Geology Reference
In-Depth Information
frequently develop where the sediment is erodible, such
as unvegetated, fi ne silts and sands on tidal fl ats. Often
a sign of an immature drainage pattern, they commonly
occur on fl ats that are regularly impacted by storms,
thus 'resetting' them either partially or totally. Such
behavior is observed in open regions of Kyenoggi Bay
(Korea) and along the Jiangsu coast (China; Lee et al.
1992 ; Ren 1986 ; described in Eisma 1998 ) , where the
wave energy is high and tidal currents are weaker in the
open-coast environment. The more sheltered regions of
Kyenoggi Bay exhibit dendritic channel networks (Lee
et al. 1992 ). Gullies in sandy sediment are generally
shallower and wider than those in fi ner sediment, and
are more likely to be ephemeral or even absent (van
Straaten 1954 ). The implication is that parallel chan-
nels, often also straight, are transient, potentially being
removed and recreated with every storm.
Areas with few or no channels may also occur in
regions that experience only infrequent tidal inunda-
tion, or freezing or arid conditions for long periods of
time, stabilizing the sediment (e.g. James Bay in south-
ern Hudson Bay, which is covered with ice for up to
6 months of the year; Eisma 1998 ) .
There are of course exceptions: in New South Wales,
Australia, there is a distinct lack of channels in the
marshes (Adams 1997 ). These systems are of limited
size, sitting landward of mangrove forests. Where
drainage does exist it is often inherited from river sys-
tems. The region is microtidal and doesn't fi t most pat-
terns as described above. It is not clear why these
regions lack channels, perhaps it is purely that the
strength of the vegetated soils are suffi ciently high, and
the size of the areas suffi ciently small, that the sheet
fl ow across the marsh surface is unable to initiate chan-
nels, but further research is undoubtedly needed.
Likewise, Hughes et al. ( 2009 ) observe a system of par-
allel channels forming and actively incising into vege-
tated salt marsh platforms across the Santee Delta (SC,
USA). The authors propose that burrowing and her-
bivory by crabs weakens the soils in the region sur-
rounding the head, allowing the creek to erode headward
more easily than on other vegetated marsh platforms.
Tidal currents are the dominant hydrodynamic forcing
in the generation and maintenance of tidal channels.
Fluvial currents (if present) decrease in infl uence with
distance seaward of the tidal limit (i.e. the landward
extent of the tidal wave). The intensity of fl uvial fl ow
depends upon river stage and precipitation, but can be
considered constant over the timescale of a tidal cycle.
Wave energy decreases swiftly with distance from
the ocean. Locally generated wind waves may occur
within very large channels and bays, producing local
erosion of the marsh edge and channel banks. This
may create gullies in tidal fl ats or a “cleft and neck”
morphology on salt marshes (Pethick 1992 ; Watzke
2004 ; Schwimmer 2001, 2008 ) . Clefts, are narrow
channel-like indents in the edge of the marsh platform
and necks are the tracts of marsh remaining between
the clefts. The infl uence of waves in smaller channels
tends to be low because of sheltering.
Tidal areas experience two peak velocities during a
full tidal cycle, which occurs once or, more commonly,
twice a day (tidal period = 25.8 and 12.4 h respec-
tively). The fl ood velocity is directed landward and the
ebb is directed seaward. Depending on the position
within a tidal system, these velocities will vary both in
absolute magnitude and in comparison to each other
( tidal asymmetry ). The bidirectionality of tidal fl ows
makes them distinctly different from fl uvial systems
and has a signifi cant impact on channel morphology.
In general, fl ows within tidal channels are often driven
by gradients in water slope rather than bed slope
(Rinaldo et al. 1999 ). In many areas channel bed slopes
are low, yet fast currents are generated by the variation
in water depth related to the tide. In small, fi rst-order
creeks and across a tidal fl at or marsh platform, how-
ever, bed slope may have more infl uence becoming a
signifi cant force driving fl ows at low stages of the tide.
Unlike rivers, maximum current velocities within tidal
channels do not necessarily coincide with maximum
stage (water depth), but instead occur at some mid-
point during the tidal cycle.
Spatial variations occur both in the magnitude of tidal
fl ows and in the asymmetry of the ebb and fl ood periods
or velocities. These variations result from: (1) variation
of tidal range (prism) across the system, (2) water depth
and its effects in terms of modifying the tidal wave, and
(3) the morphology of the surrounding intertidal area
(e.g., vegetation will retard fl ows during over-bank
events; low versus high gradients on the regions between
channels will produce different fl ow rates).
11.4
Hydrodynamics
Along the continuum from marine to terrestrial set-
tings, tidal environments experience variations in tidal,
fl uvial and wave energy (Dalrymple and Choi 2007 ) .
 
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