Geology Reference
In-Depth Information
7.3.1.2 Tidal Asymmetry
An important consequence of hypersynchronous tidal
amplifi cation is the development of an asymmetry in
the ebb and fl ood limbs of the tidal wave. In this case
the wave crest (high tide) propagates faster than the
wave trough (low tide), causing the fl ood period (low
to high tide) to shorten and ebb period (high to low
tide) to lengthen. This time asymmetry requires higher
current velocities for the fl ooding tide to accommodate
the tidal prism, and is described as being a fl ood-dom-
inant tidal system.
Given that the rate of sediment transport (
y
)
increases as a power function (
b
) of current velocity
(
x
), where
y = ax
b
with
b
= 1.6-2.0, most fl ood-dominant
tidal systems result in a net onshore-directed trans-
port of sediment, an effect called “tidal pumping”
(Postma
1967
). This effect may have fundamental
implications for the morphology and behavior of
tide-dominated delta systems (see Sect. 7.4.1), but its
nfl uence likely varies spatially and temporally with
such factors as river discharge. For example, where
river discharge is high the net fl ow and sediment trans-
port patterns may be signifi cantly altered or even
reversed from the tidal signature alone. In general low
river discharge allows a net upstream (landward) trans-
port of sediment (e.g., during the dry season), whereas
high discharge weakens this tidal-pumping effect and
forces net offshore transport. These natural patterns
in tidal pumping and sediment transport may be
considerably altered on rivers with large dams used to
artifi cially control water discharge (Wolanski and
Spagnol
2000
) .
7.3.1
Tidal Processes
7.3.1.1
Tidal Amplifi cation
At the offshore limits of the delta system, the incom-
ing ocean tide fi rst interacts with the clinoform delta-
front, where water depths shoal from 20 to 90 m at
the bottomsets to 5-30 m at the topset-foreset roll-
over point, a distance typically of a few tens of kilo-
meters for megadeltas to a few kilometers for smaller
deltas (Fig.
7.3
; Storms et al.
2005
) . Tidal currents
accelerate across this zone from <20 cm/s on the
open shelf to 30-80 cm/s on the outer delta-front
platform (ie., topsets), still tens of kilometers off-
shore. This acceleration across the prograding delta-
front represents an important morphodynamic
feedback that in large part is responsible for forming
the compound clinoform that is typical of most tide-
dominated delta systems. In this case strong bed
shear on the inner shelf (i.e., delta platform) defi nes
a zone of limited deposition that separates the pro-
grading subaqueous and subaerial clinoforms
(Fig.
7.3a
; see also Sect. 7.3.3.2 ).
After crossing the delta-front platform (i.e. topsets)
the progressive tide wave becomes channelized as it
propagates upstream of the shoreline, inducing a sec-
ond phase of energy focusing that accelerates tidal cur-
rents to velocities of 50 to >100 cm/s. This acceleration
continues for a signifi cant distance upstream (10s of
km) due to tidal amplifi cation. Although tidal energy is
lost to friction, the local tidal power is actually ampli-
fi ed by the decreasing cross-sectional area of the nar-
rowing channels. This is called a hypersynchronous
channel system, whereby tidal height and current
velocities increase steadily upstream before declining
to zero as tidal energy becomes increasingly attenu-
ated by frictional forces.
Due to this positive feedback of tidal amplifi cation
across the shallow prograding delta-front and tapering
delta-plain channels, tides actually infl uence a much
larger reach of the continental margin than they would
in the absence of the delta. In larger tide-dominated
deltas, this enhanced tidal infl uence may extend
100-200 km across the margin (Fig.
7.5
). In general
tidal-bed shear in this broad reach is suffi cient to
impart a strong infl uence on sediment transport and
deposition, although preservation of tidal signatures in
the sedimentary record is less certain (see 7.4.2 ).
7.3.2
Fluvial and 'Estuarine' Processes
The evolution of tidal hydrodynamics at the coast is not
only infl uenced by seabed and shoreline morphology
but also by interactions with freshwater discharge. In
the case of most tide-dominated deltas, the interaction
of large river-water fl uxes and meso- to macro-tidal
regimes tend to generate strong horizontal shear, tur-
bulent eddies, and vigorous vertical mixing. Such
dynamic fl ows are generally adequate to preclude
density stratifi cation and result in a well-mixed estuary
at the delta rivermouth. Therefore buoyancy-driven
gravitational circulation is not as significant in
