Environmental Engineering Reference
In-Depth Information
seems no big constraint in dispersal of organisms along
the coastline (see below). After all, the composition of
communities may change in the future as a result of
increasing temperature and CO
2
enrichment of the
atmosphere.
Restoring salt marshes requires less intensive man-
agement to get somewhere near the targets and is
essentially about reinstating dynamic physical proc-
esses. Because of the additional functions that salt
marshes provide, however - such as coastal defence
and carbon storage - above more general
biodiversity
provisioning, the importance of successful restoration
is high.
Although there are many similarities in approaches
for restoration applied in various parts of the world,
some striking different accents can be recognized. In
North America, examples are given of removal of fi ll,
amendment of the soil, sowing target species and use
of herbicides to remove dominant
Phragmites australis
or
Typha angustifolia
. Sometimes, this happens at a
large scale. However, a plea is made for restoration
through an
adaptive approach
by subdividing the site
into modules to be restored in phases. The most urgent
question should be matched to the fi rst module. An
early question might be, which species need to be
planted and in what assemblages? A second question
could be, how should the soil be amended or the topo-
graphy be manipulated to achieve the project goals
(Callaway & Zedler 2009 )?
Mitigation
projects,
including raising a site and planting
Spartina
spp.,
should not be considered restoration, as there is no
balance between tides, elevation, drainage pattern,
substrate type and vegetation, hence it is not a gradu-
ally self-organizing system. In Europe, de-embankment
and increase of tidal amplitude are mainly practised
without further assistance for the development of a
self-organizing marsh. The same holds for changes in
livestock grazing. The latter is the option to be used in
South America and Autralasia.
behind the seawall or summer dike will be greater
when the polder is intensively drained for agricultural
purposes. This will also hold for coastal systems with
accumulation of peat where great shrinkage can take
place (Roman
et al
. 1995). For the sake of coastal pro-
tection and the costs of seawall maintenance, it is
assumed that a well-inundated tidal marsh with a good
rate of sedimentation in front of the seawall or summer
dike is better than a low-lying polder without sedimen-
tation. Coastal defence and nature conservation might
be combined by de-embankment of polders and subse-
quent restoration of these former tidal marshes,
described as ' managed retreat ' or ' managed realign-
ment' (Boorman 1999). In such cases, a new seawall
is necessary inland of the present coastal defence that
will be knocked down or breached.
After de-embankment of a summer polder, renewed
contact with the sea results in rapid re-establishment
of abiotic conditions (Erchinger
et al
. 1994 ). A restora-
tion is also expected to be quickly successful for birds
during high tides, as they have few dispersal problems.
It can take a long time before a site is appropriate for
foraging on the proper type of food, or breeding in the
proper vegetation structure. However, there might be
dispersal constraints for plants. Are tidal plants still
available in the community
species pool
as persistent
seeds in the soil
seed bank
as a historic record of the
former marsh vegetation? A 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.
Percentages of target species, as related to the regional
species pool, established in 70 de-embanked sites in
north-western Europe, may amount to 70%, but most
sites show lower fi gures (Figure 19.6; Wolters
et al
.
2005). The reason is often the constricted elevational
zone for all possible salt marsh communities. Nearly
100% of the newly established species originated from
the adjacent salt marsh in the de-embanked sites.
Apparently, dispersal of diaspores to de-embanked sites
may not pose a problem.
On barrier islands in the Wadden Sea, the restora-
tion of dynamics by (partly) removing artifi cial sand
dikes (see Plate 19.1) is in the phase of planning.
Along the mainland coast of the Wadden Sea, it is
hoped that the intensive ditching pattern in seminatu-
ral marshes will transform into natural drainage pat-
terns with current rates of sedimentation. However, an
experiment with excavation of 1.5 m of clay for seawall
19.4.2 De-embankments and other
measures to repair geomorphological
conditions
Embankments interrupt not only salinity gradients but
also sediment deposition. It is obvious that continuous
rise in net surface elevation occurs on the unembanked
marsh in front of the newly created polder after
embankment. Differences in soil level in front of and
