Geology Reference
In-Depth Information
11.4.1 Tidal Range
relative water surface slope between the platform and
the channel is steeper, creating faster fl ows. As water
on the platform surface becomes very shallow, fl ows
returning to the channel may be driven by bed slope.
As a consequence, the magnitude and timing of peak
velocity during the ebb tide are altered (Friedrichs and
Aubrey 1988 ; Fagherazzi et al. 2008 ) .
While these two factors are the principle controls
on asymmetry in most tidal environments, in these
complex systems there are often other factors.
Location can be of great importance to the tidal asym-
metry and very local variations may be seen across a
channel or either side of a shoal. This is particularly
notable in meandering channels or in the deeper sub-
tidal regions of the inner estuary. Li and O'Donnell
( 2005 ) examine the behavior of fl ows is subtidal chan-
nels, comparing long and short channels. This study
neatly demonstrates that in estuaries that are long in
comparison to the tidal wave, the seaward regions are
likely to experience ebb dominance in deeper regions,
with fl ood dominance on shoals. In contrast, short estu-
aries and the upper reaches of long estuaries will exhibit
fl ood dominance in deep channels and ebb dominance
in shallower subtidal regions. This is the result of the
nature of the tidal wave, whether it behaves as a stand-
ing wave (in short channels) or a progressive wave (in
the outer part of long channels). Residual sediment
transport within an estuary will be integrated across
these local variations and, thus, it will be infl uenced by
the tidal asymmetry throughout the entire system and
calculations of this parameter should not be based
purely on measurements in the main channel.
In regions with diurnal tides (e.g. the Louisiana
coastal plain), where the K1 and O1 tidal constituents
are very signifi cant in comparison to the semi diurnal
M2 tide, tidal asymmetry (in the ebb direction) is
directly related to the ocean tidal wave rather than to
shallow water effects (known as overtides) or the hyp-
sometry of the drainage network (i.e. the relative extent
of the marsh platform or tidal fl at to the channel;
Howes 2009 ). This asymmetry of the fl ow at the tidal
inlet may propagate throughout the system, underlying
further modulations upbasin.
Finally, there is a potential infl uence of fl uvial dis-
charge, which, if signifi cant, can produce apparent ebb
dominance towards the tidal limit as the fl ows are
superimposed (Wolanski et al. 2006 ; Dalrymple and
Choi 2007 ) .
Tidal range is proscribed by the offshore tidal wave,
which varies according to latitude, the shape of the
ocean basin and the width of continental shelf (Davis
and FitzGerald 2004 ). Within a geographically exten-
sive tidal system (mega-scale), tidal range may vary in
both timing and magnitude. Given the forcing of the
offshore tidal range, the magnitude of a tidal signal in
a region is primarily a response to bed morphology. In
wide-open basins and back-barrier areas, the signal
will experience a gradual reduction in amplitude inland
( hyposynchronous ). However, a funnel shaped estuary
may experience amplifi cation of the tidal wave inland,
before reducing to zero at the tidal limit ( hypersyn-
chronous , Dyer 1997 ). This results in two zones of
similarly weak tidal infl uence occurring seaward and
landward of a strongly tide-dominated zone (Dalrymple
and Choi 2007 ), the Bay of Fundy (Canada) being the
classic example.
11.4.2 Asymmetry of Tidal Currents
Essentially there are two reasons for inequalities
between the magnitude of fl ood and ebb velocities or
the respective periods over which they fl ow. The fi rst is
the fi nite amplitude effect (also called the shallow-
water effect). In shallow water, the difference in depth
between the crest and trough of the tidal wave is sig-
nifi cant; therefore water under the crest (i.e., high
water) will move faster than water under the trough as
the celerity of a wave is proportional to the water depth
( c gμ where g is gravity and h is the water depth)
(Dronkers 1986 ; Parker 1977, 1991 ; French and
Stoddart 1992 ) .
The second cause of tidal asymmetry is morpho-
logical. The presence of extensive intertidal regions
has an impact on the timing of the fl ood and ebb (par-
ticularly in the presence of vegetation). The slower
propagation of the fl ood and the ebb over the platform
leads to both a delay in the turn to ebb and a slower
returning fl ow to fi rst-order channels. The delay in the
turn of the tide shortens the ebb, and continuity requires
that the velocities need to be faster to move the same
tidal prism during this shorter period of time. Physically
the fl ows in the channel can move more easily than
fl ows over the platform, so during the ebb tide the
 
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