Geology Reference
In-Depth Information
have created a large, isolated dune field near the shelf
edge (Heathershaw et al.
1987
).
On the shelf itself, the nature of the tide and tidal
currents is strongly controlled by the complex 3D
interaction of the tidal wave with the geometry of the
shelf and shoreline. On long, straight shelves, the tide
is dissipated by friction as it crosses the shelf, such
that tidal currents decrease in a landward direction
(Fig.
13.3
). As the shelf width increases, however, it
becomes closer to resonance with the semi-diurnal M2
tide: resonance happens when the tidal wave reflected
by the coast is in phase with the incoming wave, which
occurs where the shelf width is equal to one-quarter, or
3/4, or 5/4, etc., of the wavelength of the tidal wave,
which is a function of the water depth (e.g. Pugh
1987
).
Due to tidal resonance, the maximum tidal range
occurs when the shelf is of the order of 200-400 km
wide for typical shelf depths. The influence of chang-
ing tidal range on tidal-current speed is direct, but the
impact is not uniform over the entire width of the shelf;
the greatest change in the strength of the currents
occurs near the shelf margin because this is where the
change in the tidal prism is greatest.
The situation in embayments and semi-enclosed
seas is more complex, with the response of the tidal
wave being dependant on the specific configuration
of the sea and of its connection with the open ocean.
Most open-mouthed embayments accentuate the tide
because the cross-sectional area through which the
tidal wave passes becomes smaller in a landward direc-
tion. Consequently, the tidal range and current speeds
are generally higher in embayments than on straight
shelves. Examples are given by the English Channel,
the North Sea, and the Yellow Sea, and by the Gulf of
Bengal, which is a tectonic embayment fully exposed
to the ocean. The tidal ellipse is also more elongated
and the currents tend toward being rectilinear because
of the confinement by the margins of the embayment.
Other types of tectonic embayments where the tide is
commonly amplified include rifts and foreland basins;
in fact, a significant number of the areas with tidal
ranges greater than 10 m today are in such settings
(Archer and Hubbard
2003
). The prediction of reso-
nance in embayments can only be done using numeri-
cal modeling, with a full knowledge of the 3D geometry
of the shelf and shoreline morphology, as illustrated by
studies of the funnel-shaped Gulf of Maine - Bay of
Fundy system (Greenberg
1979
) and the Western
Channel Approaches that might have gone into and out
of resonance during the early stages of the last post-
glacial transgression (Uehara et al.
2006
). By compari-
son, semi-enclosed seas such as Hudson Bay and the
Baltic Sea are more likely to have small tides because
the oceanic tidal wave cannot propagate into them
effectively, and they are not large enough to have their
own tide. Again, the specific response can only be
determined by numerical modeling.
Local coastal irregularities such as headlands also
perturb the tide. Horizontal flow expansion and con-
striction on either side of a headland brings about a
complex 3D tidal asymmetry, which results in a resid-
ual flow that takes the form of time-averaged eddies on
either side of the protuberance (e.g. Pingree and
Maddock
1979
).
Seaways and straits that connect two larger bodies
of water are especially prone to pronounced accentua-
tion of the tidal currents because of the constriction.
Even a small difference in water elevation at the two
ends of a strait can generate strong currents (Pratt
1990
). This is the case of the Messina Strait in the
modern Mediterranean Sea, despite the fact that the
tidal range is less than 10 cm (Androsov et al.
2002
),
with dunes forming in water depths of more than
several hundred meters (Colella
1990
).
13.2.2 Residual Tidal Currents
Because each tidal constituent is oscillatory and
symmetrical, the net flood and ebb currents should be
equal and opposite. However, the examination of mea-
sured tidal ellipses show that they are not symmetri-
cal: the peak ebb and flood currents are neither equal
in speed, nor are they colinear. This is due to the dis-
tortion of the tide and/or to the interplay of more than
one tidal constituent. The most important of these is
the interaction of the M2 (semidiurnal) tide with its
first (M4) harmonic (Pingree and Griffiths
1979
; see
more below).
Distortion of the tidal wave occurs due to topographic
effects. As the tide moves into shallow water, it slows
down because of friction, but with the trough decelerat-
ing more than the crest because the water depth is less
beneath the trough. The consequence is the develop-
ment of tidal asymmetry, with the front of the tidal wave
(i.e., the flood tide) being steeper and of shorter duration
than its back (i.e., the ebb tide). This, in turn, brings
about an inequality of peak flood and ebb current speeds,
