Geology Reference
In-Depth Information
Fig. 5.8
Plots of sinuosity as a function of position within each
of four tide-dominated estuaries. See Fig.
5.1
for satellite images
of the Cobequid Bay—Salmon River, Severn and Thames estu-
aries; note that the plots shown here are oriented in the same way
as the satellite images in Fig.
5.1
. The 'sinuosity index' is the
ratio of the along-channel length divided by the straight-line dis-
tance between the tidal limit and estuary mouth. In all four cases,
the sinuosity increases inland from the mouth, commonly quite
abruptly, reaching a maximum (indicated by
arrows
) where the
sinuosity is greater than about 2.5, before decreasing to lower
values further inland. This zone of maximum sinuosity is the
tightly meandering zone of the 'straight'—meandering—
'straight' channel pattern. Note the much greater variability of
channel form in the area landward of the sinuosity maximum.
Systems that export sediment to the sea (i.e., deltas) do not show
this peak. Instead, the sinuosity increases inward
straightening of this bend occurred suddenly by means
of a neck cutoff in 1991 during a particularly large
river flood, and the river shows no sign of reoccupying
the tight bend, which is passively filling with sediment
(Bostock et al.
2007
). The South Alligator River in
Northern Australia also shows morphological evidence
that it was once more highly sinuous in the inner part
of the coastal plain and is now exporting sediment to
its mouth (Woodroffe et al.
1989
). The Ord River in
Northern Australia, which is commonly cited as a
tide-dominated delta, possesses the tightly mean-
dering zone, so it is either an estuary or has evolved
into a sediment-exporting deltaic system so recently
that it has not yet lost its estuarine channel pattern
(Fig.
5.8d
).
Flood-dominant channels flank the main ebb chan-
nel. Unlike the main ebb channel, these channels are
invariably discontinuous, terminating headward into
tidal flats or sand bars. They are separated from the
main ebb channel by an elongate tidal bar that attaches
to the shoreline or to another, commonly larger, tidal
bar. The morphology of the 'blind' flood channel and
its flanking bar looks like a fish hook, and the short,
flood-dominant channel has been termed a
flood barb
(Robinson
1960
). Overall, these channels become
shorter in a landward direction and are absent beyond
the inner end of the tide-dominated portion of the estu-
ary (Fig.
5.2
).
In general terms, tide-dominated estuaries can be
subdivided into two main morphological zones based
on the nature of the channel network:
1. A broader outer estuary with several ebb- and flood-
dominated channels that separate elongate tidal bars
and/or sand flats (zones 1 and 2 of Dalrymple et al.
1990
) that are commonly flanked by wave-generated
beaches and shorefaces (Fig.
5.2
); and
