The solution is as simple as it is cheap: Across the existing
drainage trenches, small embankments have been constructed in
suitable places in front of hollows in the ground which retain
water. During the year, most of the water will then stay in the
forest and slowly sink into the ground to become part of the
groundwater.
In the wetland created behind the embankment, a "monk" has been
installed; a mechanism which, with a simple system of pipes,
regulates the water level in the reservoir and ensures even
drainage of the surplus water to the trench downstream of the
embankment. In rainy periods, the construction retains the water in
an ever-increasing lake, until the water reaches a level at which
any surplus rainwater can run off through a larger pipe.
In this way, eight new wetlands with a total area of 7-8 hectares
will be created by the embankments in Store Hareskov. Seven of
these areas totalling 4 hectares (corresponding to eight-nine
football fields) will probably have gravitational water table all
year round. The total reservoir can retain up to 24,000 m³ of
water. This corresponds to a large retention basin.
The southeast corner of Store Hareskov with the new wetlands
marked by shading. The yellow blocks with the flags show the
location of the monks.
The construction in Store Hareskov has not yet been tested during a
proper flood. However, Kim Søderlund, forester at the Danish Nature
Agency and responsible for the construction, has no doubts that it
will serve its purpose:
"With regard to mechanics, the construction itself prevents water
from coming out of the forest in volumes that will cause problems
in Hareskovby, even during heavy cloudbursts. We are not yet able
to evaluate the impact on biodiversity of the forest.
The new wetlands have been partially filled with water,
especially downstream. We know from experience that it easily takes
2-3 years before they are filled entirely. But there is no doubt
that it will also work for biodiversity," said Kim Søderlund.
Wetter nature will automatically attract new life into the
forest, which is a popular destination for an outing for many
Copenhageners. The Danish Nature Agency expects the area to be
filled with dragonflies, water damselflies, caddis flies and
mosquitoes, which again will attract swallows and bats, as well as
a wealth of frogs, newts and grass snakes.