Geology Reference
In-Depth Information
Fig. 11.7
An example of the temporal variation in water
depth, velocity and suspended sediment concentration in a
small tidal gully in the Wadden Sea (Adapted from van Straaten
1954
; in Eisma
1998
). Water depth (H) in cm is given on the
outer left
y axis and sediment concentration (
S
) in mg/L is
displayed on the inside of this axis, velocity (
V
) in cm/s is given
on the
right
y axis. The direction of the velocity is indicated by
the
arrows
running along the
top
of the diagram
11.5
Tidal Channel Morphology
of the run off (which in tidal environments will vary
with spring-neap cycles, meteorological tides and pre-
cipitation), the infi ltration capacity of the sediment,
and the resistance of the sediment on the fl ats to ero-
sion. In intertidal environments, suffi ciently high
velocities are most likely to occur on an ebb tide
because of the stronger hydraulic gradients that can be
generated between platform and channel, however
Pethick (
1992
) suggested that some channels form as
the result of fl ood inundation. Given the relative erod-
ibility of non-cohesive versus cohesive sediment, and
unvegetated versus vegetated soils, channel initiation
will occur more easily on sandy tidal fl ats (Eisma
1998
). The initiation of channel formation on a previ-
ously bare surface could be related to a number of
potential perturbation to the system, it may be as little as
a small change in the height of the tidal fl at as sediment
is deposited, but the resulting ebb fl ow may be increased
just enough to exceed the critical value for erosion. Once
channels start to form, cross-grading (the slope tangen-
tial to the main channel gradient) and micropiracy (the
capture of fl ow by a slightly deeper channel) lead to the
combination of channels and to the formation of den-
dritic networks (Leopold et al.
1964
) . Depending on
how easily the substrate can be eroded, this develop-
ment may take a few tidal cycles or many years
(Knighton et al.
1992
; Symonds and Collins
2007
;
D'Alpaos et al.
2007b
; Hughes et al.
2009
) .
11.5.1 Initiation
Observational evidence suggests that there are two
ways in which a channel may develop: incision into a
surface or deposition, i.e., accumulation of sediment
around a channel. In the fi rst of these, initial formation
is followed by a slower elaboration (deepening or
increase in sinuosity; D'Alpaos et al.
2005
; Symonds
and Collins
2007
; Knighton et al.
1992
) . Conceptual
models describing this process have been put forward
by a number of authors (Pethick
1969
; French and
Stoddart
1992
; Steel and Pye
1997
; Allen
1997
) . High
shear stress at creek heads and the behavior of fi rst-
order channels suggests that headward erosion is the
major process in the development of a network of
channels. Thus the formation of a network is decou-
pled from any subsequent evolution (meander devel-
opment and ecogeomorphological development of
intertidal areas), which happens gradually over longer
time-scales.
In general, very shallow fl ows over a fl at surface
will occur as sheet fl ow. However, after a certain dis-
tance of fl ow the converging volume and velocity of
the fl ow will reach a suffi cient magnitude to erode the
surface of the fl ats. This is known as the
critical length
of a fl ow and depends upon surface slope, the intensity
