Geology Reference
In-Depth Information
versus tidal patterns, differences are the norm rather
than the exception once carefully examined”.
Variability may, in fact, be the primary characteristic
of channel systems in tidal environments. In tidal
marshes, multiple sub-basins may exist with quantifi ably
different channel distributions (Fagherazzi et al.
1999
;
Marani et al.
2002
) as a result of highly localized
changes in sediment type or vegetation or broader
changes in hydrodynamics. Neighboring drainage
basins may have entirely different planform morphol-
ogy and exhibit different relationships between drainage
area and channel dimensions (Marani et al.
2002,
2003
; Rinaldo et al.
1999,
2004
, c.f. Eisma
1998
) .
Some will exhibit values closer to fl uvial systems
than others.
The explanation for such variation lies in the large
number of factors that infl uence the evolution of tidal
channels. These can be broken down into either physical-
environmental constraints or hydrodynamic factors.
Physical attributes that are important in channel devel-
opment include antecedent geology, sediment deposi-
tion patterns and grain size, and the presence and type
of vegetation. These will all impact the erodibility of
the substrate and consequently the stability of the
channel features. Stability controls persistence, and
therefore evolutionary complexity, but it is also a
factor in channel shape (both planform and cross-
sectional profi le).
Hydrodynamic infl uences on channel evolution
encompass the balance of exposure to tidal and wave
forces. The tidal fl ows in a channel may either result
from external or remote forcing (i.e. the offshore tide)
or be a response to the local morphology, but it is not
always easy to separate these. For example, channel
size and shape respond to the portion of the tidal
prism that passes through it. This depends not only
on the regional tidal range and the size of the basin
being fl ooded, but also on the local morphology of
the surrounding channels, which modify the advancing
tidal wave (Marani et al.
2003
) . Other factors infl u-
encing the hydrodynamics are: the gradient over
which the drainage occurs (ranging from very abrupt,
local effects relating to a change in underlying stra-
tigraphy or local vegetation, to regional variations in
tidal range), the dominance of the ebb (seaward) or
fl ood (landward) tidal velocities, the curvature of the
channel, and, lastly, the hydraulic radius of the channel
(a function of the width to depth ratio). Assessing
these relationships is complicated by interdependent
feedbacks between factors, particularly channel
curvature and hydraulic radius. Note that here, we
will not consider the impact of meteorological tides
and waves in any detail.
Processes controlling the initiation and evolution of
channel systems operate within both the vertical and the
horizontal plane. Vertical processes include: deepening
through erosion and suspension of sediment, through
compaction, or due to sea-level rise; shallowing through
inorganic sediment deposition; or relative change due to
the erosion or accretion of the surrounding platform or
tidal fl at. Laterally, processes include channel widening
through bank erosion; 'elaboration' i.e., a change in the
intensity of meandering or channel migration; and head-
ward erosion (D'Alpaos et al.
2005
) .
Channels within the same system may not only
result from different processes, they may also function
differently depending upon their origin. The observa-
tions of Zeff (
1988
) and Ashley and Zeff (
1988
) illus-
trate this. These studies identify two types of tidal
channel within the salt marshes of New Jersey. The
fi rst type are 'through-fl owing' channels that connect
channel to channel or to lagoons; the second type are
'dead-end' channels which end within the marsh, and
often start at a through-fl owing channel.
As well as notable differences in channel size,
width to depth ratio, sediment properties, sedimen-
tary type and structure, and the variation of width
inland, there is also a signifi cant difference in hydrau-
lics between these two types of channel. Peak cur-
rents in the dead-end channels occur close to bank-full
conditions, whereas, in the through-fl owing channels,
they occur at mid to low tide. The maximum currents
are generally an order of magnitude larger in the
through-fl owing channels than the dead-end channels.
Zeff (
1988
) proposes that through-fl owing channels
are formed during the infilling of the back barrier
as the fl ood-tidal delta was stabilized by vegetation,
(i.e. they are fl ood-formed channels that are now
essentially relict). In contrast, the dead-end channels
have eroded headward into the marsh platform, post
vegetation, and as such are formed by ebb fl ows and
are still likely to be actively evolving. These two
channel forms are therefore fundamentally different,
yet proximal, with very different sediments and
resulting facies. As this example illustrates, it would
be easy to assume that smaller tidal channels are a
scaled version of the larger channels in a system, but
this is often not the case.
