Geology Reference
In-Depth Information
ebb tide. There are numerous studies of tidal creek
hydrodynamics and related suspended sediment
transport in salt marsh areas (e.g. Boon 1975 ;
Settlemyre and Gardner 1977 ; Bayliss-Smith et al.
1979 ; Nixon 1980 ; Healy et al. 1981 ; Dankers et al.
1984 ; Green et al. 1986 ; French and Stoddart 1992 ;
Reed et al. 1999 ) just to mention a few. This topic
will be treated elsewhere in the topic.
from the source, there will be a deposition minimum.
Ultimately, this will lead to a concave profi le between
two primary salt marsh creeks and between the salt
marsh front and its hinterland (Allen 2000 ) .
As is the case with river banks, levees are formed
along the banks of salt marsh creeks as a result of fallout
from suspension when sediment-laden water fl ows from
the turbulent channel region to the calmer overbank
environment. In general, however, salt marsh creek
levees are smaller than their fl uvial counterparts. The
reason for this is twofold: (1) salt marsh creeks are usu-
ally transporting relatively fi ne grain sizes with only a
small tendency for developing pronounced levee depos-
its and (2) a combination of the astronomically con-
trolled water level and the development of salt marsh
drainage systems. As the period around high water is
associated with moderate to no currents, overmarsh
tides are in general associated with less energetic condi-
tions than is the case for their fl uvial counterparts. This
means that a smaller amount of suspended sandy mate-
rial is carried over the banks of salt marsh creeks than
over fl uvial banks during otherwise similar conditions.
The less elevated salt marsh creek levees are in general
also less exposed to the formation of crevasse splays in
weak spots than is the case for their fl uvial counterparts
where the most vigorous fl ow coincides with the highest
water level. Breached locations in salt marsh creek
levees, however, attract water draining the salt marsh
during subsequent and usually relatively quick tidal-
controlled water level descends after overmarsh tides.
As a consequence, these locations easily become focus
points for small backward eroding fi rst-order creeks.
Eventually, these creeks will dominate the local area
behind breached levees as erosional features (Fig. 8.4 )
where, in the fl uvial counterpart, crevasse splays domi-
nate as small depositional systems.
Headward-eroding creek systems, from fi rst to higher
orders, represent the way in which a drainage system is
established in salt marsh areas. Some creeks will develop
faster than others and thereby 'steal' drainage area from
less fortunate creeks and leave them behind to degener-
ate. An example of this can be seen in the central lower
right of Fig. 8.4 . This example is from a creek system in
Georgia, USA, with little chances for small-order tribu-
taries to develop into larger higher-order creeks, as the
primary drainage structure has already matured. In more
juvenile salt marsh formations, headward-eroding fi rst-
order creeks will mature as higher-order creeks as the
drainage system develops.
8.3.2
Salt Marsh Platform
A fl ooded salt marsh is, in principle, without water
movement during high tide slack. Wind effects, topo-
graphical variations, and hydrodynamic delays can all
create local currents, but these will in general be small
compared to those acting in relation to a fl ooded fl ood-
plain where water is exchanged between turbulent fl ow
in the channel-near region and the fl oodplain. Thus,
fl oodplain deposits are largely related to diffusion pro-
cesses (Pizutto 1987 ), whereas the distribution of sedi-
ments in salt marsh areas is primarily related to
advection (Woolnough et al. 1995 ) . The relative impor-
tance of diffusion and advection, however, depends on
a number of local conditions such as topography and
tidal conditions. Even if it seems logical that diffusion
is more active in fl uvial than in tidal environments,
both processes are present in both places. The diffu-
sion model of Pizutto ( 1987 ) has actually been demon-
strated to be applicable in salt marsh areas (Bartholdy
et al. 2004 ). A number of empirical studies (e.g. Letzch
and Frey 1980 ; Carling 1982 ; Reed 1988 ; Stoddart
et al. 1989 ; French and Spencer 1993 ; Bartholdy 1997 ;
Bartholdy et al. 2010a ) have shown that deposition of
salt marsh sediments decreases away from the source
primarily in form of the salt marsh edge, and followed
in signifi cance by the higher-order salt marsh creeks.
The small-order creeks have little infl uence (Stoddart
et al. 1989 ; Bartholdy et al. 2010a ) . This agrees well
with a general picture of tides having an increasing
part of the inundating water derived from the salt marsh
edge as they get higher. From an initial concentration
of fi ne-grained material that is present when the water
passes the salt marsh boundary, sedimentation com-
mences and continues as long as enough material is
left in the water column or the water leaves the marsh
area. The major deposition takes place close to the
marsh edge or creek margin, where the coarsest parti-
cles settle out. This suggests that at some distances
 
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