Geology Reference
In-Depth Information
11.4.3 Overtopping and Velocity-Stage
Relationships
Where water depth is deep relative to the tidal range,
the tidal wave is progressive. Just like a wind wave,
velocities are highest under the crest and the trough.
Thus, peak fl ood currents occur at high water and peak
ebb currents at low water. Under a standing wave, by
contrast, peak fl ows occur at mid tide, which is the
common model for coastal tidal fl ows. The latter
occurs in regions where the water depth is shallow
compared to the tidal range. Where the tidal wave is
progressive, little energy, and thus tidal amplitude, is
lost over distance. In shallow regions friction acts to
reduce the amplitude of the tidal wave and so tidal
range is reduced up channel. Large systems will expe-
rience some combination of these two stage-velocity
models (progressive and standing wave conditions) as
the tidal wave moves up estuary (Wright et al.
1973
;
Hibma et al.
2004a
; Howes
2009
) .
In regions of extensive intertidal areas, the stage-
velocity is complicated further. Hydrodynamically, it is
possible to distinguish two types of tidal channel, which
represent the two end members along a continuum.
Low-order channels, in which the tidal range is signifi -
cant in terms of the channel depth, derive the majority
of the water fl ux that passes through them from sheet
fl ow leaving tidal fl ats or the marsh platform. Higher-
order channels, for which the change in volume experi-
enced over a tidal cycle is small in comparison to their
size, in contrast, receive a signifi cant volume of water
from other channels, rather than from overbank fl ow
which has been strongly effected by shallow water and
frictional effects. It is possible that these two end mem-
bers could be compared to the dead-end and through-
fl owing channels of Ashley and Zeff (
1988
) ; however,
any high-order channel within a dendritic system may
fall into the larger subtidal category. The two types of
channel will experience different fl ows. Low-order
creeks experience velocity transients (surges) at close
to bankfull conditions (Fig.
11.4
, Bayliss-Smith et al.
1979
; French and Stoddart
1992
; Fagherazzi et al.
2008
). The higher-order creeks are more likely to have
their highest velocities near mid-tide (if the tidal wave
is a standing wave), have a lower tidal asymmetry, and
experience signifi cantly higher velocities (~1 m/s at
compared to ~0.1-0.6 m/s in low-order salt marsh
creeks; Ashley and Zeff
1988
; Hughes et al.
2009
) .
Fig. 11.4
The hysteresis observed in tidal velocity versus water
depth (stage). Velocity is highly variable, but two distinct peaks
are seen, one during the fl ood just above bankfull conditions
when the water level is at the level of the marsh surface, and one
during the ebb. In terms of symmetry around either high tide or
the timing of bankfull conditions, the peak ebb velocities lag the
fl ood transients, occurring later, at a lower stage of the tide, just
below bankfull. DU indicates the difference in the height at which
the peak velocity occurs (From the observations of Bayliss-Smith
et al. (
1979
), adapted from Fagherazzi et al. (
2008
) )
This has signifi cant implications for the net transport
and erosion patterns in each type of channel; it may
also help to explain why tidal channels are not scale
invariant in the way of fl uvial systems (Fagherazzi et al.
1999
; Rinaldo et al.
1999
; Marani et al.
2003
) .
The frequency of bankfull and overtopping tides
varies; it occurs with every tide on unvegetated tidal
fl ats, but may occur as few as 6-8 times a month on the
high marsh. When overbank fl ow does occur, a distinct
hysteresis is seen in the discharge of low-order chan-
nels (Fagherazzi et al.
2008
Fig.
11.4
). During the
fl ood, a surge is seen when the platform is inundated
(as an increased volume of water is drawn through the
channel in order to fi ll the platform area). During the
ebb, fl ow peaks when the water level is at or just below
the marsh surface. As water drains from the marsh
platform, a steep hydraulic gradient between the water
on the platform and the water level in the channel cre-
ates fast fl ows and focuses the fl ow into the creeks,
particularly at the head (which serves a greater area of
unchanneled platform). The discharge within the chan-
nel will be a function of the inundated surface area (
S
)
and the water depth (
h
) (Boon
1975
) :
