Environmental Engineering Reference
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from erosion of the seawall, hence the latter will be
removed for reasons of coastal protection. Coastal
defence and nature conservation might be com-
bined by de-embankment of polders and subsequent
restoration of these former tidal marshes, described as
'managed retreat' or 'managed realignment' (Boorman
1999). For this and other forms of restoration of
halophytic communities two prerequisites need to be
fulfilled: adequate abiotic conditions and the estab-
lishment of target species.
After de-embankment of a summer-polder, the
renewed contact with the sea results in a quite fast
re-establishment of the former abiotic conditions
(Erchinger et al. 1994). A restoration is also expected
to be quickly successful for birds, as they have few
dispersal problems. However, there might be con-
straints for plants. Are tidal plants still available
in the community pool as persistent seeds in the soil
seed bank as a historic record of the former marsh
vegetation? A recent study in natural salt marshes
indicated that most salt-marsh species have a
transient or short-term persistent seed bank (Wolters
& Bakker 2002). This suggests that restoration cannot
rely on a persistent seed bank of salt-marsh species.
Apparently dispersal of diaspores to de-embanked sites
may not pose a problem. Percentages of target species,
as related to the regional species pool, established
in 70 de-embanked sites in north-west Europe,
may amount to 70%, but most sites show lower
figures (Fig. 13.8; Bakker et al. 2002b, Wolters et al.
2005).
In organogenic coastal areas such as the Baltic with
a negligible tidal range, removal of the summer-dike
might result in permanent stagnant pools containing
slightly brackish water without any vegetation, after
die-off of the freshwater community. These events were
indeed found at Karrendorfer Wiesen on the German
Baltic Coast (Müller-Motzfeld 1997). De-embankment
of an estuarine summerpolder revealed that Ph.
australis took over in the absence of livestock graz-
ing, whereas P. maritima became dominant in a
grazed site (Bakker et al. 2002b). These differences
emphasize the effect of management regime on the
outcome of de-embankment. This is a general
phenomenon. The sites that were revealed to be most
successful with respect to restoration of halophytic plant
species after de-embankment were all grazed (Wolters
et al. 2005).
35
France
Denmark
Netherlands / Belgium
United Kingdom
30
25
20
15
10
5
0
Percentage of target species
Fig. 13.8 Frequency distribution of scores of
percentage of target species related to the regional
species pool of over 70 de-embanked sites in north-
west Europe. Note the absence of monitoring in
about 50% of the sites. After Wolters et al. (2005).
13.6.3 Increase of tidal amplitude
In Connecticut, USA, tidal flows were restored by place-
ment of a 1.5-m-diameter culvert in an impoundment.
In 1988, 10 years after the start of the restoration,
T. angustifolia had declined from 75 to 15%, whereas
S. alterniflora had increased from < 1 to 45%. In
addition, high-marsh species had re-established and
covered 20%, but Ph. australis had also spread. After
1988 Phragmites declined and salt-marsh vegetation
had increased to cover 85%. Although the restora-
tion exhibits a striking result, the restored marsh only
moderately resembles the pre-impoundment marsh
(Fell et al. 2000).
The polder Beltringharder Koog, Germany, was
embanked in 1987, including 845 ha of intensively
grazed salt marsh and 2450 ha of intertidal flats. The
tidal range was reduced from 3.4 m to zero immedi-
ately after embankment. Former salt marshes were
covered by Cirsium arvense and Epilobium hirsutum
stands, and former intertidal flats by Suaeda maritima ,
Spergularia spp. and Salicornia spp. In order to com-
pensate for the losses, a lagoon system of 845 ha was
established after separating it from the new polder by
dams and allowing reduced tidal influence through two
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