Geoscience Reference
In-Depth Information
(a)
(b)
(c)
3, 4
HW
2, 5
+1
1, 6
+3
+11
+5
+9
+7
7, 12
8, 11
9, 10
Fig. 6.40 Tidal current variations with time. (a) Linear symmetrical ebb-flood with zero residual; (b) symmetrical tidal ellipse with zero resid-
ual current; (c) Irregular tidal ellipse with complex residuals.
cause appreciable net sediment transport in the direction of
the residual current. The turbulent stresses of the residual
currents will be further enhanced should there be a superim-
posed wave oscillatory flow close to the bed (Section 4.10).
A further consideration arises from the fact that turbulence
intensities are higher during decelerating tidal flow than dur-
ing accelerating tidal flow, due to unfavorable pressure gra-
dients. Increased bed shear stress during deceleration thus
causes increased sediment transport compared to that during
acceleration, so that the net transport direction of sediment
will lie at an angle to the long axis of the tidal ellipse.
A final point concerns the importance of internal tides
and other internal waves (Section 4.9), particularly in the
outer shelf region. These are common in summer months
when the outer-shelf water body is at its most density-
stratified, with a stable warm surface layer of thickness h
0.3 m s 1 ). Also it is not uncommon for
the inner shelf to shoreface to be tide-dominated during
the summer months but wave-dominated during the
winter. In any case, tidal currents and wave currents are
progressively less important offshore, so that at the outer
shelf margin it is only the largest storms that affect the
bottom boundary layer. In these areas it is common to
find a multilayer water system, with a surface boundary
layer dominated by wind shear effects, a middle “core”
layer, and a basal boundary layer dominated by
upwelling, downwelling, or intruding ocean currents
(Fig. 6.34). Winter wind systems assume an overriding
dominance on most shelves, causing net residual currents
arising from wind drift, wind set-up, and storm surge.
Wind shear causes water and sediment mass transport at
an angle to the dominant wind direction because of
the Ekman effect arising from the influence of the
Coriolis force (see Section 6.2). For example, southward-
blowing, coast-parallel winds with the coast to the left
in the northern hemisphere will cause net offshore
transport of surface waters and the occurrence of
compensatory upwelling.
From all this the reader can appreciate that outer-shelf
dynamics are extremely sensitive to the magnitude of shelf
wind systems. Depending upon dominant wind regime,
either import or export is possible: for example, cool shelf
waters can be driven far oceanward as intruding tongues
that may interfere with ocean currents like the Gulf Stream
(Section 6.4).
tidal currents (
and density
2 . They are also
common in fjords. If a wave motion is set up at the stable
density interface (due to storm-induced wind stress or the
incoming tide), the restoring force of reduced gravity , is
much smaller than at the surface and so the internal waves
cannot be damped quickly; they provide important mixing
mechanisms when they break at external boundaries.
1
overlying a denser layer,
6.5.2
Wind drift shelf currents
Although all continental shelves suffer the action of
storms, weather-dominated shelves are those that also
show low tidal ranges (
1 m) and correspondingly weak
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