Geology Reference
In-Depth Information
marsh channel, the rates that were observed were
higher in the muddier freshwater tidal channels than in
the heavily rooted salt marsh channels. This is consis-
tent with the observations made in
Sect. 11.3
.
In general, large channels are more stable than
smaller channels in a similar setting due to the relative
volume of sediment transport that is required to make
any change (Eisma
1998
). Large channels tend to be
less sinuous, and fl ow speeds are often lower in straighter
sections of a given channel (Eisma
1998
) . Elaboration
or migration of large-scale (tens of meters wide) or
macro-scale (hundreds of meters wide) channels would
be likely to occur on the scale of decades to centuries
(Eisma
1998
, c.f. van Proosdij and Baker
2007
) .
The formation of meanders is very likely to be
related to the periods of strongest fl ow as their evolu-
tion depends on erosion (Ashley
1980
) . The timing of
peak currents varies throughout a tidal environment
(
Sect. 11.4
), but in large creeks this condition is likely
to occur at mid tide, when water is lower in the chan-
nel. In smaller creeks this peak current velocity may
occur closer to high tide (just after bankfull condi-
tions; Figs.
11.5
and
11.7
). It is unclear if this has any
effect on meander evolution. The key observation to
make when considering meanders in tidal channels is
that tidal fl ows are not steady, but reverse on a rela-
tively short time scale (compared to meander evolu-
tion), and high velocities are not maintained for long
periods. This may limit the time during which erosion
thresholds are exceeded and prevent the development
of full meanders (as proposed by bend-instability the-
ories for rivers).
Meanders in fl uvial systems may be skewed, a
geometry that is sometimes termed
goose-necking
(Fig.
11.8b
, Fagherazzi et al.
2004
; Seminara
2006
) . It
occurs because the streamline of highest velocity does
not necessarily coincide with the channel axis. Thus,
the peak erosion on the outside edge of a meander may
not coincide with the apex of the meander curve. If the
erosion is suffi cient, the feature may migrate in the
direction of the skewness (Seminara
2006
) . In tidal
channels the fl ow is bidirectional, but the streamline of
the highest velocities during an ebb tide may not take
the same path as the streamline during the fl ood
(Figs.
11.8
and
11.9
). As a result, peak erosion occurs at
different points of the meander during the fl ood and the
ebb. Depending on the relative strength of the ebb and
fl ood (tidal asymmetry) the meanders may be skewed
or symmetrical (Fig.
11.8
, Fagherazzi et al.
2004
) .
Fig. 11.8
Meander morphology evolving from (
a
) an initially
sinuous channel, under conditions of: (
b
) unidirectional fl ow;
(
c
) bidirectional fl ow with a notable ebb-dominance; and
(
d
) bidirectional fl ow with equal fl ood and ebb currents. (
e
)
Shows the position of the streamline of highest velocity fl ow
in comparison to the central channel axis, for fl ood and ebb
conditions. The position of peak erosion is indicated for each
case on each meander by a
star
, here the tidal streamline is
closest to the bank and this highest velocities experienced
along the bank will occur at these point. These positions vary
notably between the fl ood and ebb fl ows (Adapted from
Fagherazzi et al.
2004
)
