Geology Reference
In-Depth Information
Fig. 11.10
(
a
) Estuarine meanders showing the mutually
evasive flow within channels and the development of mid-
channel islands (After Bird
1984
), this also illustrates the
cuspate nature of tidal meanders (
b
) Examples of meanders,
pointbars and mid-channel islands in the Rowley River, MA,
USA. The Rowley River has very little freshwater input and
a tidal range of almost 4 m during springs tides
enhancing the secondary circulation and pointbar
formation (Seminara
2006
) .
In a tidal system, the secondary circulations set up
during the ebb or fl ood are likely to be offset, acting in
different directions, and of different magnitudes. The
reversing fl ow causes deposition or erosion on the
upstream and downstream bank of a meander alter-
nately. Figure
11.9
shows measured and modeled,
depth-averaged, residual currents, in planform, over
point-bars in a meandering tidal channel. The bars all
show clear rotational residual circulations resulting
from the interaction of the differential paths of the
high-velocity streamlines during the fl ood and the ebb
(Li et al.
2008
). The inner bank of the meander experi-
encing reversed fl ow compared to the direction of the
stronger fl ows on the outer bank. Evidence for these
opposing, but offset fl ows can also be visualized by
observing the bedforms that occur on each side of the
pointbar (Fenies and Faugères
1998
) . The inner bank
of a tidal pointbar often exhibits bedforms of the oppo-
site symmetry and orientation to the dominant fl ow
(Barwis and Hayes
1979
; Barwis
1978
; Dalrymple and
Choi
2007
). This hydrodynamic regime leads to com-
plex pointbar formations (Barwis
1978
) .
In a fl uvial system small erosional channels may
form across the inside of the meanders when the river
stage is high; these are known as
chutes
(Van Straaten
1954
; Eisma
1998
). These may form blind channels or
cut entirely across part of the pointbar or meander,
shortening and straightening the channel (Seminara
2006
). In meso-scale tidal creeks and channels, similar
morphology can be observed, but will be compounded
as each side of the inner meander bend is periodically
exposed to bank-normal velocities (Fig.
11.9
) This
may lead to the creation of a tidal barb in direction of
the subordinate tidal fl ow, the dominate tidal fl ow
occupying the outer region of the meander. If the
meander is cut off completely, a secondary channel
may form carrying the subsidiary tide. Mid-channel
islands are common in large meandering tidal channels
and pointbars often exhibit some level of detachment
from the bank (Barwis
1978
) . Figure
11.10b
shows