Environmental Engineering Reference
In-Depth Information
ities, consisting of plant and animal species that are
adapted to low temperatures and constantly flowing
water, are also highly endangered in western Europe
(Wolejko
et al.
1994). Restoration measures should be
aimed at stabilizing the water outflow in the springs
and preventing pollution in infiltration areas.
When the discharge of groundwater has been
diminished or if the spring water has been polluted
in surrounding infiltration areas, the most obvious
measure is to protect the direct catchment areas and
abolish drainage and fertilization practices.
Koska and Stegmann (2001) reported on the
restoration of a severely damaged spring mire in
north-east Germany. Several measures were taken to
raise the water tables in a degraded spring-mire com-
plex (6 ha) in the Sernitz source area. These measures
included (i) construction of a series of wooden dams
in the largest drainage ditches, (ii) complete filling
of ditches with peat, (iii) reflooding parts of the mire
with spring water and (iv) perforating the impervious
gyttja layers at the base of the mire. The best results
were obtained with complete filling of drainage
ditches. Flooding parts of the mire with spring water
that had accumulated behind wooden dams in the main
drainage could not prevent severe desiccation during
the summer. The series of wooden dams in the main
drainage ditch were also ineffective, since it had only
a very local effect and these measures only slow the
drainage down, but do not prevent it. Perforating the
impervious layers to enable spring water to discharge
at the top of the spring mire had also a very local
effect and did not contribute to the rewetting of the
mire. Perforating impervious layers in spring systems
is, in general, not recommended since it can damage
mire systems further down the hydrological gradient.
Yet in the case where shortcuts have been made at
the base of still active springs, the construction of an
artificial outlet in the top of the system could be a
measure for repairing the damage at the base of the
spring. If at all possible, the outlets at the base of the
springs should be closed after installation of the new
outlet closer to the top.
and it can under certain conditions compensate for
the influence of drainage. Negative effects of deep
drainage in agricultural areas can sometimes be com-
pensated for by using existing hydrological structures,
such as a large canal with a high water level. Con-
necting the old ditch system and the remnants of river
meanders with the canal enables the continuation of
flooding in winter, while groundwater losses to the
adjoining agricultural fields are largely compensated
by a lateral (groundwater) flow from the canal and
by high water levels in the old meanders. The com-
position of the surface water is, however, crucial in
such artificial hydrological systems. Nutrient levels and
sulphate levels should be very low to prevent rapid
eutrophication of the soil. More often remnants of
former extensive flood mires are now completely
surrounded by agricultural areas with much lower
water levels. Former discharge areas have turned
into infiltration areas and acidification of the topsoil
has degraded the fen meadows to the extent that,
for instance, iron levels have become critically low.
Reflooding with surface water, even when pre-
purified by helophyte filters (reed swamps that reduce
the nutrient content of the surface water), does not
always restore the iron contents to higher levels,
since surface water usually contains very low con-
centrations of iron. If iron is fixed as FeS
2
(pyrite),
flooding with surface water may lead to a higher pH
during the winter and spring period (van Breemen
& Buurman 2002). However, when discharge of
calcareous and iron-rich groundwater is no longer
available during summer, the groundwater levels will
drop and the topsoil acidifies again due to oxidation
of FeS
2
(Lamers
et al.
2002). Permanent flooding
could prevent acidification, but it will also increase
the availability of nutrients in the soil, leading to the
very productive plant communities (Lucassen
et al.
2004). Such restored wetlands, however, have re-
gained an increased capability to capture nutrients
from the surface-water system (Hoffmann 1998, Olde
Venterink
et al.
2003).
Stimulated flooding in former agricultural areas
usually results in very eutrophic soil conditions
(Richert
et al.
2000, Roth
et al.
2001). Most flood-
plain species on eutrophic soil are very persistent in
the cultural landscape. They have remained in wet spots
and in ditches and after rewetting they can expand
very rapidly (
Glyceria maxima
,
Acorus calamus
and
Phalaris arundinacea
). Although most of these species
9.5.5 Restoration of flood plains:
stimulate traditional management
Stimulating flooding in relatively eutrophic flood
mires can be a good measure to combat acidification,
