Geology Reference
In-Depth Information
Channels are, therefore, a primary control on coastal
environments.
Tidal channels are generally defi ned by bidirectional
tidal fl ow. The term
tidal channel
can describe features
across a range of scales, from large distributaries or
cuts between tidal sand bars to small marsh creeks and
shallow runnels across tidal fl ats. Networks formed by
connected tidal channels are dynamic in nature, experi-
encing changes on timescales shorter than that of the
evolution of the tidal landscape as a whole (D'Alpaos
et al.
2005
) and at times may appear comparatively
transient. Active channel systems may refl ect present
conditions, or exhibit inheritance from paleo- or pre-
existing networks. For example, marsh systems often
develop over tidal fl ats or bars with the channels pre-
served as creeks (Pethick
1969
; Perillo and Iribarne
2003
; Temmerman et al.
2007
) . Alternatively, rapid
changes in sediment supply, sea-level, or freshwater
inputs can change the hydrodynamics of a system, and
the resulting morphological adaptation may rework
deposits, obliterating the record of past environments.
The migration and evolution of channels in response to
changing physical conditions can lead, therefore, to
complicated architecture in the resulting sedimentary
deposits, including the presence of multiple erosive
surfaces. The transgressive nature of many modern
shorelines adds to the diffi culty of interpreting tidal
channel deposits (Dalrymple and Choi
2007
) . Yet,
understanding the evolution of modern systems,
explaining changing morphology and quantifying
rates of network expansion or reduction, can provide
improved insights into coastal response to sea-level
change, both past and present.
Previous chapters have described the channels, and
the associated facies, in a number of different tidal
settings. This chapter aims to give an overview of the
evolution and common characteristics of channels
within coastal systems, drawing comparisons with
fl uvial channels. We will start with a general overview
of the nature of tidal channels and then compare sev-
eral classifi cations of tidal channel network, accord-
ing to planform, with a focus on shallow intertidal
settings such marshes and tidal fl ats. The remainder of
the chapter will examine the defi ning physical pro-
cesses and the resulting geomorphologic relationships
that have been observed for channels in these environ-
ments. Deposits created by tidal channels and the
potential for their preservation within the stratigraphic
record in specifi c settings have been explored in previous
chapters. However, in the last Section we will provide
a description of certain tidal facies that can are par-
ticular to channels.
11.2
General Characteristics of Tidal
Channel Systems
The response of a region to the repetitive fl ooding of
tidal water is to self-organize into shallow areas that are
periodically fl ooded, and channels that drain them. As
a consequence, in shallow, intertidal landscapes there
tend to be three major morphological components: (1)
unvegetated tidal fl ats or bars; (2) vegetated marsh plat-
forms or mangroves; and (3) channels, which dissect
and interconnect the other two zones (D'Alpaos et al.
2005
; Fagherazzi et al.
2006
). These channels may be
intertidal (drying out or having standing water in only
the very deepest parts during low water) or peri-subtidal
(in which the wetted perimeter of the channel is large
in comparison to the tidal range). In systems that exhibit
extensive subtidal regions, channel-shoal morphology
is often seen, in which very deep (compared to the tidal
range) channels run between bank-attached bars or
mid-channel linear sand bars, parts of which may be
exposed at low tide (for example the Wash, the Gironde
Estuary, or the mouth of the Fly River).
Although, large-scale features, such as an estuary
(a fl ooded river mouth; Dalrymple et al.
1992
) , are
undoubtedly to be considered a tidal channel, fea-
tures on this scale are complicated by extreme varia-
tions along their length. For simplicity, here we will
focus on meso-scale features; channels that fi t within
macro-scale features, such as fl ood-tidal deltas, or
mega-scale features, such as estuaries or back-barrier
basins. These tidal channels contain micro-scale
morphological forms such as bedforms or bar-forms
(de Vriend
1996
; Hibma et al.
2004a,
b
) . Their evo-
lution occurs over medium timescales (days to cen-
turies), as they equilibrate to forcing such as storm
events, sea level rise or gradual infi lling where there
is an adequate supply of sediment. Tidal inlets and
high-order tidal channels have a relatively high pres-
ervation potential (Belknap and Kraft
1985
) , while
shallower tidal features are vulnerable to erosion
during shoreline transgression.
Meso-scale tidal channels share a number of char-
acteristics: (1) some sinuosity; (2) depositional bed
morphology such as ripples and bars; (3) low channel-bed
