Geology Reference
In-Depth Information
Fig. 11.11
Four planform pointbar morphologies observed by Barwis (
1978
) in tidal channels in South Carolina (USA)
several such islands in meanders in the Rowley River, a
tidal river in Plum Island Sound (Massachusetts, USA).
Barwis (
1978
) undertook detailed investigations
of the morphology and resulting vertical succession
of deposits within tidal-creek pointbars in South
Carolina. Figure
11.11
illustrates the four common
pointbar planforms, which the study identifi ed within
the back-barrier area of an ebb-dominated, meso-tidal
barrier system. Pointbars are categorized according
to morphology and the ratio of the radius of the chan-
nel curvature (
r
) to the channel width (
w
): (a) linear
welded bars (
r/w
> > 3); (b) linear mid-channel bars
(
r/w
> 3); (c) multi-lobed bars (2.5
< r/w
< ~3); and
(d) steep apical bars (
r/w
< 2.5). As
r/w
decreases
sinuosity increases.
Unless forming on a very straight or a very tight
meander, pointbars in tidal systems tend to be elon-
gate, stretching out in the direction of the dominant
tidal current. From this, one could surmise that the sys-
tem shown in Fig.
11.10b
is ebb-dominated as the
pointbars that are visible extends seaward from the
apex of the meanders. When forming on a meander of
intermediate sinuosity (2.5 <
r/w
< ~3), pointbars are
more complex. The bars are detach from the inner
bank at all but the tip closest to the meander apex
because of the presence of a barb, which carries the
subordinate current while the main channel carries the
dominant current, in this case fl ood and ebb respec-
tively (Fig.
11.11b
, Barwis
1978
). A value of
r/w
closer
to 2.3 produces a pointbar with multiple lobes. Multi-
lobed pointbars also display segregation of currents.
This is caused by topographic shielding as the high-
velocity streamlines occur in different positions during
the fl ood and ebb.
It is interesting to note the similarities in pointbar
and barb morphology of large tidal channels which
occur in varied tidal settings, and the 'braided' chan-
nel-shoal networks seen deeper subtidal regions in the
middle of estuaries (Hibma et al.
2004a,
b
; Dalrymple
and Choi
2007
). Comparisons can be also be drawn
between the mutually-evasive channels observed mid-
estuary and the mutually evasive streamlines in mean-
dering tidal channels (Figs.
11.9c
and
11.10b
). This
suggests a continuum where similar processes act
under slightly different forcing conditions.
The evolution of estuarine morphology has been
modeled (using a 2-D depth-averaged model of fl ow
and non-cohesive sediment transport) by Hibma et al.
