Geoscience Reference
In-Depth Information
(a) (b)
Low
tide
Low
tide
High
tide
High
tide
Time
Time
0
0
Critical velocity
for sediment transport
Critical velocity
for sediment transport
Sediment transport 'window'
Fig. 1.8
Graphs showing changes in ebb- and flood-tide velocities within (a) symmetrical and (b) asymmetrical tide-cycle settings.
Within each phase of the tidal cycle sediment transport occurs only when velocities exceed the critical thresholds for transport. In
symmetrical settings, sediment is moved back and forth but there is no net sediment transport direction. In asymmetrical settings, a
stronger ebb- or flood-tide phase may result in a net direction of sediment transport.
As flow velocities increase, upper flow regimes
(Froude number
frictional effects of high vegetation cover on the
mangrove flats (see Chapter 9).
Sediment transport is also initiated in near-
shore (marine or lacustrine) environments where
wave-generated water motion interacts with
the shoreline substrate. Waves are generated by
the frictional effects of wind and this initiates
water particle motion within the upper part of
the water column. The orbital particle motion
decreases with depth (Fig. 1.9a) until it reaches
effective wave base (defined as half the wave-
length), below which there is no wave-induced
water motion. In open, deeper water, water
motion therefore exerts no influence on sea-floor
substrates, but as the water shallows nearshore
the oscillating water particles start to interact
with the sea-bed (Fig. 1.9b). As this occurs, the
water particles move in an increasingly ellip-
soidal fashion and initiate on- and offshore
movement of sediment.
Transport in aqueous fluids may also occur in
density currents. These are associated both with
traction and suspension transport, and occur due
to density differences between two fluid bodies.
Within aqueous environments, density differences
commonly result from variations in temperature,
salinity or suspended sediment load where two
bodies of water meet. Where the fluid entering a
body of water has a higher density, for example
where sediment-laden water enters a lake, the
denser fluid will flow beneath the less dense fluid
1), characterized by turbulent
flow, are reached and under these conditions the
sediment bedforms are initially smoothed out
to form planar beds and eventually antidunes
which may migrate upstream (Fig. 1.6b). As flow
reduces, a reverse sequence is followed. Hence
through cycles of river flooding, the mechanisms
and processes of sediment movement change
with flow velocity (see Chapter 3).
In contrast, shallow marine environments are
characterized by bi-directional flow, although
the magnitude and frequency of the flood- versus
ebb-tidal phase varies depending upon local
tidal regime and nearshore geomorphology.
The potential for sediment transport changes
through each tidal cycle as flow velocity in-
creases through either the ebb- or flood-tide
phase and then decreases approaching either low
or high tide ('slack' water). In settings where the
tide cycle is symmetrical (Fig. 1.8a) sediment
will be reworked first seaward and then land-
ward, but there will be no net transport in either
direction. It is more common, however, for tidal
cycles to be asymmetrical (Fig. 1.8b), and under
these conditions there will be a net sediment
transport direction. This situation is common in
many estuaries where fluvial outflow exerts an
influence on the tidal cycle (see Chapter 7), or in
mangrove settings where strong ebb-tide flows
in the mangrove creeks can occur owing to the
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