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is highly problematic. Theoretically, the sedimentation
should ultimately decline to almost zero. This would
happen fi rst in the outer part and later the inner part
where after the two levels should even out. But even
after 5,000 years (not shown), the modelled difference
between the two locations has only declined to a little
under 20 cm with a growth rate of approximately
0.5 cm per 100 year and a level of the outer part of
about 1.9 m above
HAT
.
The concept of salt marshes being able to reach
dynamic equilibrium in a rising tidal frame under a
relative sea-level rise has been suggested by Allan
(
1990
) and discussed by others such as French (2006).
Bartholdy et al. (
2010a
) tested this idea by letting
MHWL in Eq.
8.12
¢ follow a rising tidal frame of dif-
ferent sea-level rise scenarios using conservatively
raised high water statistics similar to the one used
above. The results are presented in Fig.
8.18c
(inner
marsh) and Fig.
8.18d
(outer marsh). The starting point
is the same as that above, with the marsh at its present
elevation after about 100 years of simulated deposition
on top of the bare sand fl at. Simulated levels are plot-
ted as the difference between the modelled salt marsh
level and the rising
HAT
. When this difference becomes
less than ÷0.5 m (corresponding to the actual
MHWL
of 0.8 m DNN with the present
HAT
of 1.3 m DNN),
the salt marsh is assumed to degrade back to unvege-
tated tidal fl at, as this is the level of salt marsh initia-
tion. There might be a hysteresis effect keeping already
established salt marshes 'alive' below this level, but to
what extent this is the case is unknown. The constant
rising salt marsh level in relation to
HAT
in the stable
case (a sea-level rise of 0.0 mm year
−1
) is similar to the
results given for the two locations in Fig.
8.18a
. The
overall impression of the different sea-level rise sce-
narios is that there is a signifi cant difference between
the topographical reaction of the inner and outer marsh,
which in principle rules out any possibility of topo-
graphical equilibrium within a realistic timescale. For
each site, there is a specifi c sea-level rise where the
equilibrium concept of Allan (
1990
) actually exists.
This is for the inner marsh close to 0.5 mm year
−1
and
for the outer marsh close to 1.0 mm year
−1
. With a sea-
level rise of 0.5 mm year
−1
, the inner marsh would
'mature' to a constant level in the rising tidal frame
about 1,000 years after salt marsh initiation, and with a
sea-level rise of 1.0 mm year
−1
, the outer marsh would
reach the same stability in about 500 years. It is inter-
esting that both marsh areas reach stability at the same
Fig. 8.17
Map of the distribution of the characteristic concen-
tration difference available for deposition, ΔC in mg l
−1
, on the
central part of the Skallingen backbarrier in a tidal period with a
high water level of 1.3 m. Areas above 1.3 m DNN and areas
associated with creeks have been cut out leaving the underlying
orthophoto visible (After Bartholdy et al.
2010a
)
the backbarrier as well as in the areas along the major
creek systems. This pattern, as will be discussed
beneath, complicates the concept of salt marsh
equilibrium.
Based on high water statistics for the period 1949-
2007, the model by Bartholdy et al. (
2010a
) was run
for longer periods with a stable sea level simply by
using multiples of this distribution. An inner and an
outer position relative to the salt marsh edge with typi-
cal b-values and a sand base level of 0.80 m DNN
(close to the actual conditions on the Skallingen back-
barrier) were chosen to illustrate developments in the
salt marsh level from the time where deposition started
on top of the underlying sand fl at.
The difference between the outer and inner salt
marsh (Fig.
8.18a, b
) continues to grow during the
marsh development for a long period. After about
1,700 years, it reaches a maximum of a little less than
21 cm. Both the outer and the inner area continue to
grow with a few cm per 100 year after this where the
level above the highest astronomical tide (
HAT
= 1.3 m
DNN) has reached 44 cm and 63 cm, respectively. A
stable sea level for such a long period is unrealistic,
and the result is therefore in every respect highly hypo-
thetical. It shows, nevertheless, that the idea of equilib-
rium salt marsh topography in this type of environment
