Geology Reference
In-Depth Information
sediment is input from the ocean, whereas small
estuaries and deltas will have a low efficiency. The
trapping efficiency is also a function of grain size, with
estuaries exporting fine-grained suspended sediment
to the ocean earlier than sand during their transition to
a delta.
similar hydraulic function. The main ebb channel gen-
erally becomes more sinuous in a landward direction.
Near the mouth of the estuary, it can be essentially
straight, but the radius of curvature of the meander
bends decreases (i.e. the bends become tighter) and the
sinuosity increases in a landward direction (Dalrymple
et al.
1992
; Billeaud et al.
2007
; Burningham
2008
)
(Figs.
5.1
and
5.8
). Qualitative observations and quan-
titative measurements indicate that the main channel
reaches a peak sinuosity that exceeds a value of about
2.5 (and may be greater than 3) some distance inland,
after which it becomes less sinuous again near the limit
of tidal influence (Ichaso and Dalrymple
2006
). The
sinuosity of the river above the limit of tides varies
widely between examples, and can be quite sinuous,
but rarely reaches a value as high as 2.5. Dalrymple
et al. (
1992
) was the first study to note the presence of
this pattern, which they termed 'straight'—meander-
ing—'straight' (SMS; Fig.
5.1a
), where 'straight'
refers to a channel of relatively low sinuosity and not
to a truly straight channel. Subsequent quantitative
studies reveal that the SMS pattern even exists in small
tidal creeks (Fagherazzi and Furbish
2001
; Solari et al.
2002
; see also Chap. 11), provided there is little or no
fluvial influence. Systems that are known to be pro-
grading and, thus, are 'deltas' in the sense used here,
do not show this pattern (Ichaso and Dalrymple
2006
;
see also Chap. 7). Instead, there is a progressive
straightening of the channel from the river to the mouth
of the estuary (Dalrymple et al.
2003
, their Fig. 6). As
a result, the presence or absence of a short zone (typi-
cally only one or two meander-bends long) with very
tight and generally symmetrical meanders appears to
be an easy way to distinguish between estuaries and
deltas. The reason for this SMS pattern is not known
with certainty, but observations in the Cobequid Bay-
Salmon River estuary (Zaitlin
1987
; Dalrymple et al.
1991
) show that the tightly meandering zone lies
approximately at the location of the long-term (i.e.
multi-year) bedload convergence, a suggestion sup-
ported by observations reported by Ayles and Lapointe
(
1996
). As the estuary fills and the bedload conver-
gence migrates seaward, the zone of tight meanders
should migrate with it, but gradual migration of the
meandering zone is apparently not possible. In the
Fitzroy estuary (Bostock et al.
2007
; Ryan et al.
2007
),
for example, the point of bedload convergence, as indi-
cated by the facing directions of large subaqueous
dunes in the main channel, lies approximately 10 km
seaward of the very tight meander bend. The predicted
5.3
Morphology of Tide-Dominated
Estuaries
5.3.1
General Aspects
Tide-dominated estuaries show the typical funnel-
shaped geometry that characterizes all coastal systems
in which there is appreciable tidal influence (Myrick
and Leopold
1963
; Wright et al.
1973
; Fagherazzi and
Furbish
2001
; Rinaldo et al.
2004
). This exponential
decrease in width in a landward direction (Figs.
5.1
-
5.3
) is a result of the landward decrease in the tidal flux
(Myrick and Leopold
1963
; Wang et al.
2002
), which
reaches zero at the tidal limit. By comparison, river
channels are nearly parallel sided and show only a very
slow seaward increase in width in the coastal zone,
because there is only a small increase in fresh-water
discharge, derived from small tributaries, direct pre-
cipitation and groundwater discharge. In the end-mem-
ber case of strongly tide-dominated estuaries (Fig.
5.1
),
the tidally created funnel extends right to the open
coast. However, as the wave influence increases, long-
shore drift becomes capable of building a spit into one
or both sides of the estuary mouth, producing a con-
striction. Gomso Bay, which has an 'incipient barrier'
(Yang et al.
2007
), represents a situation that is close to
the tide-dominated end-member of the wave-tide spec-
trum of estuary types. The Gironde estuary, France
(Allen
1991
), with its tide-dominated bayhead delta
and muddy 'central basin' that is enclosed by a wave-
built spit, and the Westerschelde estuary, the Netherlands,
are more mixed-energy settings because of the pres-
ence of a wave-built barrier-inlet complex at their
mouth (Dalrymple et al.
1992
). For more on such bar-
rier-inlet systems, see Chap. 12.
Every river entering an estuary possesses a main
channel that continues seaward through the estuary as
an ebb-dominated channel. Main channels issuing
from tributaries join the main ebb channel, but seaward
branching of this channel in a distributary-like pattern
is not obvious, although the
swatchways
that dissect
the elongate tidal bars in the estuary mouth serve a
