Environmental Engineering Reference
In-Depth Information
control drainage of the hinterland. It consisted of
intensively grazed salt marsh and intertidal fl ats. The
tidal range was reduced from 3.4 m to zero immedi-
ately after embankment. Former salt marshes were
colonized by tall fresh forb communities and former
intertidal fl ats by salt marsh species. In order to com-
pensate for the losses, a lagoon system was established
allowing reduced tidal infl uence through two sluices.
From 1990 onwards a 0.3 m tidal amplitude was estab-
lished with stormfl ood simulations of 0.8 m tidal
amplitude twice a month in the nonbreeding period of
birds. The restoration was successful in the lower inun-
dated parts where a typical salt marsh zonation estab-
lished. However, the highest parts remained fresh
grassland, and started to get overgrown by shrub
species, in the absence of livestock grazing (Wolfram
et al
. 2000 ).
between seawalls or artifi cial sand dikes in the hinter-
land and intertidal fl ats can be prevented by realigning
landward. This should not be carried out by the exten-
sion of new sedimentation fi elds into the intertidal
fl ats, because this would imply a reduction of the area
of intertidal fl ats with their characteristic conservation
issues such as waders. Along the mainland coast of
Europe, seawalls and summer dikes have successfully
been removed and new salt marshes have developed in
former polder areas. On barrier islands in the Nether-
lands, removal of artifi cial sand dikes is planned, but
experiments are needed. Increase of the tidal ampli-
tude in embanked polders through sluices is not always
successful, and especially the higher parts of the eleva-
tional gradient may be lost for salt marsh development
when only part of the tidal amplitude can be rein-
stated. Intensive ditching patterns in salt marshes with
sedimentation fi elds or mosquito control do not trans-
form into meandering creeks. Consequently, reduced
abiotic variation may establish, and hence habitats for
plant and animal communities.
Increased
sea level rise
caused by global warming
might have a drowning effect on salt marshes. However,
the salt marsh elevation may follow the level of Mean
High Tide without negative consequences for increased
sea level rise. Is current vertical accretion enough for
salt marshes to keep their relative position in the tidal
frame? The current sea level rise in western Europe
is about 3 mm yr
− 1
. Recent modelling studies suggest
the overwhelming importance of tidal amplitude and
concentrations of suspended sediment (Kirwan
et al
.
2010). At future sea level rise of about 10 mm yr
− 1
salt
marshes in regions with a tidal range greater than 1 m
and suspended sediment concentration greater than
20 mg L
− 1
will not be permanently submerged. When
future rates of sea level rise beyond 20 mm yr
− 1
occur,
only salt marshes in regions with tidal ranges over 3 m
and sediment concentrations above 30 mg L
− 1
are
expected to keep pace with future sea level rise. In the
latter case, many marshes will permanently submerge
despite their tendency to accrete more quickly (Kirwan
et al
. 2010). Knowledge on vertical accretion rates at
the scale of catchment areas (creeks with their drain-
age area) in salt marshes is poorly developed. Back
barrier marshes with a low vertical accretion might
suffer earlier than mainland marshes with a higher
vertical accretion.
Mangrove forests occur lower in the
zonation
than
salt marshes. Sea level rise will increase the inundation
frequency, reduce soil salinity and increase transport
19.4.4
Changes in the grazing regime
Cessation of livestock grazing can result in a decrease
of the number of plant and animal species, as dis-
cussed above. In an experiment of resuming livestock
grazing in a formerly grazed back barrier salt marsh in
the Wadden Sea, the changes in the vegetation after
abandonment
turned out to be reversible within 10
years at a relatively high stocking rate of cattle (Bakker
1989). At the larger scale of the entire elevational gra-
dient, the number of plant communities increased,
whereas at the small scale of each elevational level, the
number of plant species per unit area increased, mainly
because of the removal of the tall grass
Elytrigia
atherica
in saline marshes and of
Phragmites australis
in brackish and fresh marshes (Bakker 1989; Bakker
et al
. 2003). However, at the low marsh the number of
species decreased. The soft, frequently inundated sub-
strate is trampled by cattle and only annual pioneer
species can be maintained. It is estimated that the
numbers of brent geese in the entire Wadden Sea in
May can be a factor of 4-8 higher if all salt marshes
were to be grazed by livestock, than in the absence of
grazing (Bos 2002 ).
19.5
PERSPECTIVES
Targets for the successful restoration of salt marshes
include changes in the
geomorphology
and
succes-
sion
of plant communities. Squeezing of salt marshes
