Geology Reference
In-Depth Information
(
2003
,
2004a, b
). The study discussed but did not
determine the process of this evolution. Non-linear
interactions in the model lead to a stable regular pat-
tern developing from an initial perturbation, produc-
ing realistic estuarine morphologies that change
progressively up estuary from alternating bars in the
outer estuary, to channel-shoal mid-estuary and mean-
dering channels with bars in the inner reaches. The
decrease in meander wavelength (and thus shoal size)
inland seems to be a response to changing depth and
width to depth ratio. The braided channeling mid-
estuary occurs where the ebb and fl ood currents both
reach high velocities at approximately the same water
depths, whereas the inner estuary is more likely to
exhibit peak fl ows closer to high tide. This variation in
velocity-stage relationship along the length of the
channel is a result of the gradual change from a pro-
gressive to a standing tidal wave within a long estuary.
This could perhaps explain the resulting morphology,
however, questions remain. The use of differing sedi-
ment-transport formula in the model produces differ-
ent scales of morphology and the actual processes
causing these morphological responses to the tidal
wave are still not understood fully. The model is also
yet to include cohesive sediments or vegetation
(Hibma et al.
2004a
) .
Seminara (
2006
) questions the similarity of the pro-
cesses forming meandering channels in cohesive and/
or vegetated soils, to those in more easily eroded sedi-
ment. He conjectures that in small, dead-end, salt
marsh channels, meandering may occur purely by ero-
sion. Often in the smallest fi rst-order creeks no deposi-
tional features, such as pointbars, are seen. A
symmetrical cross-section might limit morphological
feedback with fl ow and thus the position of the ero-
sional maxima, potentially creating a slightly different
shape of meander. These questions warrant further
investigation.
2006
). Migration of a channel and creation of lateral
deposition requires a suffi cient supply of sediment
and fl ows capable of eroding sediment from channel
margins (Braudrick et al.
2009
) . The latter condition
will be a function of both the fl ow conditions and
the erodibility of the sediment. As a consequence
the migration of tidal channels is related to setting
as well as the size of the channel, which will control
the rate at which it can migrate (larger channels are
more stable).
Tidal channels in salt marshes are considered
highly stable and lateral movement ranges from a
few centimeters a year to imperceptible depending
on the vegetation and the channel size (Redfi eld
1972
; Garofalo
1980
; Gabet
1998
) . On the contrary,
Hood (
2010
) observes active development of mean-
ders in tidal channels in a deltaic setting, with lat-
eral channel migration varying with channel width
but on the order of meters per year. In the mid and
outer reaches of estuaries, where sediment is more
likely to be non-cohesive, channels may be more
dynamic. Likewise, in channels that periodically
experience a strong fl uvial infl uence may also expe-
rience periodic migration or channel bank erosion
(Allen and Duffy
1998
) .
In general the rates of migration decrease toward
the tidally-infl uenced sections of river systems (French
and Stoddart
1992
; Gabet
1998
; Fagherazzi et al.
2004
). Reworking of the sediments by tidal channels is
signifi cantly lower than that in river systems in com-
parison to vertical accretion (Howard
1996
) . This
explains why the morphology of meander bends in
tidal systems is unlikely to display the typical scroll
bar deposits observed in fl uvial systems.
11.6
Geomorphic Relationships
A number of relationships has been determined to
quantify the morphology of tidal channels in tidal fl ats
and salt marshes using a combination of aerial photog-
raphy and fi eld surveys (Rinaldo et al.
1999
; Fagherazzi
et al.
1999
; Marani et al.
2002,
2003
) . The relation-
ships reported here describe channel dimensions and
network distributions in shallow intertidal settings.
Where stated, they may also apply to subtidal environ-
ments, but will not necessarily scale up to deeper
coastal zones, such as the outer reaches of an estuary
(Rinaldo et al.
1999
) .
11.5.4 Channel Migration
Migration of channels has the potential to produce
signifi cant depositional features through lateral
accretion. In fl uvial systems, migrating meander
bends may produce a series of asymmetrical ridges,
parallel to the meander described as
scroll-bars
,
however, these features are less common in tidal
environments (Howard
1996
; Seminara
2006
; Hood
