Geology Reference
In-Depth Information
11.6.1 Channel Width
In salt marsh networks, channel width is consistently
seen to reduce towards the head of a channel (Fagherazzi
and Furbish
2001
). Marani et al. (
2002
) compare the
reduction in width with distance along-channel for
seven meandering tidal channels in three locations
globally (Venice Lagoon, Barnstaple Marsh MA, USA
and Petaluma CA USA). They fi nd a tendency toward
an exponential relationship, but this
e-folding
relation-
ship (i.e. the length of channel over which the width
decreases by a factor of
e
), is not consistent amongst the
channels. The ratio of e-folding length to total channel
length is larger for shorter channels, indicating that
they widen at a faster rate than longer channels.
On much larger scales, estuaries also demonstrate a
similar exponential decrease in width, or funneling,
towards the inner estuary. Macrotidal estuaries exhibit
longer, relatively narrower funnels, while in mesotidal
estuaries the shape is broader and shorter (Wright et al.
1973
; Pethick
1984
; Eisma
1998
). A channel with a
purely progressive wave is likely to exhibit parallel
banks (Wright et al.
1973
) .
Fig. 11.12
Plot of width versus depth showing the two discrete
populations of tidal channels. Channels on vegetated salt marshes
show a distinctly different width-depth ratio (
) than
channels over tidal fl ats, which tend to behave more like their
fl uvial counterparts
β
=
2
Bh
(Feagin et al.
2009
), these are conditions that support
development of wider, shallower channels.
11.6.2 Width-to-Depth Ratio
11.6.3 Channel Cross-Sectional Area
While there is great variability throughout tide-domi-
nated systems, the channel width-to-depth ratio
(b = 2B/h) can be split into two populations: marsh
creeks (5 < b < 8) and tidal fl at channels (8 < b < 50)
(Fig.
11.12
, Zeff
1999
; D'Alpaos et al.
2005
) .
This bi-modality of channel type has implications
in terms of hydraulics and implies that vegetated creeks
and channels in bare fl ats respond differently to ero-
sional and depositional processes. Factors contributing
to this distribution of width to depth ratios include the
different processes and rates of bank erosion, e.g. the
tendency for undercutting and slumping when channel
banks are heavily rooted near the marsh surface where
the live root biomass is most dense (van Eerdt
1985
;
Huat et al.
2009
; Howes et al.
2010
) . Vegetative baf-
fl ing of fl ow will also retard currents once the water
level overtops the channel bank, leading to increased
deposition close to channel edges and the potential for
enhanced accretion close to the bank (Leonard and
Luther
1995
; Brown
1998
) , thus increasing channel
depth. Within lower tidal fl ats, sediments are coarser,
potentially non-cohesive and are more easily eroded
The existence of a relationship between cross-sectional
area (W) and tidal prism within tidal inlet channels is
widely accepted, such that
Ωα
aP
b
(11.2)
where
P
is the volume of the spring tidal prism and a
and b are empirically derived constants (Escoffi er
1940
; O'Brien
1969
; Jarrett
1976
, see Chap. 12 discus-
sion). This relationship suggests that there exists a
dynamic equilibrium whereby cross-sectional area will
adjust in response to discharge given that a set volume,
V
, of water must pass through the area during the fi xed
period of half a tidal cycle. This produces erosion or
deposition within the channel. Friedrichs (
1995
) noted
that, although this relationship is complicated at tidal
inlets by exposure to wave energy and littoral drift, in
more sheltered regions in the interior of a tidal embay-
ment, the cross-sectional area of the channel is more
closely related to shear stresses resulting from tidal
currents alone. As the nature of this equilibrium would
suggest, tidal prism may be substituted with peak
discharge (
Q
), a value more easily derived or measured
