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(b)
High
Flood
LT
H T
LT
Low
Ebb
High
(c)
Peak current velocity
High
High current velocities -
mud kept in suspension
LT
HT
Low current velocities -
mud settles out
Flood tide
Low
Time
Mud
Sand
Mud
Mid-tide sand shoals
Fig. 7.5 (Continued)
asymmetry, this point of maximum velocity
need not be at the half way mark). The patterns
of sedimentation will reflect this (Fig. 7.5c).
Where the current velocities are lowest, clays can
settle out, but when they flow faster, clays are kept
in suspension and only the coarser sediments
can settle. Bearing in mind that this variation in
current velocity is paralleled by a rising and
falling of the tide, these patterns of deposition
are shown spatially by areas of sediment accre-
tion, such as subtidal muds (below low water),
mid-tide sand flats where velocities keep clays in
suspension, and high-tide mud flats where clays
can again settle out.
A further aid to increasing sediment transfer
from the water body to the sediment surface is
the presence of vegetation (Fig. 7.5a). Sediment
surfaces tend to be relatively smooth in respect
of the ability to cause friction with the overlying
water body. Hence, the energy lost by waves
moving over a mud flat is relatively small. In
contrast, vegetation is rough and provides much
more of a barrier. Waves running over a veget-
ated surface will, therefore, use considerably
more of their energy, which means that they
will lose speed and drop more of their sediment
load. This ability for vegetated surfaces (see also
mangroves, Chapter 9) to dampen wave energy
is important in estuaries and deltas because it
not only facilitates vertical sediment accumula-
tion, but also reduces the wave energy reaching
the landward limit of the marsh (often a sea wall).
Work on the North Norfolk (UK) marshes
(Moeller et al. 1996; Shi et al. 2000) has shown
that after crossing 180 m of a vegetated marsh
surface, waves typically lose up to 80% of their
energy. Not only does this represent an aide to
coastal defence in the area, but it also means that
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