Geology Reference
In-Depth Information
Fig. 11.9
Residual
circulation over tidal
pointbars. Residual velocity
vectors calculated from
observations in: (
a
) in the
Satilla River Estuary, GA,
USA (conducted on 17-18
November 2004); and (
b
)
modeled using FVCOM
(Finite Volume Coastal
Ocean model) for a meander
in southeast Louisiana, USA
(Chef Menteur Pass)
(Adapted from Li et al.
2008
). The position of the
high velocity stream lines
during the ebb and fl ood,
respectively are illustrated
using
grey lines
Likewise, erosion at two points on a meander bend may
lead to the formation of
cuspate
(or box) meanders,
also described as 'pinch and swale', seeing this plan-
form morphology on meanders is a clear indication of
a tidal infl uence (Figs.
11.9
and
11.10
)
The bidirectionality of fl ow also impacts the resulting
cross-sectional morphology of meandering tidal channel.
As fl ow moves around a curve, momentum draws the
streamline of high velocity towards the inside bank of
the channel, before forcing it to the outside of the curve
where it erodes the bank. When this high fl ux of water
is forced to the outside of the curve, it creates a suffi cient
gradient in the water surface that a secondary circula-
tion is set up, moving water and sediment towards the
inside of the curve, building up a
pointbar
. The hydraulics
and morphology of fl uvial pointbars are well docu-
mented (Abad and Garcia
2009a,
b
; Parker et al.
2010
),
however, studies concerning bars in tidal channels are
scarce. Under unidirectional fl ow, the growth of the
pointbar creates a shallow zone close to the inner bank.
This reduction in depth also acts to direct the streamline
of high velocity toward the outside of the bank further
