Environmental Engineering Reference
In-Depth Information
accumulated behind wooden dams could not prevent
severe desiccation during summer. The series of
wooden dams in the main drainage ditch were also
ineffective, since they only slowed down drainage,
rather than preventing. Their impact remained very
local. Perforating the impervious layers to enable
spring water to discharge at the top of the spring mire
also had a very limited effect and did not contribute to
the rewetting of the mire.
degrade slowly, such as
Phragmites australis
and various
tall sedges (
Carex
spp.; Richert
et al
. 2000 ).
16.5
PERSPECTIVES
In densely populated areas,
ecological restoration
of
mires is almost impossible since hydrological condi-
tions of natural mires are usually dependent on the
larger hydrological systems of the surrounding land-
scape. If restoration implies restrictions to regional
water use, support from society will often be lacking
entirely. Targets for restoration projects should there-
fore be set carefully and clearly in order to obtain politi-
cal support (Swart
et al
. 2001). In practice, mire
ecosystems can only be restored on a very local scale,
and although features of natural mires may return,
the new ecosystems will differ from the ones that have
been destroyed in former times. Successful projects
have in general been executed at sites little affected by
intensive agriculture and drainage.
Successful restoration in densely populated areas
often implies repairing damage at high costs. From an
economic perspective, conservation of still-existing
undisturbed ecosystems is much more cost-effective.
However, there are increasing numbers of situations
where pressures and motivations are suffi cient to
trigger - and economically justify - mire restoration
elsewhere. Failures to repair damaged elements of
mires are as numerous as the successes, but they are
usually not well documented. Failures are often caused
by an incorrect diagnosis of the restoration prospects
of the site (Grootjans
et al
. 2006), lack of knowledge
on ecological processes affecting the site negatively,
and expectations that are too high or unrealistic.
Reduction of the amount of N deposition remains
a prerequisite for successful restoration of many
nutrient-poor ecosystems, such as bogs, fens and
several types of fen meadow. In some European coun-
tries, atmospheric N and SO
2
deposition has declined
considerably during the past 20 years, due to more
strict environmental legislation. Freshwater ecosys-
tems have benefi ted considerably from drastic reduc-
tions of phosphorus emissions in the surface water, but
in many streams and rivers the amount of sulphate is
still much too high and causes eutrophication in ter-
restrial mire systems when they are fl ooded.
Additionally, leaching of nitrate to the groundwater
should receive much more attention, especially for its
potentially large-scale mobilization of sulphate from
16.4.5
Restoration of fl oodplains
Stimulating fl ooding in eutrophic fl ood mires can be an
effective measure to combat acidifi cation and can,
under certain conditions, compensate for the infl uence
of drainage (see also Chapter 17). Nutrient levels and
sulphate levels should be very low to prevent rapid
eutrophication. Remnants of former extensive fl ood
mires are often surrounded by agricultural areas with
much lower water levels. Former discharge areas have
turned into infi ltration areas and
acidifi cation
of the
topsoil has degraded the fens and fen meadows to the
extent that iron levels have become critically low (van
Duren
et al
. 1998 ; Lamers
et al
. 2002 ). Refl ooding with
surface water, even when pre-purifi ed by
helophyte
fi lters
(reed swamps that reduce the nutrient content
of the surface water), does not always restore the iron
concentrations to higher levels, since surface water
usually contains very little iron. If iron is fi xed as FeS
2
(
pyrite
), fl ooding with surface water may lead to a
higher pH during winter and spring, but during
summer, when groundwater levels drop, the topsoil
acidifi es again due to oxidation of FeS
2
(Lamers
et al
.
2002 ). Permanent fl ooding could prevent acidifi cation,
but will also increase the availability of nutrients in the
soil, leading to very productive plant communities
(Lucassen
et al
. 2004). Such restored wetlands,
however, regain the capability to capture nutrients
from the surface water system (Olde Venterink
et al
.
2003; Richardson & Hussain 2006), and thus provide
habitats for large populations of waterfowl (Middleton
et al
. 2006). Flooding is also important for the redistri-
bution of seeds within restoration sites (Wheeler
et al
.
2002 ; Jansen
et al
. 2004). Flooding does not automati-
cally restore peat formation. Many species that respond
positively on renewed fl ooding, such as
Glyceria maxima
,
Acorus calamus
and
Phalaris arundinacea
, do not form
peat. For peat formation, the vegetation should consist
of wetland species with stiff or fi brous tissues that
