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platform tends to be typified by horizontal
sediment layers (topsets), the finer sands of the
delta slope by seaward dipping (10° to 25°)
layers (foresets), and the finest sediments of the
prodelta by horizontal to gently inclined layers
(bottomsets) (see Fig. 7.4).
Although it is relatively simple to under-
stand how a sand or gravel particle can settle,
the settling of clay particles is more complex.
Estuarine sediments typically comprise large
volumes of intertidal or subtidal muds (mud
flats). Clearly, the finer the grain size, the longer
a sediment particle will take to settle. Stoke's
Law, a method of estimating such settling
times, states that for fine particles less than
100
It is this process that explains why estuaries are
such active sinks for muddy sediments. Import-
antly, this ability of clays to attach to other things
in the water body is not only restricted to other
clay particles. Clays may also attract to a range
of contaminants, thus making muddy deposits
potentially rich in a range of environmental
contaminants (see section 7.4.2).
Although flocculation appears a good explana-
tion for the occurrence of mudflats in estuaries,
many researchers have identified a major discrep-
ancy in that there appears to be too much mud
deposited if flocculation was the only mechan-
ism. McCave (1970) details studies from the
German Bight that help explain this. He iden-
tified that the bottom of the water column is
characterized by a viscous layer that moves just
above the mudflat surface. This layer contains not
just recently flocculated material that has sunk
into it from above, but also material brought
in by the tide, and reworked from elsewhere in
the estuary. From this viscous sublayer, quasi-
continuous sedimentation occurs throughout the
tidal cycle (Fig. 7.5a). Thus, deposition of clays
need not be restricted to the head of the salt
wedge or to periods of slack water.
Flocculation and quasi-continuous sedimenta-
tion, therefore, make the formation of extensive
deposits of mud within a relatively high-energy
environment possible. There are, however, other
controls over sediment deposition and reten-
tion. Even though clay flocculates and forms
larger agglomerations, these will fall through
the water column only when current velocities
are low enough. On a tidal cycle, water moves
at different rates depending on the stage of
the tide (Fig. 7.5b). When the tide is fully out
and on the turn, water velocity is at its lowest
(theoretically, this has to be zero for a period of
time for the water to stop moving out, and start
moving back in). As the tide floods, it picks up
speed before starting to slow again towards high
tide. Similarly on the ebb tide, speeds increase
as the tide ebbs, before slowing towards low
tide. Current velocities, therefore, will be at their
lowest at high and low water, and at their highest
at some point between (note: owing to distor-
tion of the tidal wave and the production of tidal
m (silts and clays) the settling velocity of
a particle is proportional to the square of the
grain diameter. For coarser particles of over
2 mm diameter (coarse sands and above), the
settling velocity is proportional to the square
root of the diameter. The implications of this
are that fine clay particles in estuaries will settle
very slowly. Pethick (1984) demonstrates that
according to Stoke's Law, a 2
μ
m clay particle
will take 56 days to settle through 0.5 m of still
water. This is clearly not possible in a natural
environment because water is only really still
for very short periods. Theoretically, therefore,
there is no way a clay particle could ever settle
in estuarine or deltaic situations. To explain this
apparent discrepancy a mechanism is required
to enable fine clay particles to settle through
significant depths of moving water.
The answer relates to the fact that estuaries are
areas of brackish water. Clay particles behave
rather differently from sand or silt grains in
that they possess a surface attraction, which is
amplified by the presence of only a few parts per
thousand of salt in the water. Hence, when clays
suspended in fresh water enter an estuary and
mix with saline water, their attraction to each
other increases. As clay particles adhere to one
another they form agglomerations of particles
called flocs. The name for this process is floccula-
tion (Krank 1973, 1975). As more clay particles
stick together and the flocs increase in size, their
effective settling weight increases and so the flocs
can settle much faster than the individual grains.
μ
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