Geology Reference
In-Depth Information
and do not truly scale with size. The complexity and
variability of tidal channels is ultimately a function of
the heterogeneity of the fl ows, sediments and ecosys-
tems within the intertidal and subtidal systems. Perhaps
it was this that led Rinaldo et al. (
2004
) to describe
tidal channel networks as: “arguably a consequence of
a frustrated tendency towards critical self-organization”,
because so many factors act to inhibit this self-
organization and thwart scaling within the system.
Despite the scale invariance seen within intertidal
channel networks, all tidal channels consistently main-
tain an equilibrium between channel cross-section and
tidal prism. Likewise, there seems to be a continuum
of bar-channel morphology within estuaries, although
further research is needed to explore this hypothesis.
Classifi cations of network morphology differenti-
ate between channels based on the level of elabora-
tion, and the shape of a network (if a network is indeed
formed). Initiation of channels and the formation of
networks occur through both erosive and depositional
processes. Active channel systems may refl ect present
conditions, or exhibit inheritance from paleo-channels.
Residual circulation patterns and presence of bidirec-
tional fl ow create high spatial variability in hydrody-
namics and there is, as a consequence, a great deal of
potential for overlap in both the processes and the
resulting morphology and stratigraphy that are
observed in tidal channels.
Sinuosity is common and is exhibited by channels
in all tidal environments, however, channels tend to be
more sinuous where the substrate is vegetated or more
diffi cult to erode (cohesive sediments). The process by
which meanders form is still not well understood. It is
also unclear how mobile the meanders in channels in
salt marsh or in very cohesive sediments may be. Salt
marshes have the same width to meander-length rela-
tionships as other channels but have an individual pop-
ulation when width is compared to depth. This perhaps
supports the hypothesis that they inherit their form
from tidal fl ats or fl uvial systems, vegetation stabilizing
their original meander geometry while the channel bed
deepens and the banks accrete vertically (Marani et al.
2003
). This observation, however, does not seem
consistent with the higher sinuosity associated with
meanders in vegetated environments, which implies
increased elaboration after colonization by vegetation.
Furthermore, it is unknown whether meanders in small
salt marsh creeks experience a different evolution
because of the lack of morphological feedback through
the formation of pointbars. Additional research is
necessary to address these questions.
In planform, tidal channels can often be identifi ed
by cuspate meanders, associated with the mutually
evasive fl ood and ebb fl ow paths. Tidal point bars are
often skewed in direction of dominant fl ow, and
detached from the channel bank, with a subordinate
current barb forming at the inner meander bend.
Preservation of tidal channels occurs through infi ll-
ing as tidal prism changes over time and or lateral
accretion as a channel migrates. Deposition occurs in
particular at tidal pointbars, making our understanding
of meandering in these channels all the more impor-
tant. The range of facies expected within a pointbar
varies with morphology and with setting (according to
mud content) but the presence of IHS bedding and
moderate to high levels of bioturbation are two key
indicators of deposition in a tidal channel environment.
The three-dimensional internal architecture of the tidal
pointbars has not yet been extensively examined and is
another topic that warrants further research.
References
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Abad JD, Garcia MH (2009b) Experiments in a high-amplitude
Kinoshita-generated meandering channel. 1: implications of
bend orientation on mean and turbulence fl ow structure.
Water Resour Res. doi: 10.1029/2008WR007016
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