Geology Reference
In-Depth Information
11.5.2 Secondary Processes of Initiation
or Evolution
Once a channel system has formed, fl ow convergence,
and thus the erosive forces, will be focused at the head
of the channels (Fig.
11.6
), which receive water from
the broad area of the platform beyond the channel as
well as from the sides. If shear stress is suffi cient,
channels may erode headward. Rates of headward ero-
sion reported in the literature vary, the highest rates
being reported by Knighton et al. (
1992
) in Northern
Australia, where tidal inundation of a fl at coastal plane
and the reoccupation of paleochannels led to channel
growth of up to 500 m/year. Symonds and Collins
(
2007
) monitored the development of channels over a
tidal fl at in the Wash (UK), fi nding 'natural condition'
extension of 15 m/year. After the managed breaching
of a seawall, channel extensions of 400 m/year were
measured because of increased sheet fl ow across the
fl ats due to insuffi cient capacity of the existing chan-
nels given the enlarged tidal prism. Shi et al. (
1995
) ,
report that fi ve channels in the sandy salt marshes of
the Dyfi Estuary (UK) extended at an average rate of
2.5 m/year. Newly formed channels in the muddy salt
marshes of South Carolina are extending by 2 m/year
(Hughes et al.
2009
) .
Channels on tidal fl ats and marshes are not always
formed through erosional processes (Eisma
1998
) .
Depositional models for channel formation in marshes
have been put forward by Hood (
2006,
2010
) and
Temmerman et al. (
2007
). Vegetation is seen to colo-
nize tidal fl ats, creating raised 'islands', and ultimately
extending the marsh edge seaward. Accumulation
rates on the marsh platform are enhanced in com-
parison to those on the tidal fl ats or in the channels, by
the contribution of organic material by vegetation
(primarily root development) and increased baffl ing
of tidal waters, enhancing inorganic deposition. Both
scouring at the edge of vegetation patches and inheri-
tance of pre-existing tidal flat channels produce
conduits where fl ow is focused, prohibiting accumu-
lation of sediment, while the marsh islands grow up
around them. This process is central to the formation
of channels within the numerical models of Kirwan
and Murray (
2007
). While inheritance from an ante-
cedent network is not a necessary part of this para-
digm, it is likely the most common underlying cause
of this phenomenon in nature. Salt marshes have been
observed to inherit their channels from both tidal fl at
systems (as they prograde seaward; Pethick
1969
) and
fl uvial systems and streams (as they expand inland;
Adams
1997
) .
Secondary processes operate to alter existing networks,
playing a part in their elaboration. These processes
include the connection of existing channel sections
and the extension or blocking of channels by the col-
lapse of blocks from the channel bank (Allen 1965;
Pestrong
1972
; Collins et al.
1987
; Eisma
1998
) . In
the high marshes of New England, fi rst-order chan-
nels are seen to fl uctuate in length in conjunction
with ponding and drainage on the marsh surface
(Wilson et al.
2009
). These processes operate over
moderate time scales, changes being seen over a
number of years, sometimes decades. It is possible
that marsh channels envolve through geochemical as
well as physical processes. Ponded water on the
marsh surface can lead to increased salinity, and thus
changes in vegetation (Perillo and Iribarne
2003
) ,
and may also alter the rate of decomposition of
organic matter. These changes can change both the
topography of the marsh surface, infl uencing fl ow
patterns, and the erodibility of the sediment through
reduction in rooting.
A similar phenomenon is observed by Perillo and
Iribarne (
2003
) and modeled by Minkoff et al. (
2006
)
in salt marshes in Argentina, where the interaction of
crabs and vegetation cause bare patches on the marsh
surface. These de-vegetated regions coalesce to create
creeks. Analogous behavior is seen in the marshes of
South Carolina, whereby straight creeks erode head-
ward into a mature marsh platform as a result of low
soil strengths within transient de-vegetated regions
that move with the head of the creeks, again as a result
of crab herbivory and burrowing (Hughes et al.
2009
) .
The continued existence of a channel is a balance
between erosion and deposition. If the tidal prism
changes (due to sea-level rise or fall, anthropogenic
basin modifi cation, or changes in sedimentation) such
that velocities in the channels are reduced, then the
channel will infi ll (Symonds and Collins
2007
) .
Likewise, events such as heavy precipitation, storm
surges and increases in tidal prism may also lead to
erosion of sediment due to increased fl ows across the
tidal fl at or marsh (Murphy and Voulgaris
2006
;
Hughes et al.
2009
) .
The impact of changing salinity and ecology within
tidal channels is an additional consideration. Recent
research into channel elaboration has focused on the
