Geology Reference
In-Depth Information
meanders in the main channels is to be expected, but
detailed documentation of such changes are rare, so it
is not known whether there is a systematic behavior of
the meander bends. The swatchways also migrate,
apparently preferentially in a headward direction
because of the flood-dominated sediment transport that
prevails. In the Cobequid Bay estuary, one large
swatchway (relief ca. 5 m) has been documented from
sequential air photos to have migrated 2.1 km over a
35-year period (average rate 61
m/a
), with a maximum
rate of slightly more than 80
m/a
(Dalrymple et al.
1990
). Smaller swatchways with a relief of only about
1 m migrated more than 150
m/a
.
In most tide-dominated estuaries, the zone of elon-
gate tidal bars passes gradationally into the narrower
inner part of the estuary. This transition involves the
gradual simplification of the channel—bar morphol-
ogy through the loss of channels, until there is only a
single, main ebb channel (Fig.
5.9
). The Cobequid
Bay—Salmon River estuary appears to be unusual, if
not unique, in having a 'braided' sand-flat area (i.e.
'zone 2' of Dalrymple et al.
1990
) (Fig.
5.10b
) between
the zone of high-relief elongate tidal bars and the 'sin-
gle-channel' inner estuary. In this area, which owes its
existence to the shallowness of the estuary, the very
strong tidal currents that exist here, and the fine sand
that characterizes this area (see below), cause the wide-
spread development of upper-flow-regime conditions.
The resulting morphology consists of an apparently
disorganized 'braided' network of subtle, only slightly
elongate bars, most of which show a headward (flood-
dominant) asymmetry. The relief of these bars is typi-
cally less than a meter, but can reach as much as 2 m,
and slopes are rarely more than 0.5°.
The areas along the margins of the outer part of
tide-dominated estuaries tend to be wave dominated
(Fig.
5.2
) because waves can penetrate into the estuary
at high tide, and because tidal-current speeds are mini-
mal in the upper intertidal zone at that time. As a result,
the margins have a concave-up shoreface profile, with
a beach at the high-water level if coarse sediment is
available (Dalrymple et al.
1990
; Pye
1996
; Tessier
et al.
2006
). If the estuary mouth is transgressing, this
shoreface is erosional (Fig.
5.10a
): this erosional trans-
gression can continue even though the margins of the
inner part of the estuary are prograding (Allen
1990
;
Dalrymple et al.
1990
; Dalrymple and Zaitlin
1994
;
Allen and Duffy
1998
; Pye
1996
; Tessier et al.
2006
).
At some point in the estuary, the beaches end abruptly
and are replaced by tidal flats and salt marshes; a good
example of this has been documented in the Dee estu-
ary, England (Pye
1996
, his Figs. 2.11-2.13). The
location of this beach-marsh boundary commonly lies
near the headward end of the elongate sand-bar com-
plex, but presumably depends in part on the evolution-
ary stage of the estuary, migrating further into the
estuary as the estuary transgresses.
5.3.3
Inner Estuary
The axial channel system in the inner part of tidal-
dominated estuaries consists of a single ebb channel
that connects to the river(s) that feed into the estuary,
and displays the 'straight'—meandering—'straight'
channel pattern discussed above (Figs.
5.1
and
5.8
).
The depth of the ebb channel is deepest on the outside
of each bend and is shallowest in the cross-over areas
(Jeuken
2000
). In those portions of the channel where
there is appreciable tidal influence (i.e. in the outer
'straight' reach [zone 3A of Dalrymple et al.
1990
]),
the channel shows a repetitive pattern of channel bends,
flood barbs and elongate tidal bars (Fig.
5.1
; Jeuken
2000
; Schuttelaars and de Swart
2000
). Each 'estuary
section' or 'estuary compartment' comprises a single
channel bend between two successive inflection points
and consists of a point bar or alternate bar that is cut by
a flood barb. The flood and ebb channels are separated
by an elongate tidal bar that can be either simple and
continuous (Barwis
1978
), or a complex series of bars
separated from each other by one or more swatchways
(Jeuken
2000
; Schuttelaars and de Swart
2000
). These
flood barbs and adjacent tidal bars become progres-
sively shorter in a landward direction because of the
decreasing wavelength of the meanders (Fig.
5.9b, c
);
the number of swatchways also decreases inward as the
bars become shorter (Fig.
5.11
; Jeuken
2000
). On occa-
sion, the flood channel and a swatchway can become
large enough that they assume the role of the main
channel for a period of time. This can lead to the alter-
nation of channel location between two discrete loca-
tions (van Proosdij and Baker
2007
; Burningham
2008
),
and the episodic creation of channel-center bars.
The meander bends tend to be asymmetric, or
skewed, with a tendency for the asymmetry to alternate
between landward-directed and seaward-directed in
successive bends (Burningham
2008
). Overall, there
might be a tendency for the meanders to be skewed
