D.H., Murrell, M.C., Gartner, J.W.,
and Wright, S.A., 2006, Constancy of the relation between floc size and density
in San Francisco Bay. In: Maa, J.P., Sanford, L.H., and Schoellhamer,
D.H., Eds., Estuarine and Coastal Fine Sediment Dynamics, 540 p.,
Elsevier, Amsterdam, Netherlands.
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